Air Conditioning Auto Repairs

Can't fix your car problem on your own? Ask a live mechanic!


Sometimes air conditioning auto repairs are not as complicated as they may first seem. Let me give you a quick example of what I’m talking about.

A customer recently visited me for a car air conditioning repair. He informed me that the air conditioning stopped working towards the end of the warm weather in 2008.

He thought for sure that this would be an expensive problem and decided to hold off on the diagnosis and repair of the air conditioning system until the warm weather returned. This is probably what most automobile owners would do in this situation.

Automotive air conditioning diagnosis

automobile-air-conditioning-diagramMy first step in any auto repair troubleshooting is to perform a visual inspection. But I let myself down and did not perform this step. In my mind I thought that the Freon had leaked out, and that’s why the compressor was not coming on. This is the most common situation when it comes to automotive air conditioning not blowing cold air.

I connected my manifold Gauges to the air-conditioning system. I found that not only were the pressures the same on both sides as they should be with the compressor off, but the vehicle currently had a full charge of Freon. My customer was buzzing around me trying to figure out why I had a surprised look on my face.

I said to him, I have good news and bad news. The good news is your air-conditioning system is filled with Freon and ready to blow cold air. The bad news is the compressor is not coming on and it is apparently an electrical problem.

My customer said oh my god, not an electrical problem. I said calm down and let me start poking at it and see what I find. I checked the air-conditioning fuse and it was good. So I returned to my normal diagnostic procedure. I performed a visual inspection of the electrical wiring and connectors to the cars a/c system.

Air conditioning auto repairs

A/C Compressor
A/C Compressor

I had the key on engine off and decided to check for power at the A/C compressor. When I grabbed the electrical connector, which was a two wire simple power and ground connection, the compressor clicked on and engaged. So the problem turned out to be a poor connection to the air-conditioning compressor. No actual parts and a very small amount of labor repaired the A/C system.

But that is not the end of the story. The reason for the poor connection was that the connector was actually damaged. The plastic housing that holds the terminals had a crack in it and allowed the terminals to back out. I question my customer about how strange this was that there was damage to this connector.

My customer said, oh I forgot to tell you. Last year my belt broke and I had it replaced while I was out of town at a local gas station. So actually what happened was when the belt let loose it damaged the air-conditioning compressor electrical connector.

But yet another lesson to be learned about air-conditioning auto repairs. It is not always a good idea to put off diagnosing and repairing this system even if you will not be using it for months. My customer knew that his air was not working and suffered with his dark secret all winter long.

He actually told me that he was dreading that summer was approaching, because he knew the A/C repair would be necessary. So why spend five or six months worrying about something that may be repaired very easily. Also if the situation was different and there was a major A/C leak and the system was left empty for that long, further damage may have occurred and increased his auto repair costs.

This story is one reason that I decided to post a DIY auto air-conditioning repairs section to my new car repair website. You will find many pages that will not only explain the theory of operation, but also discusses common automotive air conditioning problems.

Can't fix your car problem on your own? Ask a live mechanic!

57 Replies to “Air Conditioning Auto Repairs”



  2. mrg

    Hello Scott: This sure is a strange air conditioning problem. The fact that the compressor stays on only when in the defroster mode is bizarre. The subject of my post was air conditioning auto repairs are not always complicated. But in your case it appears to be complicated.

    My next step If I was diagnosing your vehicle would be to connect a manifold gage set to the high and low side service ports. Then monitor the pressures. Your car ac may have a restriction or a clog in it. The best way to check for this is to monitor ac system pressures.

  3. marc

    Hi I filled up my 99 Pontiac grand am with a freon and it works great for about 2 or 3 weeks. Then the air started to get warmer. I am assuming it’s a leak. If it is a freon leak how can i resolve the ac problem.

  4. michael

    hi i filled my 99 grand am gt up with freon the compressor runs fine but my a/c fan wont kick on and the air is not really cold what could the problem be thanx

  5. mrg

    If you are talking about the ac condenser fan not working? It could be a bad fan motor itself. I have seen this often. But the best bet would be to follow a diagnostic tree chart for the ac fan inoperative. This will test the circuit that sends power to the fan motor.

