rocketTT

The short version of the story: the AC is not working on my 2001tt. The shop tried to refill it a couple of times to no avail. They told me that the high pressure side builds up but the low pressure side is not doing what it supposed to do. The bottom line is that it is not working.

The long version of the story: I took the car to the shop to get dyno tuned because the car was new to me and I wanted to make sure the tune that came with the car was safe. The coolant pipes leaked during the tune. I had them drop the motor and pin the pipes. We also updated the turbos and intercoolers while they were at it. After they put the motor back in, the previously working AC system is no longer working. They have tried swapping expansion valve and the compressor and it is still doing the same thing: the air would blow cold for about 1 minute and then stops working. I was told that the high side pressure keeps building and the low side was staying constant. It sounds like some kind of blockage to me. The shop is a good tuning shop that has just started working on Porsches. They are good with the drive train bits and they were able to tune the car to have very smooth power with no knocks at 498 whp/487 ftlb. However, I am not sure if they have a lot of Porsche knowledge though to deal with this AC issue. I would like to hear what the collective wisdom on the forum would suggest how I should proceed forward with this. Any suggestions I should forward to the shop? This feels like it should be something simple/trivial, not mechanical since the AC was working fine before.

Thanks in Advance.

neanicu

First step : make sure there are no leaks in the system. They can use dye to find out. Secondly,they probably need the factory diagnostic tool(PIWIS) because there's a chance the control unit might need to be reprogrammed.

PS. I would also suggest to have them go back and check their work on the engine installation. In these situations it can happen that something was done incorrectly.

993GT

what stops working?
Is the A/C compressor shutting off? or it just stops blowing cold air and the compressor is still engaged?

BostonDuce

SWAG's;
Assuming "blowing cold" is 15-20 below ambient temp:

bad low pressure switch or loose/missing connection; failed receiver/dryer (should have been routinely replaced when they opened the system); air in system/insufficient freon charge.

ask if they 'vacuumed' the system before the recharging after replacing the expansion valve and compressor. If not, you need to have that done as well as a nitrogen flush before freon is put back in.

BD

993GT

^+1

Carlo_Carrera

^+2

Yes, it sounds like the AC system was not properly vacuumed out and re-charged when the motor was re-installed.

rmc1148

I would assume all your cooling fans are working properly?

Kevinmacd

If your condenser's are overheating than your high side pressures will build. You should hear the cooling fans running on high speed when the AC is turned on! Next di did they crimp any of the aluminum tubing when they removed the engine.

dprantl

If the compressor runs until the high side pressure rises so high that it gets cut off by the high pressure switch and the low side pressure is static (assuming a new compressor and expansion valve and no refrigerant leaks), and since it happened after they dropped and re-installed the engine, I would guess that they bent an A/C line and it is now creating a partial or complete restriction in the system. Trace all A/C lines in the system to verify they are not damaged. Did they also replace the receiver/drier? What were the exact pressures before the air out of the vents started warming up?

Dan

rocketTT

The shop decided to send the car to the local dealer to have them troubleshoot. It turns out to be the fans. When the fans are not working, the pressure on the high side keeps going up and then it get shutoff by the switch.

The dealer wants $2800 to replace both of the fans! I don't know what is entailed in replacing the fans. But $2800 sounds a little high just for fan replacement.

rmc1148

Do it yourself easy job and big money saved= thought it was the fans been there myself .

jpflip

The fans are about $400.00 each when brand new from the dealers. $2000.00 in labor??? Better found an honest shop to get this work done. It is not a bad DIY in fact it is a simple straight forward job!

Note many used one and cheap one on Ebay part# 99662412700 (search for porsche 996 turbo fan)

Kevinmacd

As one of my things I suspected. As I said you should hear the fans run on high when the AC is turned on, there is no missing it. Good thing you found it early! Trying to run the AC with very high high side pressures can blow a seal in the compressor.

TurboTodd

How high is high exactly?

Anyone know what the low and high side pressure should be?

dprantl

It depends on many factors, but generally on a well running R134a system the high side should not exceed 350psi or so in the short term. Usually you want the system running under 300psi high side in the long term. Low side pressure is directly related to how cold the evaporator is. With R134a you want the low side to be in the low 30's or even as low as 28 psi on a perfectly running system that has reached equilibrium if it is not too hot/humid outside. The low side will never be lower than that unless the anti-freeze switch is not functioning correctly.

Dan