DIY Auto Service: AC System Diagnosis by Symptom

Updated on January 25, 2019
Michael M Thomas profile image

Michael Thomas, automotive and diesel technician, is the author of the McGraw-Hill textbook "Truck and Trailer Systems."

AC System Diagnosis

The typical complaint for an AC problem is the AC blows warm air. This condition has a few possible causes: the AC system could be low on refrigerant, an electrical problem may not be allowing the compressor to turn on, there could be an internal problem in the system, or perhaps there is a problem with the heater/AC controls. Since the most common problem is usually the refrigerant, we will start there.

Three-Part Series

This series is divided into three parts. The AC system operation, AC system service, and the diagnosis of a malfunctioning system. To be able to service and repair an AC system, read through all three articles.

DIY Auto Service: Air Conditioning (AC) System Operation with TXV or Orifice Tube

DIY Auto Service; AC System Service and Component Replacement

DIY Auto Service; AC System Diagnosis by Symptom

Static AC Gauge Pressures

Static Pressure means the system is not running. Compare this pressure with the ambient temperature to gauge the amount of refrigerant in the system. Less than the chart indicate low refrigerant charge and higher may indicated an overcharge.
Static Pressure means the system is not running. Compare this pressure with the ambient temperature to gauge the amount of refrigerant in the system. Less than the chart indicate low refrigerant charge and higher may indicated an overcharge.

Static Pressures

There is a relationship between the pressure in the system and temperature. A temperature pressure chart can be used to compare the pressure of the refrigerant at a given temperature. Since there is no way to know exactly how much refrigerant is in the system, without removing it and measuring it, we will use the pressure and compare it to a pressure temperature chart.

  • Hook up a gauge set or recovery machine.

  • Record the pressures. High and Low Side should be equal.

  • Measure the ambient temperature.

  • Does the pressure match the temperature pressure chart?

    1. If the pressure matches this means the refrigerant level is close.

    2. If the pressure is lower than the chart, the refrigerant level is low. The system is probably leaking.

    3. If the pressure is zero, that is atmospheric pressure and means the system has a big leak.

    4. If the pressure is higher than the chart, it is either overcharged or may have air (non-condensable) in the system.

Leak Testers and Refrigerant Identifiers

If the Static Pressure is low, a leak is probably present. Use the leak tester for larger leaks and the Dye with a black light for smaller leaks. Identify the refrigerant before hooking up to a recovery machine.
If the Static Pressure is low, a leak is probably present. Use the leak tester for larger leaks and the Dye with a black light for smaller leaks. Identify the refrigerant before hooking up to a recovery machine.

Running System Pressures

Start the engine and operate the AC system. Perform a visual check to see if the compressor is running.

  1. Was a click heard when the AC was engaged? Is the center hub turning?

  2. Observe the pressure gauges. Does the low side pressure start going down and the high side going up?

  3. After the system runs for awhile, raise the RPMs up to about 1,500 RPMs.

  4. The system should be about 25 to 30 psi on the low side and 200 to 250 psi on the high side. If the ambient temperature is higher or lower than normal (70 to 80°F) the pressures will go up or down accordingly.

Low and High Side Pressure the Same

No change in low side and high side pressures, the pressures are still equal.

  • Double check to see if the compressor clutch is engaged. If the center hub is turning, the compressor is not pumping.

  • Shut the engine off and rotate the center hub by hand. Is there any resistance?

The compressor may be faulty.

Higher Pressure on the Gauges

Higher Pressure on both the High and Low side may indicate an overcharge or lack of cooling at the Condenser.
Higher Pressure on both the High and Low side may indicate an overcharge or lack of cooling at the Condenser.

Higher-Than-Normal Pressures

Higher than normal pressures on both high and low side with the correct amount of refrigerant, could mean a problem with air flow thru the condenser.

  • Is the condenser or cooling fan working?

  • Does the condenser have bent or dirty fins?

  • Is there blockage inside the condenser?

Lower High and Low Pressures

Lower pressures may indicate low refrigerant level or a weak compressor.
Lower pressures may indicate low refrigerant level or a weak compressor.

Lower Low and High Side Pressures

Lower than normal pressures on both high and low side, with the correct amount of refrigerant, could mean a problem building pressure in the system or too much heat is being removed at the condenser. Is the ambient temperature low?