  6. Mark

    Running into the same issue on 99 Grand Am GT. Put gauges on and system has pressure. Cannot get compressor to engage. Checked Relay, (swapped with horn relay) changed one fuse (10 amp for AC was blown).. Switch light in dash glows as if it is getting energized, What other electrical back yard mechanic checks can be done?
    Any check for high side switch? Check for BCM activating relay?

    Thanks for any help..

  7. mrg

    Mark on Your 99 Grand Am ac problem you mentioned the fuse was blown. If you replaced it and it didn’t blow again? I would have someone check for power at the compressor.

    If you do have power at the compressor clutch with no engagement it could be a bad clutch coil. I have seen this before where the ac clutch coil takes out the fuse when it goes. It was not on a 99 Grand am though. And it could be a lot of other things as well just thought I would mention it!

    I put up a video I recommend for basic electrical diagnosis tips. The video actually shows you how I use automotive wiring diagrams to fix electrical car problems.

  8. Yvonne

    The AC in my 99 Grand Am also only blows warm air. I took it to a mechanic and they checked for leaks and evacuated the system. Now they are telling me that it is an electrical problem. They say I have to replace the ECM (electronic control module) and have it reprogramed but I am not sure about this. What do you think?

    Thank you.

  9. mrg

    It is possible for the ECM to cause the ac not to work. It’s rare but I have seen it before on a Grand Am. What makes me think that a second opinion is in order is the fact they evacuated the system.

    If the clutch was not engaging do to an electrical problem, the shop should have never touched the freon side of the ac system. They could check pressures without evacuating. Second opinions on expensive repairs is always a good idea.

  10. Melissa Hayes

    I have a 1997 Lincoln Town Car, the air compressor won’t come on. If you put 12 volts to the low pressure switch it will come on and blow cold air, as soon as I disconnect the wire it quits. Are there any other switches or sensors to check? Thanks

  11. mrg

    Melissa: Sorry I would have to see a wiring diagram. If the Town Car has automatic climate control that system can be quite complicated. If you do have ATC the factory auto repair manuals are the best way to go. They will have diagnosis and repair diagrams for the electrical side of the system.

  12. Mike

    I bought a recharge kit – 2 cans of R12 and gauge and R134 fitting with blue cap. The low side went from 30 pounds to the recommended 38 pounds, but when I removed the quick coupler, I discovered that the valve was damaged in the process and leaks. So I replaced the valve. This was not done quickly because of rust in the stem. I lost some refrigerant ( and maybe some mineral oil? ) When I tried to recharge the system again, the low side would not go below 90 pounds. Do I need to add mineral oil? Is there an air lock now? Would I have better luck at night when it is cooler outside? What do I do?

  13. mrg

    Rick: If I can’t find the freon charge capacity on an under hood label I usually start with a 2 lb charge, check pressures and add from there. I have worked on a few 97 town cars for ac problems and I think that the charge is 2.4 lbs but don’t quote me on that.

  14. jim merry

    I have a new Honda Civic, it blows cold air when accelerating, but it warms when deaccelerating. Honda says that is normal ?
    This is after they evacuated and rechaged sytem (added.76 lbs) because it was never cold from the factory.
    I have experienced warm air when increasing speed (on small car) because shutting off the compressor helped acceleration. The oposite doesn’t make sense to me.


  15. automotive jobs

    The air-conditioning was working okay. But, now when I turn it on, the a/c knob lights up for about three to four seconds and then starts flashing. Now the compressor will not work.

  16. Luis

    I live in South Florida and have a 2005 Jeep Liberty. A/C worked fine until last week when the a/c just suddenly started blowing warm air. However, when I accelerate it starts to blow cold again. It only blows cold during acceleration. I looked under the hood while the engine was in park and at idle. Found the clutch engaging and disengaging in 5-10 second intervals.

    I used one of those cheap gauges that come with a charge can i had lying around from a previous vehicle and it read full charge. I need help.. Its so hot and humid down here. Money is so tight that i am afraid to go to a tech.

  17. mrg

    Luis: It sounds like maybe a condenser fan type problem. The faster you go the more air that flows across the condenser. Its the job of the condenser fan to pull that air across the condenser at idle.

    Without air flow across the condenser the high side pressure shoots up. The cycling clutch would be a symptom of that also. The high pressure cut off will disengage the compressor to prevent damage from high pressures.

