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Compressor Rapidly Cycling On and Off in Car Air Conditioner
Friday, 10 June 2011 12:24

This week I've done some work on my 2001 Honda Civic EX.

Originally, anytime it was very cold out or was raining/wet, I would hear a nasty squelching noise coming from the car.  Then, I attempted to turn on the air conditioner on in the car last week and again - very bad squelching noise coming from the car when the AC was on.

So I took it down to a guy I know that works on cars for a very affordable price.  I took the car to him a few years back to have him replace the timing belt and serpentine belt.

Turns out there is a second belt - an accessory belt - that controls the A/C compressor.  He got that replaced and also replaced the serpentine belt while he was at it.  Problem fixed - no more squeek!  But, he did tell me that my AC was cycling on and off and it needed to be charged.

So, down to the local automotive store.  I bought one of those low-pressure gauages for about $15 and a can of R-134a refrigerant with stop-leak in it.  I know folks online say not to use this - so I won't be using that going forward.

Got home and began to slowly put the R-134a into the car air conditioner system with the AC turned on and the temperature set fully to cold and the blower fully on.  The compressor was fully on and the car just was barely cooling before I began adding the freon into the car - but it certainly was not keeping up and was not cool enough.

As I was adding the freon into the A/C system, I heard a popping-like noise and noticed that the compressor kicked off.  Then, it happened again, and again!  The air conditioner compressor was rapidly cycling on and off.

I turned the A/C off and stopped putting R-134a into it becuase the compressor cycling was almost causing the car to die from the load it was putting on the engine.

Onto Google I went.  Everywhere I turned, people said that if the compressor was cycling on and off quickly, that meant the compressor was low on freon.

OK - so explain this to me - why was the compressor not cycling at all before I put the refrigerant into the car - and only started happening when I got about 3/4 a 12-ounce can into the car?  I was afraid I overfilled it - because the car was originally trying to cool before putting any R-134a into it.  All the forums and posts I saw indicated that people that had this problem had no cooling coming out of the car.

Called up my mechanic and he basically told me the same thing - he said that if the compressor was rapidly turning on and off, it was because it was undercharged.  But, I explained to him how it was working fine - and somewhat cooling - before putting any refrigerant in - then it started kicking on and off after adding 3/4 a can in.  I said - "Is it because I overfilled it?"  He agreed and thought that I may have overfilled it - but said he hasn't ever noticed a car doing that before.

The Honda Civics can hold up to 22 ounces of R-134a in them.  A 12-ounce can would be just more than half of it's capacity.

Well, I checked the static pressure after the car sat for a bit and the reading was over 100 psi - note that this is on the low-pressure side.  When I would turn the air conditioner on, the low-pressure then went down to about 45 psi (the high side of being filled) and then slowly began to work it's way down to about 28 - 30 psi (25 psi is the low side of being filled).  Then the compressor would pop and kick off again.  Then it would pop and kick on again.  Over and over maybe five seconds apart.

I then called a co-worker that had a full set of hoses that can be used to check both high and low pressure.  At this point, I was pretty sure that it was overfilled and I wanted to see if the car was being kicked off because of high pressure - not low pressure.

So we hooked the hoses up and the high pressure climbed very rapidly when turing the A/C on.  it hit 350 - then 375 - then 400 - then just shortly after, the compressor popped and shut off.  Then high pressure then started going down - then the compressor kicked on again.  So, the compressor was rapidly turning on and off still.  As expected - the high pressure was too high.

Some of the freon was taken out of the vehicle and the guy said there most likely was some air in the line that wasn't helping things.  After taking some out, the compressor stopped kicking on and off - and the high pressure then stayed fairly stable between 325 and 350 psi.

Test drive time!

Didn't even make it very far and the compressor started kicking on and off again!  So, back to the hoses.  This time, I revved the car up to about 1,500 to 2,000 RPM (since the Civic seems to idle at around 500 rpm).  Sure enough, he said that the low pressure was being sucked down to about 20 PSI and the high-pressure was kicking way up again past 400 PSI.

So more was taken out until the high pressure would only be about 325 PSI at 1,500 RPM.

Alright - test drive again!  It worked!  That was the problem - it was simply over-filled.  The car no longer cycled the compressor rapidly after taking some R-134a out of the system and it cools very well.

So, the car only needed a bit more than 1/2 a container of 12-ounce freon and it was working like a champ!  With recirculating the air and the outside temperature around 80 degrees, the car was putting out about 44 degree air from the A/C vents.

So next time that your air conditioner compressor rapidly turns on and off in your car, it may not be due to low freon levels - but because it is over-charged!  Those cheap low-pressure testers that come with the charge kits only tell you part of the story.  When I was charging mine, I was well within the acceptable range - but the high pressure (which is what I couldn't see) was telling an absolute different story!


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