Hello,

wondering if anyone can help with this 1997 Audi A8 quattro problem.

0. AC had been running fine last summer, and to the best of my knowledge, no work has ever been done on the AC system. But AC hasn't been used since last summer, because winters are dry and cold here in Ohio.

1. now AC not blowing out cold air

2. low side pressure is about 80psi when compressor is off and about 60psi when on. measured by taking off the low pressure sensor and using the adapter referenced in other threads. the compressor does run when pressure sensor harness is jumpered in this condition.

3. i can hear hissing in the low pressure line near the pressure sensor near the driver side hood hinge when compressor running (I think the expansion valve is just a little bit upstream from that point)

4. internal drain lines are not clogged, but no condensation when AC is turned on.

only thing i can think of is the mixer door is permanently switched to the heat setting, but I'm not sure how to test for that. and even if that were the case, wouldnt i still get condensation drainage from the condenser coils?

Thanks in advance,
1997 Deep Green A8 Quattro

 

It sounds like your compressor is not functioning properly, did you check you high side? That would be helpful in a diagnosis.

 

Thanks, I didn't check the high side yet but i see the connection in front of the radiator. Do you know what the expected high and low side readings should be for proper operation?

 

Usually the low side should be between 40-50psi range after the system stabilizes (a minute or two) and the high side should be 220-300 range depending on the heat load.

The idea with a compressor is when it is pumping the low side will go down and the high side will increase...
Also, generally 100psi is good standing pressure with the system off and equalized.

Pressures are relative to outside temp and humidity... So there is not a magic number that is exactly spot on. This is why when you normally have a leak it is a good idea to do a complete evacuation and recharge of the system by weight.


-Richard

 

Even 25-40 psi is plenty (I never charge past 40 psi) on the low side with engine hot, running and AC set to max. Anywhere over 25 psi should be belching out cold air.

And yes, engine off, the standing pressure (low pressure side) should be right at 100 psi.

 

The issue here in Texas is if you are on undercharged (closer to 25psi) on the highway cooling is not good, around town it will be...

Maybe my low side reads higher due to our higher ambient temps outside..
I know if you undercharge a system you are putting the hurt on your compressor. Whenever I here a compressor short cycling it's the sound of death to me. If you are low on refrigerant the system is low on oil and that's never a good thing.

 

Thanks everyone, I think you've given me what I need to better understand the problem.

I did find this link, which seems to be pretty helpful also:

http://www.idqusa.com/faqs.php?faq=7...category_id=18


Thanks again,
Gorky

 

That's a great reference! Let us know what you find.

-Richard

 

So not exactly as straight forward as i was hoping.

Anyway, when i hooked up both sides of the system, the result was a little different than before. I know it shouldn't have mattered, but maybe i didn't have one of the valves closed previously.

Initial:
1. about 80psi static pressure
2. with compressor on, by jumpering at the low side pressure sensor, the low side was jumping back and forth between 25-30psi, and the high side was only about 90psi.

Initially, that suggested to me that the i might be low on refrigerant and/or the compressor is shot.

So I tried adding some refrigerant from the low side, but despite letting it sit there for about 5 minutes with the system running, it only drew down about 8oz/200grams. The high side pressure at that point had only risen to about 100psi and the low side was still bouncing around between 25-30psi (roughly coincident with the idling frequency).

Since it doesnt appear to want to take in any more R134a, I'm guessing that is is probably a compressor problem, unless there is another issue that might cause the slow uptake of refrigerant.

Since i will have the system open, i plan on replacing the drier/receiver too. But does anyone know if the A8 has an expansion valve in addition to the orifice restricter? Does anyone know where the restricter is located? I might as well replace it too.

And I think the compressor comes with about 5 oz of PAG oil, but do i need to add more than that? what is the rec'd load of oil for the system?

Thanks again,
Gorky

 

Oh yeah, it was about 72 degrees F today and about 55% humidity