AC Compressor Problem

[wlong]

My AC is not cooling although the compressor comes on as expected. A quick pressure check indicates plenty of refirgerant (150 high side and 75 low side) but the compressor is not pumping much. I think my swash plate is stuck in the minimum displacement position. Only one mention of this possibility in previous posts.

Any ideas or suggestions or should I figure on a new compressor?

Thanks in advance

[maxnix]

What is the temperature at the coils? Presume your interior fan is working properly? If so, then I am thinking expansion valve.

[wlong]

Maxnix - Thanks for the input.

Outside temp around 90°F, interior a little hotter. Yes, interior fan, displays, controls, etc. appear normal. Anything else to check?

If it is the expansion valve (stuck closed), why would the low side pressure be so high (75 psig+ versus an expected <30 psig)? I would also expect the high side to be much higher in this case. It just looks like the compressor is in the low displacement mode.

[maxnix]

Why I said to take the temperature at the coils if the interior fan is working.

If they are not at 34°F, then expansion valve is not working (ambient temperature) or refrigerant may be low or drier partially blocked.

Your plate theory may be correct, but I am not well versed enough to comment on how it would fail in that mode.

The only other thing would be to get a drier from Joe and do acomplete evacuation and repalcement of refrigerant and lubricant. Ever been done? You may find your refrigerant is low. If the coils check out as cooling, but not sufficiently, that would be my next step.

[qship96]

The biggest mistake you can make is to guess at A/c problems and throw parts at it.Much cheaper and easier to spend the money having an ac specialist properly diagnose the problem,and have them or yourself replace the parts they determine need replacing-your pressure numbers mean nothing if not recorded EXACTLY as the FSM conditions mandate.Expansion valve,dryer,compressor,blah blah blah-might be one or all,or none of these-and whatever you do,do not just"top it off" to see if it helps

[wlong]

The pressure readings I quoted were obtained "EXACTLY as the FSM conditions mandate" and the FSM concludes that the compressor is failing its performance test and should be replaced (not a guess). Since the compressor is operating and not making a lot of noise and was just recently working fine, I proposed a theory as to what had failed (swash platte stuck) and was hoping that someone on this board perhaps had a similar experience and potentially an easy fix. The point of the forum is to share these things and hopefully avoid spending money on an AC specialist to diagnose/repair the problem. I don't think the AC specialist would offer much beyond the procedures outlined in the FSM. My experience is that they typically offer less.

[qship96]

well,maybe you should do a search on this forum,as I recently posted on my similar a/c problems with my 96q along with the fix that cured it-however the best diagnosis begins with a evac/recharge to be sure regrigerant level is perfec,without that factor being spot on,its all a crap shoot.You can measure pressure all day,but a weak fan clutch,clogged condenser,restricted air flow,internal restriction,etc will create different pressure readings-leading to possible wrong diagnosis-get the freon level spot on,and go from there

[maxnix]

Well, if you go to a mechanic who has a refrigerant recovery machine, who knows what you get.

But if you go to a shop that specializes in automotive refrigeration, usually they have an expertise that far surpasses any other people not in the daily practice.

Now back to the questions. Did you?

[wlong]

I read your thread and I am glad your repairs worked out so well. Apparently your problem was inadequate cooling as well and a new compressor was called for - and it worked. Did you end up flushing the condenser?

It looks as if I am in the same boat.

[wlong]

My AC problems in the past have been obvious and the guy with the recovery machine does the job. You and qship96 do have a good point and I'll look around for a real specialist. Now that I think about it , I recall one that I have also recommended based on work on my car. I'll let you know what they say when I get time to get to their shop.

[maxnix]

Great. Might save you a lot of time and wrong parts money. Please let us know.

