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    How to Tell if an Auto AC Compressor is Bad?

    Automotive AC-CompressorDennisB © Summary: This article is written by an ASE Master Tech with many years experience working on automotive air conditioning in Florida. A common question is “How can you tell if a car or truck’s A/C compressor is bad?”. This short easy to understand article answers  this question once and for all.

    Most AC compressors only act as pumps. Very few have some sort of regulation of pressure built-in. The V5 compressor in the image is one exception. It has a variable displacement. But for most compressors just think of them as a simple pump. The question below was asked regarding a compressor that was replaced on a system because they thought it was bad. They thought that since it pumped too high of pressure, it had to be faulty. As you’ll read, this couldn’t be further from the truth.

     Dennis: I am having issues with too much high side pressure, system kicks out on high pressure on ranger at 400-450 and the high side line is extremely HOT, low side seems fine 35-45. The system has been recovered and the compressor dryer and orifice tube replace ( the issue mentioned above was before repairs had been made ) After replacement of parts mentioned above, dropped in the recommended oil half in compressor and other half in dryer and pulled vacuum on system. Then dropped in recommended amount of R134a on Ford Ranger is 30 oz. when you start to get close to filling the system and it starts to cool a little the high side goes up again and causing the compressor kick off and on due to high pressure and the line is still very HOT. Had the system broken down again and had a feeling the condenser was a parallel and was stopped up I replaced condenser in front of radiator and started over again and lo and behold the same thing has happened. I’m at a loss and im no AC expert but an average home mechanic working on his own vehicles. This issue is baffling to me and I don’t understand how I can have the same issue 3 times with new compressor, drier, orifice tube and condenser, note when I replaced major components the evaporator coil was flushed several times and blew out with dried air for a while, till all moisture was gone. This repair has cost me a lot of money even a remanufactured compressor and aftermarket replacement parts also freon WOW, don’t know what else to do other than wishing I had taken it somewhere else to start with. This simple to moderate repair has been a major headache!!

    Well let me start with saying that 45 on the low side is high and you already know the high side is far above normal. Most of the time two things cause too high of pressure;

    1. The system is overcharged (with Freon, flush or oil).
    2. There’s not enough air through the condenser to cool it sufficiently.

    The high pressure line being very hot is normal. The higher the pressure the higher the temp of the line. You have the same problems because of the wrong diagnosis. Keep in mind that even though you’ve opened a system, there could still be refrigerant in it. So if you add a full charge plus what’s already in it you can have an overcharged system. Also the flush, if not removed completely can result in an overcharged system. It is very difficult to remove ALL of the flush solution just from blowing air through the components. I recommend not dumping a lot of flush into a system if not absolutely necessary. You’re better off with an added on in-line filter in the suction line if the system is really contaminated badly. If the system isn’t’ really dirty (black oil) the new accumulator or dryer should be sufficient. Keeping the vacuum pump running for an extended period may help remove excess flush and Freon. The vacuum lowers the boiling point and allows the vaporized flush and refrigerant to be removed with the vacuum pump.

    If the cooling fan (or condenser fan) is not working properly the pressures will climb above normal. Also if the vehicle’s engine is overheating, there can be heat transfer from the radiator.

    In conclusion; If you have a compressor pumping up to 4-500 pounds of pressure – it’s not bad. It’s pumping great. The problem will still be there if you put another… and another compressor on. The problem is most likely the system being overcharged or there’s not enough airflow to cool the condenser. Good Luck.

    How to tell if your A/C compressor has failed?

    1. Not pumping. If the compressor clutch is engaged and both pressure readings are the same, the compressor has failed. Say the both high and low readings with the compressor engaged are 80-100 psi. this means the compressor is not pumping.

    2. Leaking. If the compressor seam or front seal are leaking, the compressor is bad. Yes, they can be rebuilt or resealed, but most of the time you are better off replacing the unit. This is because special tools and knowledge are needed to successfully rebuild an AC compressor. Quite honestly, a do-it yourselfer will not have much of a chance of pulling off a successful rebuild. Even professional techs are few and far between that possess the ability to rebuild a compressor successfully.  Most prefer installing a replacement that has some sort of warranty anyway.

    3. Locked or seized compressor. If the shaft of the compressor cannot turn, it is said to be locked up or seized. This is one of the easiest problems to diagnose. Plain and simple.


    Comment from les
    Time August 6, 2015 at 7:28 pm

    What if the fan clutch does not disengage and is engaged all the time, what would be the cause.

    Comment from Dennis Bandy
    Time August 7, 2015 at 7:56 am

    Les, this is not common. If someone has jumped the cycling switch or if there was a short it could happen, but with the car turned off the clutch should disengage. The other possibility, which I’ve seen happen is the clutch is simply worn. Normally there’s an air gap. The clutch plate has a sort of a spring action when the electro magnet (coil) is energized, which closes the gap and turns the compressor. If the spring portion of the clutch plate (the outer section) is worn or damaged by heat, the clutch may stay engaged, which results in the compressor pumping all the time while the car is running. As long as the nose of the compressor has not been damaged (like by a spun bearing) the clutch assembly can be changed to fix this problem. The clutch consists of the coil, the hub part that the belt rides on and the clutch plate/disc. Usually this can be changed without discharging the AC refrigerant if you can access the front of the compressor, either by leaving the compressor mounted or by unbolting it and pivoting it if the hoses allow. Good Luck.

    Trackback from Tom J
    Time September 7, 2015 at 6:07 am

    Thanks for the article. Just what I needed to determine if my compressor was bad or not.