  18. Elisabeth

    Hi! I was driving today and my AC suddenly stopped blowing cold air. I had someone help me put freon in it. As we were doing it, the air would seem to start to get cooler and then would stop. Once the car appeared to not take anymore of the freon, we stopped, with still no cold air. the compressor seems to be trying to kick on and then stops repeatedly. Easy answer?

  19. Mark

    Elisabeth, The way I would approach your problem from where your at Now is. The freon that’s in there needs to be recovered. Then vacuum needs to be applied to the system for about 30 min. This removes air and moisture that may be causing problems. Then the exact charge required can be installed. Next the system can be leak checked and diagnosed as needed.

  20. joe

    I recently had 120k service done on my 03 Lexus LS430. The next day the the dash vents stop releaseing air yet I could hear the blower was running on high. This problem went away only to re-occur he next day.

    Just a few days later while driving to work the air con stopped cooling for about 10 minutes then it began working again. This happen several times since and now it seems to cool only for very short periods of time if it does at all.

    The coincidence of these 2 problems along with the recent service makes me wonder if there is more to my problem than the compressor, which the repair service company has told me needs to be replaced.

  21. Mark

    Joe : It sure does seem like a strange coincidence and you do have good reason to be cautious. As a mechanic there are 2 words I try to steer clear from “Always” and “Never”. Because of that one automobile out there that can through you a complete curve ball!

  22. Jeff

    Hi, I have a 2005 Jeep Liberty; ac has been working sporidically (meaning it runs some days, some days it does not). I have put the guages to it and the low side is very high and high side is low. What are your initial thoughts based on this? At first I thought maybe a clog but then it shouldn’t work on the other days no?

  23. Mark

    I am leaning towards a compressor going bad or maybe a clog or even both. Sometimes material from the compressor can cause clogs in the system. Clogs can shift and move and cause intermittent operation.

  24. Richard

    Here’s a strange issue:

    My compressor had seized on my 2000 Town Car and broke the belt. The compressor and the belt were replaced. The air runs perfectly cold. however, it seems as if the system is always on. The high-pressure side from the compressor to the can gets completely frozen even when the A/C is not turned on. When it is, it freezes up after about 20 minutes and no air comes out from the vents. 15 minutes of the car turned off and melts and blows fine until it freezes up again. Also, there is no clicking from the compressor AT ALL, even when turned on.

    No one seems to be able to figure this one out. One mechanic told me it seems to be always on. What are the dangers to the system if this were to continue?



  25. Mark

    Richard: 2 things come to mind. First the compressor always being on. If the air gap between the clutch and compressor pulley is not correct you may not get disengagement when it is turned off. This would also account for the no clicking situation. The air gap may need to be adjusted on the replacement compressor. The air gap is simply the space between the compressor clutch and compressor pulley with the vehicle off.

    The freeze up issue. When a compressor seizes it usually spreads tiny pieces of metal throughout the system. It is always recommended to flush the system before replacing the compressor. Maybe a tiny piece of the old compressor got stuck in the expansion valve? This would not allow the system to respond to the freeze up by controlling pressures.

  26. bill sorvillo

    it seems like the pulley on the a/c compressor keeps seizing up and making the belt rub when the car is idling then will go again and seize up again. any ideas what this is. the car is a 2005 jeep liberty sport 6 cylinder 4×4. thanks

  27. Max J. P.

    First; hello & thanx in advance :o)

    I am the proud owner of a 2000 Pontiac Grand Am SE w/ the 3.4 V6. I say PROUD owner because she’s been a really good car [ bought w/ 47,000 miles … now enjoying just over 155,000 & still running great ].

    So, my question: For filling the AC system, both Chilton’s & the factory sticker under my hood recommend 1.35 pounds of R134 — What is the psi equivalent for this in my vehicle so I can correctly monitor the fill gauge I bought or is it just a matter of simple math regardless of what this guage reads [ i.e. 1.35 X 16 oz of physical refrigeratent / 21.6 oz R134 ] ??

    Thanx again,


  28. Mark

    Hello Max: Can’t give you an exact PSI because that number will fluctuate with outside temperatures and many other factors. That’s why the factory provides the exact physical amount in weight to ensure the right amount of freon is installed. The proper amount is key to proper cooling.