[qship96]

In MY situationI did not flush the condenser,as the oil drained from old compressor was crystal clear with no particles,however you must inspect yours before deciding how to proceed-some debate as to whether this type of condenser is flushable ,or better to replace-always install new drier anytime system is opened due to moisture intrusion and the fact that the drier also acts as a mechanical filter trapping debris

[qsiguy]

Not sure where in the FSM you are reading 150 PSI is good for the high side. The lowest PSI shown in the FSM is 178-208 and that is if the ambient temp is arount 77 degrees. If it's around 86* High pressure side would need to be 192-232 PSI, 95* would be 220-267 PSI, and 104* would be 255-307 PSI.

Expansion valve requires that the high side be high enough and the low side be low enough to properly work. I need you should add a little freon and see if you get any improvement. That is the cheap and easy way to check.

Just noticed yours is a '96 but I've repaired several A/C systems and those pressures were typical for each.

[wlong]

Thanks for the comment and the suggestion. My reaction that there was plenty of freon was based in part on this and in part on the pressure in the system with the car off. I new it wasn't very low, if any and this did not explain the problem. The problem is that the compressor is not pumping like it should and the high and low pressures are out of spec like you pointed out. These symptoms are not what I would expect with low freon. On the other hand, I don't usually work on systems with a variable displacement compressor. I just followed the FSM and it says the compressor should be replaced. I would have a hard time adding more freon since the low pressure side is so high (75psig).

I plan to get a professional opinion tomorrow and let you know.

[qsiguy]

The pressures are irrelevant unless the car is on and the compressor is running. When the compressor is off the high side will drop and the low side will rise as the pressures try and equalize.

[wlong]

Well, I just know from experience that if the pressure in the system when the car is off is below, say 100 psig, the freon level is low. The static pressure in the system with the AC off is usually >150 psig with a full charge. FWIW

The pressures are irrelevant unless the car is on and the compressor is running. When the compressor is off the high side will drop and the low side will rise as the pressures try and equalize.

[qship96]

I think the pressures with the system "off" will equalize to the ambient temperature when using r134,not 150psi for example if it is 78 degrees outside,the pressures,both high and low will equalize to 78psi.

[maxnix]

The whole problem with gas pressure is it is used merely to infer refrigerant amount in the system which is what is primarily of concern here. It is not a direct meansurement of the refrigerant per se. Published pressures assume perfectly working system at ambient temperatures with some fudge factor for humidity, and a perfect refrigerant fill.
Modified by maxnix at 7:45 AM 7/16/2007

[qship96]

Exactly! Why I keep stressing the FIRST step in any a/c diagnosis is to insure the system is filled with exactly the correct amount of refrigerant-which the ONLY way to determine is to do an evac/recharge and then leak test---one of the worse things to do is to start "topping off"here and there

[texasoil]

symptoms are that of low compressor capacity OR expansion valve failed open. Try running the engine at higher speed and see what the pressures do--if they move to normal suction pressure and cooling is OK, tends to confirm low capacity (stuck in low )How to fix that? Replace compressor is certain fix. no 'good' way to unstick and keep it from sticking again shortly.

[wlong]

Running the engine at a higher rpm lonl lowered the suction pressure a few psi (<10) and it never felt like it was even trying to cool. The specialist I visited on Friday agreed that the compressor needs to be replaced.

Thanks to all for the help and comments.

[wlong]

Compressor puchased from fellow NICO member and replaced in late July. It took a couple of hours. Recharged and continues to work great - maybe even a little better than just before the original unit stuck in the low displecement mode. I removed the little shuttle valve that controls the pressure on either side of the swash plate, no problems were evident. I emagine I'll eventually take the compressor apart to see if there is an obvious reason it failed.

Thanks again

[maxnix]

Good luck and please update this thread so we can see the life of the used compressor.

[qship96]

never understood the logic of replacing a worn compressor with another worn,used compressor from a junker or elsewhere=the cost/time to RR and evac recharge,and ability to enjoy the cooling for an extended time period without further problems makes it a huge gamble-bite the bullit,spend for a brand new part, and do it correctly ONCE!