DIY Auto Service: AC System Diagnosis by Symptom

Updated on March 21, 2016

AC System Diagnosis

The typical complaint for an AC problem is the AC blows warm air. This condition could be caused by; the AC system being low on refrigerant, an electrical problem not allowing the compressor to turn on, an internal problem in the system or 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 Hubs.

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 Blowing Out the 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 3 to 4 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.

Comments

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    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      18 hours 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 

      2 days 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 

      8 days 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 

      8 days ago from Phoenix, AZ

      Sounds like a fan problem or blocked condenser.

    • profile image

      Gabe 

      8 days 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 

      5 weeks 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 

      5 weeks 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 

      5 weeks 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 

      5 weeks 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 

      7 weeks 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 

      7 weeks 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 

      7 weeks 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 

      7 weeks 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 

      8 weeks ago

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

    • profile image

      red hill 

      8 weeks ago

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

    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      2 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 

      2 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 

      2 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 

      2 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 

      2 months ago from Phoenix, AZ

      Glad I can help.

    • profile image

      Salina Hocutt 

      2 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 

      13 months 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 

      13 months 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 

      14 months 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 

      14 months 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 

      14 months 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 

      14 months 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 

      2 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 

      3 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 

      3 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 

      4 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 

      4 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 

      4 years ago

      Thanks, hope it is helpful.

    • goodnews11 profile image

      OSBERT JOEL C 

      4 years ago from CHENNAI

      nice hub.. voted up.. Interesting..

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