    You want to start with a empty system and then add the recommended amount. (3) 8oz cans would most likely be perfect because you usually loose a little when changing cans and making connections. As far as when the system is running you should have a low side psi of 30-35 psi

  29. John

    hello i have an 02 vw beetle tdi my pressure readings are low side high and high side low to normal and ideas an where to start ? thanks in advance ~JOHN~

  30. Bill

    At the end of the winter season I noticed that my A/C wouldn’t blow any hot air….only cold. It has been like this for two months now. Just last night (now that it’s warm) it started to blow only hot air, even though the compressor is still engaging. Quick fix? Time to see the mechanic?

  31. joshua

    I have a 1996 chevy 3500 the air compressor comes on when you turn the blower to off but as soon as you turn it to any other setting than off the compressor kicks off help please

  32. Darryl

    i have a 1995 GCL. The air conditioning is not blowing cold air but seems to have good lowside pressure I think. (When testing it it is low while the clutch engages and then returns to the good section as soon as it disengages) I tested it using a refill kit with gauge. The ac clutch does engage but only for about 2 seconds and then off for 10 seconds then back on. I tried jumping the low pressure cable but it did not engage. I used a test light and I see the same thing. light goes on for 2 seconds then off for 10 and continues with this cycle.

    Any suggestions on what to test next would be great.

  33. Mark

    Darryl: It sounds like your system is either still low on charge or has a restriction or clog in the low side. Recommending professional diagnosis at this point because for further diagnosis you would need to connect gauges to both the high and low side.

  34. Eric


    I really enjoy your concise and clear descriptions of A/C systems. I have a situation I have been trying to diagnose and would love some input from you.

    I have a 2008 Dodge Ram 2500, 6.7L diesel. Recently, the air started blowing warm. I was told by the previous owner that he had used one of the large cans of freon with the cheap gauge to recharge the system because it wasn’t blowing cold and that it fixed the problem at the time. Now I can’t seem to get the compressor to kick on. Here is a list of the things I have checked:

    Fuses – OK
    Voltage at compressor – Seems OK (using a crappy analog multimeter)
    Low Side Pressure w/ compressor not running – 300 – 350 psi
    High Side Pressure w/ compressor not running – 150 – 200 psi

    I am trying to test everything before replacing the compressor. There is a pressure transducer switch (has three prongs inside) on the high side line going into the condenser. Do you know if there is a way to jump the switch to see if I can get the compressor to kick on? Or could I run a ground and power wire straight from the battery to test the compressor? Would that even kick it on and what kind of damage could that cause?

    Or do you have another idea to test?



  35. Mark

    Eric: I recommend that you get a professional diagnostic test on the ac system before you replace the compressor. I’m actually thrown off by your pressure readings you supplied. Not sure how they could be that high with the compressor off? An initial diagnostic should cost you less then a 100$ and could save much more! we have ac shops here in Florida that will do it for free in the hopes of getting a new customer.

  36. Kevin

    I have a 1999 Sierra and with the mainfold gauges in place and running, a/c recirculating switch on, I get low side readings fluctuating between 26-76 psi. The high side reads 145-110 psi. With the a/c recirculation switch of the readings are 85 low and 90 high and the compressor clutch is not on. Engine off the pressure is 75 psi on both sides of the gauge set. Had to jumper the cycling switch to fill the system. Still turns on and off even after I replaced the cycling switch. I am stumped!

  37. Mark

    Kevin: The fluctuating low side readings from 26- 76 could be caused by several things including an internal compressor problem. But with your other readings it seems like there may be other issues like a partially clogged orifice tube. It may be in your best interest to pay for a diagnosis from a trusted AC repair shop.

  38. Kevin

    Thanks Mark, I was headed that way since this seems to be very odd, and I do see frost build up from the accumulator to the orifice and slightly beyond it. Thank you and honor and praise to our men and women in uniform.

  39. Monica

    I have a 90 Cadillac Deville and my a/c is blowing but it is not blowing cold air. We recently had freon put in it about a month and a half ago. We took it to a mechanic and he said he thinks it is the high and low switch. Is that what it could be? Is it a costly repair? Also the service transmission message keeps lighting up. When I shift the car into drive is usually the only “kick” I get from the car. We have just replaced the spark plug wires. And one more thing, the mechanic says it smells like out catalytic converter is going out. Does that have anything to do with the transmission? I thought it was an exhaust problem.

