RKA

Okay, here's the short version (details will follow for anyone interested in reading).

My AC works, but seems to be less efficient than it once was. One 90+F days, it seems to have a hard time keeping up. Highway driving improves the situation. Around town driving is where I notice the problem the most. AC has a full charge. All fans are working. What should I look at?

Long version:
1. Car wasn't driven much between summer 05 and summer 06. The AC probably wasn't used for over 6 months. When I started the car in June 06, and used the AC it was really weak. Connecting a gauge to the low pressure port confirmed that the pressure was really low, so I recharged it. It's on the upper end of what's acceptable now...about 43 psi IIRC.

2. All fans are working (they are turning) when the AC is on. Whether they are turning at the speed they should is unknown to me.

3. No visual obstructions in front of the AC condensor, or between the condensor and radiator. Engine coolant temps are normal. Airflow does not appear to be an issue.

4. AC was ice cold last year, under the same conditions. In fact, (last year) as soon as I started the car and drove off...cold air was coming out of the vents. The car has tint which really helps the AC keep up with the demand in 90F (well, last year it was enough, now it's barely enough). This year it seems like I need to get the car moving in order to get any cold air...and it doesn't really start to get cold until I get >30-40mph.

5. At night when the sun drops, the AC has a much easier time keeping up, and the air is in fact ice cold.

It just seems like I've lost some efficiency in the system between last year and this year. I'm wondering if there is a way for me to test the high pressure side out of the compressor housing to see if the compressor is operating within spec. I'm due for a timing belt job in the next month or two, so if there is an issue with the compressor, I'd like to definitively diagnose it now, and have the parts ready when I'm doing the timing belt work. If it's a physical obstruction of airflow, I'll likely stumble on the problem when I take the front end off for the TB service, so that's no big deal.

Anyone have any insight on...
1. What's wrong.
2. How I can confirm/test for #1.

Gotta run...I'll check the responses late tonight. Thanks guys.

Joey T

And having pressure does not equal having AC.

qfrog Ti20

we bill audi for it by the ounce iirc. like ZVW-000-134 I think for one ounce.

B5Davem

Dependent on the outside temp , the low side pressure will have to be higher with a higher outside temperature. One thing Raj also needs to consider is that he may have had a small leak and if moisture entered the system , h2o will decrease the system efficiency . Thats the reason for vacuuming a system before charge to remove as much moisture as possible from the a/c reciever/drier . Phil you need to consider that Raj isn't using a commercial a/c machine which can evacuate the system , measure the amount of refrigerant it contained and then refill it and add the specific weight needed to bring it back to full charge.Using the pressure and ambient temp method will be his best route. RKA if you read this and know what your outside temp is around when you are checking the low side ,I'll post the pressure from the chart I have in my garage. If you are in eastern PA ,I'll estimate its about 80 at night and you should be between 40-50 psi on the low side.

A4SilverT

Flaps/doors stuck in the wrong position?

Get anything out of the climate control diagnostics? VAG-com diagnostics?

A4SilverT

Also, bad sun sensor on dash?

RKA

It was during one of the "warm" weeks we've had, but I did it at night which puts the ambient around 80 or so.

RKA

At night when the sun isn't strong the temps are regulated just fine, so I wouldn't expect to find anything here. I'll check anyway.

RKA

The system never completely discharged. It was down to about 20 psi or so, so it was always under pressure. Air shouldn't have entered the system, I just lost a little refrigerant. Your theory makes perfect sense if I lost all pressure.

RKA

That wouldn't explain why it kicks in at highway speeds though.