With the system running place a fender cover in front of the condenser and observe the pressure.

  • Does the pressure go up? Can the compressor build the pressure? If not the compressor may be worn out.

  • Is the cooling fan running constantly? A pressure switch or sensor could be bad.

Lower Low Side and Higher High Side Pressures

This condition usually means there is a restriction in the system or the TXV/Orifice Tube is blocked or closed. Feel the lines, is there a pressure/temperature drop at the TXV/Orifice Tube?

  • With a drop, there may be a restriction at the TXV or Orifice Tube.

  • With no drop, there may be a restriction upstream from the TXV or Orifice Tube. Follow the liquid line back towards the condenser to see if you can feel a pressure/temperature drop.

Higher Low Side and Lower High Side Pressures

Too much refrigerant is flowing into the evaporator.

  • The TXV is stuck open or the thermal bulb is not sensing the temperature correctly.

  • The orifice tube is too large of an opening or the o-ring sealing the outside of the orifice tube is not sealing.

System Pressures Are Normal, but Warm Air Is Blowing Out of Vents

Some times the refrigerant part of the system is not where the problem lies. The AC and Heater operate out of the same box or plenum typically under the dash. Doors control the direction of the airflow created by the electric blower motor. Depending on the system, there are at least three to four doors controlled by cables, vacuum actuators or electronic motors.

These doors are:

  • Hot/Cold temperature door - controls the amount of airflow through the heater core. During AC (Cold) operation, this door may force the air to bypass the heater core. When the temperature lever is set to Hot, all the air is forced through the heater core to use the heat from the engine coolant to heat the cab.
  • Fresh Air/Recirculate door - This door selects whether the air is drawn from the outside or circulate the air inside the vehicle. Recirculating the cooler drier air inside the passenger compartment can help maintain a cooler temperature instead of cooling the hot moist outside air. Typically some outside air is always added to keep the passenger compartment pressurized to prevent exhaust from entering the passenger compartment.
  • Floor/Vent/Defrost door or doors - This door or a combination of doors controls where the cooled or heated air is sent. Floor mode typically for heater operation since heat rises. Vent out the center of the dash for AC operation. Top of the dash to defrost the windshield during cold weather. There may also be combinations of floor/vent or floor/defrost to accommodate different situations.

AC Heater Plenum

The AC Heater box or plenum is located under the dash. Electronic actuators with position sensors are very popular to move the doors to direct the airflow and control temperature.
The AC Heater box or plenum is located under the dash. Electronic actuators with position sensors are very popular to move the doors to direct the airflow and control temperature.

With or Without Hot Water Valve Systems

There are two basic designs for the airflow:

  1. All the airflow goes thru both the evaporator then the heater core. A hot water valve closes the flow of hot coolant thru the heater core during the AC operation. If this valve doesn’t close, the AC cold air will be heated by the heater core. If the valve sticks closed, there will be no heat. The result is hot air out the vents. In the defrost mode, the air will be dehydrated by the evaporator then heated by the heater core for hot dry air on the windshield.

  2. The airflow goes thru the evaporator and a door directs the air either thru the heater core or around the heater core directed by the HOT to COLD selector. During AC COLD operation, the “Blend Door” routes the cold air around the heater core. During Heat operation, all the air passes thru the Evaporator and the Heater Core. When the temperature is set in between, part of the air will pass thru the Heater Core. No hot water valve is needed because in cool mode the door closes off airflow thru the heater core.

When the Air passes thru the evaporator and heater core as in example 1 above, a hot water valve is used to shut off the HOT water to the heater core. These Hot Water Valves can be; Cable operated, Air Operated and Electric Solenoid operated.