  40. Chris

    Hello, I am trying to troubleshoot a sudden A/C failure on a 2006.5 Kia Optima EX V6 with auto climate control. I took it to a trusted shop and the tech said that he found two things. One: The fuse for the compressor clutch circuit (10A) was blown. Two: The pressures seemed a little high, however they were even on both sides. He replaced the fuse and did an evac and recharge and the system was working, however two miles down the road it failed again. The fuse had blown again. I replaced with a 15A spare and it worked just long enough to start blowing cold air before the fuse failed again. I have a basic knowledge of electrical circuits and own and know how to use a multimeter. I did measure with power off each leg of the fuse recepticle against the negative cable on the power distribution box under the hood and one leg is open and the other leg is reading short. I know that the open side most likely goes to the A/C relay so that should be normal. I’m just wondering if the other leg should read some resistance or straight short to ground. Luckily, I do have access to all wireing schematics and TSBs due to Kia making everything available online to anyone who wants to register. Any help would be greatly appreciated.

  41. Mark

    Chris: A common cause of blowing a compressor fuse after a short run time is high resistance in the ac clutch coil. This is the device that when energized creates a magnetic field that pulls the ac clutch into the engaged position. You should be able to use your meter to get a resistance reading (HOT or after the fuse blows) and then compare that to the specifications in your manual. Note that other things can cause the fuse to blow. Just something you can check.

  42. Chris

    Thanks, I think I’ll take these steps (as soon as it stops raining here). I’ll unplug the compressor and try to turn on ac and see if the fuse blows at that point. I think that would rule out a grounding out problem and then measure from the hot wire on the compressor to the outer casing of it to ohm it out. Thanks for the reply. I read that depending on the manufacturer of my particular compressor it should ohm out to 3~4 ohms. I hope it’s something simple, because I don’t want to take that compressor out. It’s in there pretty good. Worst case though, KIA does sell the individual components for the compressor. The clutch disc, pully and coil all have separate part numbers.

  43. Chris

    OK, checked everything out and I think I have determined that it is the field coil. It measures 1.6 ohms from the positive side to the outer casing of the compressor.

  44. Stephen Goode

    Hello Mark,
    At one point my a/c was working fine but over a period of a few months I noticed that whenever I would run the a/c, it would blow cool air for about an hour before it would eventually get warmer. I would then turn the a/c off for a short time and then turn it back on. It would cool again but eventually get warmer. Now the air does not cool at all. How would you diagnose this?

  45. Mark

    Stephen: The way you started to describe your ac problem “at first” it sounded like a classic case of an evaporator freezing up. This is often caused by a defective expansion valve. But now that the condition has changed its hard to say. I would begin diagnosis with getting some low and high side pressure readings. I wrote about 10 articles on my other website about Diy auto ac repair

  46. Andrew Vilailack

    I own a 2000 Mercedes ML 430 SUV and the AC works intermittently. The AC will blow cold air at low to med high speed setting but not at high setting. If I drive drive the vehicle with AC running and turn on the AC later, the system will not blow cold air. When the AC is working the temperature of the blown air is 48 to 68 degree. I have to charge the freon once a year. Please help, I am confused.

  47. Mark

    Andrew: The AC systems on a Mercedes can be very complicated. It might be best to have a dealer run a diagnostic test to confirm the problem. The list of possible causes is huge and I would not want to start guessing from the info you provided.

  48. Markos

    Dear Mark,

    I am driving one of the first Lexus models (1990) and the A/C is blowing super-cold, even at the midway level (between hot/cold) when it has freon. But it runs out in two to three days. I have no idea about cars and I’m kind of broke. Please give me advice (perhaps it may be fuse, connection?)…hopefully not a big leak!

    Thank you,


  49. Deb

    We just had a new compressor and dryer replaced in a 04 Chrysler Pacifica.
    The air is cold but not as cold as it used to be with the old unit. The air feels the same no matter what setting its on. It has full range of temps.
    We are confused because it’s all brand new parts. Any ideas?
    Thank you 😉

  50. nick

    Hi Mark I got a 2003 Nissan Maxima with 50k miles. The a/c blows cool but not super cold like it used to. The pressures are 80 on the high side and 90 on the low side. I tapped on the expansion valve thinking it was possibly stuck but nothing changed. Bad compressor? what do you think?

Leave a Reply

Your email address will not be published. Required fields are marked *