DIY Auto Service: AC System Diagnosis by Symptom
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
Static AC Gauge Pressures
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.
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Hook up a gauge set or recovery machine.
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Record the pressures. High and Low Side should be equal.
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Measure the ambient temperature.
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Does the pressure match the temperature pressure chart?
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If the pressure matches this means the refrigerant level is close.
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If the pressure is lower than the chart, the refrigerant level is low. The system is probably leaking.
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If the pressure is zero, that is atmospheric pressure and means the system has a big leak.
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If the pressure is higher than the chart, it is either overcharged or may have air (non-condensable) in the system.
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Leak Testers and Refrigerant Identifiers
Running System Pressures
Start the engine and operate the AC system. Perform a visual check to see if the compressor is running.
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Was a click heard when the AC was engaged? Is the center hub turning?
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Observe the pressure gauges. Does the low side pressure start going down and the high side going up?
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After the system runs for awhile, raise the RPMs up to about 1,500 RPMs.
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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.
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Double check to see if the compressor clutch is engaged. If the center hub is turning, the compressor is not pumping.
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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-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.
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Is the condenser or cooling fan working?
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Does the condenser have bent or dirty fins?
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Is there blockage inside the condenser?
Lower High and Low Pressures
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.
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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?
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With a drop, there may be a restriction at the TXV or Orifice Tube.
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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.
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The TXV is stuck open or the thermal bulb is not sensing the temperature correctly.
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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
With or Without Hot Water Valve Systems
There are two basic designs for the airflow:
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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.
- 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
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
Related
Comments
AUTHORDoug
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.
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
- AUTHOR
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.
- AUTHOR
Sounds like a vacuum actuator problem. May have an electric vacuum pump or vacuum leak problem.
- AUTHOR
Bean Motorhome
Sounds like the system is undercharged and the compressor is either not kicking on or a bad compressor.
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.
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.
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
- AUTHOR
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.
- AUTHOR
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.
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.
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
- AUTHOR
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?
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
- AUTHOR
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.
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
- AUTHOR
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.
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?
- AUTHOR
Not enough information but, sounds like a stuck open TXV/H-block.
low side is 85 high side is 150. any suggestions?
- AUTHOR
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
2004 Chevy truck is blowing hot air from passengers vents
I don't know about your high side but it sounds like a fan problem.
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.
- AUTHOR
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.
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.
- AUTHOR
Devin
No pressure change means either the compressor is not turning on or bad compressor.
- AUTHOR
jg
search goggle and Youtube
You can also buy a Mitchell or if you know someone with access look up the procedure.
Hissing noise then you star the car. Refrigerant inidcates full and ac lines are about the same temperature.
- AUTHOR
Michael
The pressure are wrong.
It could not be higher on the low side.
- AUTHOR
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.
I have a 99 Honda Civic the low side is 350 and the high side 150.And still blows cold air out 44 degrees
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
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.
Without knowing the vehicle or system type, it sounds like a restricted or sticking H-block.
It is random with the engine at around 1500 rpm. Sounds like it's coming from the cowl around the expansion valve area
- AUTHOR
Not sure without hearing it.
Does it change with engine rpm or if you turn ac button on off.
Could be compressor noisey.
My ac is making a fog horn noise after it had run for a while. Any ideas?
- AUTHOR
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.
How hot should the high side line be if you put a IR temp device on it? 140 to 200°
- AUTHOR
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.
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?
- AUTHOR
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
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?
- AUTHOR
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.
- AUTHOR
Sounds like a fan problem or blocked condenser.
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!
- AUTHOR
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
- AUTHOR
David
Sounds like a fan problem or blocked condenser fins. Does the AC work better on the highway verses intown stop and go?
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.
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?
- AUTHOR
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.
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.
- AUTHOR
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.
- AUTHOR
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.
what would cause driver a/c vents to blow cold but passenger vents to blow warm
my system starts out cold then the a/c clutch disengages .relay or clutch ?
- AUTHOR
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.
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.
- AUTHOR
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.
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
- AUTHOR
Glad I can help.
Thank you for sharing this information. I am a beginner and don't have much knowledge about this stuff.
- AUTHOR
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.
The Readings depend on the type of refrigerant, did you mention the type, I don't see it?
- AUTHOR
Roadmech
The AC machine charges liquid thru the high side, which is why the system needs to be off.
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.
- AUTHOR
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.
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.
- AUTHOR
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.
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
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
- AUTHOR
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.
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.
Thanks, hope it is helpful.
nice hub.. voted up.. Interesting..
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