Door Controls and Actuators

To control the airflow thru the heater box or plenum a series of doors are used. To control these doors a variety of actuators have been used such as:

  • Manual cable operated doors have been around a long time. Cables connect levers in the dash control to the doors. When a lever is moved back and forth, like the hot/cold lever, it moves the door from one position to another. Typically moving the lever quickly from one extreme to the other results in a thumping sound as the door hits the stops. The Hot/Cold door would control airflow around or thru the heater control. Cables popping off or improper adjustments are typically what will go wrong with this style of control.
  • Vacuum operated actuators, used on cars and light duty trucks use (gas) engine vacuum to act upon a diaphragm which is connected to the doors. As the controls are moved, a hissing sound is heard. Vacuum leaks are the biggest problem with this type of control. If the vacuum supply is disconnected, the controls usually default to putting air on the windshield. Diesel engine light trucks using vacuum controlled systems use an electric or belt driven vacuum pump to supply the vacuum.
  • Pneumatic actuators operate similar to the vacuum type, except the trucks air system pressure is used to move the actuator diaphragms. Hissing under the dash could be from a leaking actuator or connection.
  • Electronic actuators use an electric motor to position the doors. These electric motors usually have a sensor built in to allow the system to know the location of the doors. When the controls are moved, a small electric motor buzz can usually be heard.

Service Tip: A common problem with all the actuator types are paper clips, pens, keys and other foreign objects falling down into the heater box from the dash defroster vents and jamming the doors. This could also strip out the door connection to the actuator rod.

Cabin Filter

Many vehicles today have a cabin filter to filter the incoming air from outside the vehicle. The filter looks like a small air filter from a car. Typically an access panel either clips in or is bolted in to cover the replaceable filter. The filter may be located under the dash or on the heater box in the engine compartment. This filter is often overlooked until it is entirely plugged up and causes very little airflow into the cab.

If the cabin filter is dirty the result will be low airflow. This may cause the AC or heater output to be reduced. Many times this is confused with poor AC or Heater performance due to the fact that the low airflow cannot keep the vehicle cool/warm.

Dirty Evaporator

The dirt on this evaporator has restricted the airflow and caused a lack of cooling.
The dirt on this evaporator has restricted the airflow and caused a lack of cooling.

Dirty Evaporator

The airflow across the evaporator is used for cooling the air. If dirt, fuzz, leaves or plastic bags are covering the surface, the airflow will be reduced along with the cooling capacity. The AC system may be in good working order but the air cannot have the heat removed as it should. The complaint will be lack of cooling the same condition as a plugged cabin filter.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

  • How about if the static pressure is not equal. 50 psi on low pressure and 180 psi on high pressure with an outside of temperature of 75F, what does this mean for diagnostics of an AC system?

    With the system not running the pressure should equalize thru the Orifice Tube or H-block. When you shut off the AC you hear a hissing noise. I would say there is something wrong with the gauges. What happens when the system runs, do the gauges both move?

  • I have a 2008 Rav4. Twice I have tried to charge the A.C. system to the proper pressure. When the proper pressure is reached, the vents blow warm air and the compressor drops out. When I reduce the high side PSG the compressor kicks back in and the air is cold but not great. Could it be the hi pressure switch?

    1st we don't charge to a pressure, the system is filled to the proper capacity in pounds or kilograms. The system must be evacuated before charging.

  • How do you tell if the condenser is restricted ?

    By the temperature in and out. Hot in, cold out is restricted. Usually, the high side is really high and low side really low.

  • I recently replaced my ac compressor, drier condenser, and front expansion valve on my 2006 Dodge Durango and my ac compressor clutch does not fully engage and squeals. What should be my next step? 2006 dodge Durango slt 4.7l with front and rear ac.

    3 things to check

    1. Clutch plate air gap between the plate and the pulley in specs.

    2. Voltage at the compressor, should be 12V

    3.Voltage drops in + or - side of the AC circuit, bad connections or corrosion (in the bottom of fuse panel) causing voltage drops.

  • I have a 1998 suburban with rear a/c. The pressures are 30psi on the low side and 165psi high side. It cools to about 45 in the front and 60 in the rear. The compressor clutch stays engaged and the pressure readings stay the same. It has a new compressor, orifice tube, and accumulator. It had this problem prior to replacing these parts. I pulled a vacuum on the system prior to charging. Any thoughts?

    Usually, the problem is reversed. The rear ac uses a TXV/H-Block and it sounds like the valve is not working. The systems with rear ac hold more refrigerant, usually about 3.8 pounds. Should be a tag on the radiator support.

Comments

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    • profile image

      Andy Jones 

      3 weeks ago

      Hi

      I took my 2006 Mazda 5 in for a diagnostics as it hasn't been blowing an cold air for past few months. They did a diagnostics and say the high side pressure is too high and suggest taking it back for a flush, clean and re-gas. The funny thing is since the diagnostic it working fine. Do you think I need to get the work done or can I get away without it? The car is old so reluctant to spend too much on it if I can get away with it.