    Comment from Dennis Bandy
    Time September 7, 2015 at 9:40 am

    Tom be sure to check out our main website for your automotive tool needs,

    Comment from Marcus
    Time June 21, 2016 at 11:41 pm

    Tried to recharge my air today but I keep hearing this ticking sound when I turn the air off.An when I run the air it’s fine please help fix.

    Comment from Dennis Bandy
    Time June 22, 2016 at 1:17 pm

    Marcus, this may not be enough information for me to make a good guess. One tick could be the compressor clutch disengaging. If ticking persists I’d try to pinpoint where it’s coming from to diagnose. Good Luck.

    Comment from Steph
    Time July 11, 2016 at 6:50 pm

    My ac went out a month ago. I brought it into the dealership and the told me the compressor failed and would need to be replaced. I had it fixed and last week my ac stopped working. I am now being told that my ac compressor relay has failed and needs to be replaced. I don’t know anything about cars but it just seems odd that both parts fail within one month of each other. Does this seem right/is this typical?

    Comment from Dennis Bandy
    Time July 12, 2016 at 8:12 am

    Steph, stranger things have happened. It’s entirely possible for two parts to fail closely together. The compressor failing could have even overheated the relay causing it to weaken and fail prematurely.

    Comment from Jason Hartman
    Time July 14, 2016 at 5:14 am

    I have a 2007 jeep liberty. The ac has a charge but the clutch is not engaging. With the engine off I can turn the compressor shafts so I don’t think it’s locked up. I’ve unplugged the coil on the clutch and plied it into my radiator fan and my fan works so I know there’s power at least to my clutch. What do you think does it sound the coil.

    Comment from Dennis Bandy
    Time July 14, 2016 at 7:09 am

    Jason, I think you meant to say since you CAN turn the shaft that the compressor is NOT locked up. That’s right as long as you are turning the shaft and not just the clutch disc. I’d check for power directly to the clutch coil (maybe you have if I follow correctly) because there are other parts of the circuit that could be causing a an open besides the relay. Could be a bad pressure switch, a broken wire, control head etc. Hopefully it’s just a bad coil though. If you’re sure there’s power and ground at the coil it’s going to be a bad coil. Good Luck.

    Comment from james
    Time July 18, 2016 at 6:52 pm

    Got a 2015 6.7 ram 102k miles, ac stopped blowing cold yesterday. Clutch kicks in and out with switch and rpms drop when it engages, 35 on low side. Checked with digital thermometer gun today after running for 10 mins with ac on the compressor hit 478°F. Any ideas? I think something went wrong in co pressor.

    Comment from Dennis Bandy
    Time July 19, 2016 at 1:27 pm

    I’d check BOTH high and low readings. 478 degrees sounds like no oil in system or clutch slipping. Good Luck.

    Comment from Erica Osborn
    Time August 9, 2016 at 3:31 pm

    My air is going on and oit, it was working fine. When I put freon in it because over charged and the belt began to squeal every now and again when I tried to run the air. When I let some freon out the air got cold again but then instantly ran off and on again between hot and cold air. But stopped squealing. If I what to do

    Comment from Dennis Bandy
    Time August 9, 2016 at 5:18 pm

    Erica it is illegal and really bad for the Ozone to just release refrigerant into the atmosphere. You most likely let too much out now. I recommend taking it to a shop so the charge can be adjusted safely.

    Comment from billy mac
    Time August 12, 2016 at 2:54 pm

    new evaperator, new drier reman compressor. flushed out comdensor. high side only gets to about 50 psi and starts dropping off. low side seems a little high. 50 or 60. psi. HELP

    Comment from Dennis Bandy
    Time August 12, 2016 at 2:58 pm

    Ambient pressure with system off should be around 80-100. If compressor is running, the low side should be around 30. See our ac gauge readings explained for more info.

    Comment from Mike
    Time March 30, 2017 at 2:06 pm

    Hearing a grinding or rattling sound from front of engine. Noise goes away when I turn the AC on. I was told the compressor clutch probably needs replacing. Is this logical and, if so, would it be best to replace the entire compressor or just the clutch?

    Comment from Dennis Bandy
    Time March 30, 2017 at 3:20 pm

    I would replace just what’s needed. As long as the clutch didn’t cause damage to the front of the compressor I’d replace the coil, hub and disc. Good Luck.

    Comment from Ethan
    Time June 1, 2017 at 2:05 pm

    My AC is only blowing warm air. I can see that the clutch is engaging when i turn the AC on, so this would tell me that the electrical is ok, the clutch is operating as it should and there is sufficient pressure in the system for the clutch to pickup and engage the compressor.
    I am now at the point where I need to determine if my compressor is in fact functioning or if the clutch has broken off from the compressor.
    To do this I would like to measure the pressure on the low and high sides but I do not know what those pressures should be.
    If my compressor is good that would indicate a leak in the system; how should i proceed; inject some dye and see where its leaking out? Or would it be possible that there is a blockage in the system? how would I determine that?

    Comment from Dennis Bandy
    Time June 2, 2017 at 9:19 am

    Pressure readings?

    Comment from ron
    Time June 24, 2017 at 3:42 pm

    Hello, I have a 96 Tahoe. my compressor goes through a cycle of on and off every 6 seconds. The car only blow warm air and the Freon is full. Any thoughts on this? I hear it could be orifice tube. I was thinking of replacing the compressor since its easy and cheap but I don’t want to toss parts and too broke to pay for a mechanic :/ help.

    Comment from Dennis Bandy
    Time July 28, 2017 at 7:22 am

    Compressors are not cheap! Could be an orifice tube but no way of guessing without checking the system out. Check gauge readings, then see our AC gauge readings explained. Good Luck.

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