      Thanks

      Andy

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      5 weeks ago from Phoenix, AZ

      Terri

      Sounds like the blend door is not sealing all the way. Something may have fallen down inside the plenum.

    • profile image

      Terri Kim 

      5 weeks ago

      have a 2013 GMC Acadia with dual temp system.

      passenger side is not blowing cold air.

      checked the blend door actuator and it is working fine.

      removed the actuator and manually turned the door. air goes from hot to cool air but never as cold as the driver side.

      what are other possible causes beside the blend door?

      thank you

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      6 weeks ago from Phoenix, AZ

      If the system is prooerly charged, sounds like a bad compressor

    • profile image

      robb 

      6 weeks ago

      need help with s-10. pressure low side 84lb. pressure high side 150Lb. blowing warm

    • profile image

      Dale 

      2 months ago from South central Oregon

      Hey-

      I have an '06 Dodge cummins. A/C was working fine, turned off engine for 10 mins. and started engine again but A/C refused to cool.

      I layed underneath and could turn clutch by hand, ¼ turn or so of clutch felt like I hit a detent. It's not a smooth feel as I rotate clutch by hand.

      I'm sure that's not normal.

    • profile image

      richard scott 

      3 months ago

      really helpful chart. decided to pursue my ase. all the way to master and b. degree. gonna start with ac. sounds like you know what your talking about

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      4 months ago from Phoenix, AZ

      Doug

      I would start from scratch and drain evacuate and recharge the system with the correct amount of refrigerant. This will make sure there is the right amount of refrigerant and no contaminants (air).

      I assume this is a Pickup so no rear ac. If this is a TXV the compressor doesn't cycle.

    • profile image

      Doug 

      4 months ago

      Hi

      Have 03 Dodge Ram

      My static pressure was good

      80 at 77 degrees

      But operating temps were low

      25 low

      And 125 high.

      Hooked up refrigerant and before I added

      The clutch wouldn’t disengage

      And high side went to almost 400.

      I let it sit

      Let some refrigerant out and in the end got to 45 low

      175high

      Clutch would engage and only low side would move.

      Turned off and static pressure is 78 low

      And 125high after sitting.

      The high side will not move ever

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      4 months ago from Phoenix, AZ

      Luke

      The ac light blinking means the computer has found a fault. This is usually an electrical problem, like a bad sensor. The high side pressure should be equal to or higher off and running.

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      4 months ago from Phoenix, AZ

      Sounds like a vacuum actuator problem. May have an electric vacuum pump or vacuum leak problem.

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      5 months ago from Phoenix, AZ

      Bean Motorhome

      Sounds like the system is undercharged and the compressor is either not kicking on or a bad compressor.

    • profile image

      Luke 

      5 months ago

      Hello I have a 2010 tundra with a/c light blinking and compressor not turning on.

      Already has a new compressor and a/c relay.A/C gauge reading was low side 90psi and high side 50psi.just wondering if you have any idea.

    • profile image

      Dale McLean 

      5 months ago

      Have a Ford Triton V-10 engine in my motor home. The air conditioning works fine on trips but after about 30 minutes the air stops coming from the dash vents and is diverted to the defroster vents for about 1 minute (varies), then back to the dash vents for maybe 4 0r 5 minutes . After it starts it continues this cycle for the duration of the trip.

    • Bean Motorhome profile image

      Bean Motorhome 

      5 months ago

      Have an 2008 Tiffin Allegro Bay RV basically the same AC as any car (I'm Guessing)

      Bought a set of gauges (but first did the cheep way fro Walmart a recharge kit) but found my high side is 50(should be around 250) and low side is 45 and per spec's not sure whereto start to look any advice planing a trip from FL to CT and need ac to work and would appreciate and suggestions where to look

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      6 months ago from Phoenix, AZ

      Any time you are losing refrigerant, you should add black light dye on the next recharge. After running the ac for awhile, use the blacklight to find the leaks. the leak will show up as yellow green because it will be carried out by the oil leaking out. On small leaks it may take a week or so of running, sometimes longer.

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      13 months ago from Phoenix, AZ

      How hard are you accelerating?

      Max accel ca. Shut off the ac but should return when you let off the gas.

      May be a loose connection. Try moving the wires with it running.

    • profile image

      Ali 

      13 months ago

      Hello, I own a bmw X1 E84 and I have an issue with AC. When I accelerate, the AC doesn't blow cool air and I noticed that the cooling fan stops spinning. I have to restart it to get it working again.Please would you tell me what could be the problem. Thanks in advance. Have a good day.

    • profile image

      Jeremy 

      14 months ago

      Mike,

      Thanks for taking the time to share your knowledge, It was a bad relay on the Honda. I was also having the same problem with my 93 Toyota pickup, started looking around and found a hidden in-line fuse that went bad, fixed them both.

      Thanks again,

      Jeremy

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      14 months ago from Phoenix, AZ

      If the center clutch hub is turning the pressures should go up on high side and down on low side. Sounds like bad compressor.. when the compressor kicks in ios there any change engine RPM?

    • profile image

      Mike Bittner 

      14 months ago

      2003 honda element. New compressor,new condensor ,new expansion valve. Compressor kicks on but the pressures remain the same .They do not change.Low and high side remain the same.got me stumped.Could it be a bad new compressor.I did vacuum for 30 minutes.

      Thanks

      Mike Bittner

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      14 months ago from Phoenix, AZ

      If the pressure is at 250psi without the compressor running the underhood temperature could have raised the static pressure if it was hot from running the engine. At 100 degrees, the static pressure should be 125psi approx. if the vehicle has cooled off.

      The compressor doesn't engage, sounds like the problem. The clutch doesn't click in? Check the electrical for the compressor.

    • profile image

      Jeremy 

      14 months ago

      Mike,

      Wife was driving the car,a/c stopped blowing cold, threw the gauges on it, low side pegged out retard, high side around 250, around 100* outside. Compressor does not engage, spins freely by hand, static pressure is also pegged out on low side and 250 on high. Do you think the expansion valve is stuck open or check valves in the compressor. Thanks

      2010 Honda odyssey

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      14 months ago from Phoenix, AZ

      Too much oil takes up space that should be refrigerant.

      Oil is thruout the system.

      You may try lowering the amount of refrigerant by 15%, GM used this in retrofits when old oil was left in and new oil was added.

      The other option is pulling the compresser drain and measure the oil. Add recommended amount. That still doesn't tell you how much is in the rest of the system.

    • profile image

      NC Mike 

      14 months ago

      Mike,

      I have a 2004 Honda CRV. Compressor was replaced, but old was not exploded. Drier material and expansion valve were replaced. Upon initial startup, the compressor appeared to be locked up with belt slipping on it. After shutdown and re-start of vehicle, the compressor cycled as normal once the proper refrigerant amount was reached. The cabin temperature was never great, just below where it should be and barely keeping up. The condenser fan was not kicking in, so it was replaced.

      So now for the problem. The compressor clutch now does not appear to be cycling on/off, and the temperature is still not very cold at vent. Low side line is cold to touch where it should be, and high is very warm. I then checked the resting pressure of system, and the low side pegs out all the way to the highest point. The high is around 200. I do think there is more oil than was called for when installing the new pump: Like an idiot, I forgot to empty and replace a metered amount into the pump. Could it be too much oil? Is this a clutch issue or compressor issue? Can you help?

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      14 months ago from Phoenix, AZ

      Not enough information but, sounds like a stuck open TXV/H-block.

    • profile image

      agb3 

      14 months ago

      low side is 85 high side is 150. any suggestions?

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      15 months ago from Phoenix, AZ

      Chevy trucks use electric actuators. There is a left and right temp control. Try cycling the passenger temp lever from hot to cold several times. It should switch to heat then cold. If not the actuator probably needs to be replaced

    • profile image

      Harvey 

      15 months ago

      2004 Chevy truck is blowing hot air from passengers vents

    • profile image

      Mike 

      15 months ago

      I don't know about your high side but it sounds like a fan problem.

    • profile image

      jglassaz 

      15 months ago

      2007 buick rendezvous

      everything in the system has been replaced except the a/c lines and pressure sensors. compressor, evaporator, condenser, TXV and drier. I've evacuated the system. Put freon in. The system runs but not very cold. After running for 10-15 min the compressor stops and the low side has pegged the needle and there aren't any readings from the high side.

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      15 months ago from Phoenix, AZ

      Teo

      At 90 degrees the low side should be 29 to 31 and the high side 200 to 275 typically. There is no adjustment. The Hblock TXV controls the flow into the evap at about 30 psi.

    • profile image

      Teo 

      15 months ago

      My subcool is lower than superheat, is undercharged. My ambient air temp (Outside air temp) is 90F degrees, the superheat should be 8F-12F degrees. My problem here is I need to refill the refrigerant so my low side is around 35psi. But no matter how I refill, it won’t increase, still stay at 30psi. For your info, my high side is 200psi. My gauge(checked) is ok. Can I know whats the problem. Thanks.

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      15 months ago from Phoenix, AZ

      Devin

      No pressure change means either the compressor is not turning on or bad compressor.

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      15 months ago from Phoenix, AZ

      jg

      search goggle and Youtube

      You can also buy a Mitchell or if you know someone with access look up the procedure.

    • profile image

      Devin 

      15 months ago

      Hissing noise then you star the car. Refrigerant inidcates full and ac lines are about the same temperature.

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      15 months ago from Phoenix, AZ

      Michael

      The pressure are wrong.

      It could not be higher on the low side.

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      15 months ago from Phoenix, AZ

      Scott

      If the low and high side are about the same, the compressor is not working.

      The high side should go up and the low side down when the compressor kicks on.

    • profile image

      Michael Day 

      15 months ago

      I have a 99 Honda Civic the low side is 350 and the high side 150.And still blows cold air out 44 degrees

    • profile image

      jglassaz 

      15 months ago

      I'm trying to remove the dash to change out the evap core. Haven't been able to find any diagrams.

      Vehicle is 2007 Buick Rendezvous CXL 3.5l V6

      Thanks

    • profile image

      Scott Johnson 

      15 months ago

      I put my air conditioning unit on my gauges And the low side buried the needle all the way to the right And my high side is about 150 and the needle will climb to to 170 and stay there.

    • profile image

      Mike Thomas 

      15 months ago

      Without knowing the vehicle or system type, it sounds like a restricted or sticking H-block.

    • profile image

      Jrod 

      15 months ago

      It is random with the engine at around 1500 rpm. Sounds like it's coming from the cowl around the expansion valve area

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      15 months ago from Phoenix, AZ

      Not sure without hearing it.

      Does it change with engine rpm or if you turn ac button on off.

      Could be compressor noisey.

    • profile image

      Jrod 

      15 months ago

      My ac is making a fog horn noise after it had run for a while. Any ideas?

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      15 months ago from Phoenix, AZ

      Steve,

      The temp of the lines depends on the pressure inside. 75 degree day the line will be real warm to touch, 95 degree day it will be hot to the touch because high side pressure is higher.

      The low side should feel cold if everything is working no matter the outside temp.

    • profile image

      Steve Crow 

      15 months ago

      How hot should the high side line be if you put a IR temp device on it? 140 to 200°

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      16 months ago from Phoenix, AZ

      Teo

      If your gauge readings are accurate, there seems to be blockage between the low and high side. This could be a bad TXV/H-block or blocked Orifice Tube depending on your system.

    • profile image

      Teo 

      16 months ago

      My car AC static pressure is not equal. The low pressure side is 80psi and the high pressure side is 105psi. The ambient temperature is 32 degree celsius. Actually I should get both side 105psi. Can I know what is the problem?

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      16 months ago from Phoenix, AZ

      David,

      The readings seem to be very high for the system not running. The ambient temperature affects the readings and a hot engine compartment could also raise temperatures. Overcharging the system will also raise pressures. More information is needed.

      Ambient temp

      Engine Compartment temp

      Refrigerant charge

    • profile image

      David 

      16 months ago

      2011 Toyota Avalon

      Just took readings as compresso clutch was not engaging. High side reading at about 130 and the low side was between 120 and 350 in the retard zone. Expansion valve?

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      16 months ago from Phoenix, AZ

      Greg your VW Rutan also sounds like a fan or blocked condenser problem. If driving at higher speed seems to allow the sysyem to work, that would confirm it.

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      16 months ago from Phoenix, AZ

      Sounds like a fan problem or blocked condenser.

    • profile image

      Gabe 

      16 months ago

      I recharged my a/c, wirked for about 10mins and went hot. Now the pressure on the line is past the red, its trying to make a seond trip around the guage. Any clues!

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      17 months ago from Phoenix, AZ

      Richh52

      90psi should be enough static pressure (depending on outside temp) to operate the system. Your other pressure readings are not correct. It seem like you have 2 low side pressures. Rehook the gauges, system off, and make sure the valves are closed and the knobs are both turned clockwise on the connectors to open the fittings. The high side should go up and the low side down from the static readings with the compressor running.

      Since the system works some times, there is an intermittent problem causing the problem or low air flow across the condenser

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      17 months ago from Phoenix, AZ

      David

      Sounds like a fan problem or blocked condenser fins. Does the AC work better on the highway verses intown stop and go?

    • profile image

      Richh52 

      17 months ago

      My a/c works part time. I checked the refrigerant pressures. When the compressor runs high side and low side are the same at 42lbs. When the compressor is shut off the pressures are the same at 90lbs. Could this be due to a faulty valve or is it the compressor. When it is working, it is very cold.

    • profile image

      David D 

      17 months ago

      R134a at 95 degree F should be 50-55 low side and 275-300 high side. When I check static pressure, it takes 15 minutes for gauges to equalize at 130 psi. With compressor on the initial readings are both low 30/250 and creeps to 27/235 blowing 42 deg F air. After about 20 minutes low reads low and high reads high 32/320, blowing 52 degree air, Increased load from AC causes Tentionier/Ribbed belt to vibrate violently. It this a partial blockage or is it freezing up somewhere creating a blockages or poor cooling fans? What is your guess?

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      17 months ago from Phoenix, AZ

      Jack Leg

      Glad you found your problems. Ask the shop if they put dye in with the refrigerant. If they did, use a black light flashlight and the leak will show up bright yellow/green. Depending on the size of the leak it may take several weeks to show up.

    • profile image

      Jack Leg 

      17 months ago

      Mike,

      As promised - follow-up. Two problems. First, fan module was kicking in and out causing the fan to work only sometimes. In 92 degree heat fan finally quit completely and got a check engine code for overheat. Direct wire to fan showed fan worked OK. So, replaced fan module with junk yard module ($25 for part). Problem one solved. Second, as you suggested, at a good shop had system completely recovered, evacuated and system held vacuum ok. Took 11 oz of 134a (27 oz total system). So, it was pretty low. Cost - $120 including the 134a (cost was much better than buying a recovery unit). Might have a very small leak, but if it lasts 6+ months it's better than the cost and effort to fix the leak. Thanks again for the pointer.

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      17 months ago from Phoenix, AZ

      Warm mailman

      It sounds like your vehicle has dual zones. There is a separate actuator for each side. If you have dual controls try moving it from

      Cold to Hot (blue to Red). Should feel warm air on hot and cold on Cold. Try several times. If that doesn't work the actuator need to be replaced.

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      17 months ago from Phoenix, AZ

      Red hill

      Sound like the compressor is being shut off by the computer or a bad connection. If the system runs for a short time and shuts off, I would check for cooling fan operation.

    • profile image

      warm mailman 

      17 months ago

      what would cause driver a/c vents to blow cold but passenger vents to blow warm

    • profile image

      red hill 

      17 months ago

      my system starts out cold then the a/c clutch disengages .relay or clutch ?

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      18 months ago from Phoenix, AZ

      Is the accumulator cold with the system running? If it is it could be the blend door actuator in the heat position. If it is barely cool or warm it is an ac system (low Refrigerant) problem.

    • profile image

      Jack Leg 

      18 months ago

      Thanks Mike. I'll have to get that done outside. I don't have the recovery unit. Happened all of a sudden so there might be a leak developed that is big enough to cause a sudden issue. Don't see any evidence of a leak, but visual is no guarantee.

      I will let let you know the result. Jack Leg.

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      18 months ago from Phoenix, AZ

      With the temp being 85 to 90 degrees the static pressure should be higher. This indicates low refrigerant. The quick cycling of the compressor is also an indicator. Recover the system and measure the refrigerant. I would bet it is less than half charged.

    • profile image

      Jack Leg 

      18 months ago

      Mike, great article on gauge reading.

      Question: 2008 Merc Grand Marq, a/c system is the completely auto type. R134a system - Here's the readings: Engine off , a/c off - System static pressure is High side 75, Low side 85. Engine on and a/c running on max cooling - High side 175, Low side 25 to 28 (the compressor is drawing down ok on low side when it cycles). Ambient temp is 88 to 90 degrees, about 65% to 75% humidity. R134a does not appear to be low. Here's the problem: Engine running and a/c on max - blower works fine, but the compressor kicks on for about 5 seconds, then kicks off for about 15 to 20 seconds. This cycle just keeps repeating. Clutch looks OK, not heating up so compressor appears to engage properly when it cycles on. However, no cooling at all - the compressor does not cycle on long enough - dash vent temp is about 100 to 110 degrees. I think I'm OK with the refrig part of the system, but I'm not experienced with the automatic electrical stuff in the newer type cars. My guess is some type of sensor or relay causing the compressor to cycle on and off rather than anything with actuators...your diagnosis and suggestions of where to start looking would be much appreciated. Thanks in advance. Jack Leg DIYer

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      18 months ago from Phoenix, AZ

      Glad I can help.

    • profile image

      Salina Hocutt 

      18 months ago

      Thank you for sharing this information. I am a beginner and don't have much knowledge about this stuff.

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      2 years ago from Phoenix, AZ

      Mac

      The pressures for all 3 refrigerants are very close.

      With TXV or H-Block about 30psi on the Low Side and 125 to 275psi on the High Side depending on temperature outside.

    • profile image

      MAC 

      2 years ago

      The Readings depend on the type of refrigerant, did you mention the type, I don't see it?

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      2 years ago from Phoenix, AZ

      Roadmech

      The AC machine charges liquid thru the high side, which is why the system needs to be off.

    • profile image

      Roadmech 

      2 years ago

      Thanks for your reply. Please confirm if both high and low port charge on the vehicle will be open throughout the procedure of using an ac recovery,recycle, evacuation and recharge machine.

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      2 years ago from Phoenix, AZ

      Roadmech

      When using a Recovery/Recycling/Recharging Machine the engine is always off before recovering, evacuating and recharging. The machine can be used as a set of AC Gauges to measure running system pressures and to measure static pressures.

    • profile image

      Roadmech 

      2 years ago

      Thanks for these piesce. When using a recovery, recycle and recharge machine, you did not state wheather the car engine will be running or the engine will be switch off throughout all the phases or recovery,recyclying, evacuation and recharging process.

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      4 years ago from Phoenix, AZ

      Usually the only way to get to the evaporator is tore move the heater box under the dash. This can be a lot of work and the AC system and cooling system need to be drained first.

    • profile image

      YhuNnusz 

      4 years ago

      i really like fwiololng your blog as the articles are so simple to read and follow. excellent. please keep up the good work. thanks. lista de emails

    • profile image

      lynnchase9 

      5 years ago

      I've had a dirty evaporator forever and I can't figure out how to fix it. I've tried to clean it myself, but it hasn't worked. Do you have any other suggestions? I may just take it into a car service place if I can't figure it out. http://www.ipswichcitymechanical.com.au

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      5 years ago from Phoenix, AZ

      A system that is low on refrigerant is the #1 cause for systems not working. This is usually due to a leak in the system.

      One of the main reasons systems are overcharged is the can that taps into the low side only which are available at the local parts stores. There is no reason for a system to be overcharged if using a machine.

      Today's systems have no way of knowing how much refrigerant is in the system without recovering and recharging. Once the systems is recovered, evacuated and recharged to specifications then the operation can be evaluated.

      The other problem with the low side only hookups is that when you hook the can up the hose is full of air which is then pushed into the system.

    • hardlymoving profile image

      hardlymoving 

      5 years ago from Memphis, TN

      This article is the most simple, straight forward trouble shooting guide to automotive A/C systems I have every read.

      Only thing I would change is the beginning paragraph where you stated low pressure will result in the A/C not working. I've dealt with 2 cases were an overcharge resulted in the A/C not working. I've found a overcharge condition can be avoided by allowing the new freon to circulate for 5 to 10 minutes before taking a low pressure reading.

    • profile image

      Mike 

      5 years ago

      Thanks, hope it is helpful.

    • goodnews11 profile image

      OSBERT JOEL C 

      5 years ago from CHENNAI

      nice hub.. voted up.. Interesting..

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