PaulW

You have only one switch, which is for both high and low pressure. The 97's up to week 44 had seperate high and low switches.....from the Bently below.

There is a whole troubleshooting procedure for the AC system using VAG-COM, you can see the cut off conditions in measuring blocks, you can do an output test of the AC clutch, etc.

I still don't understand why people get the shareware version and don't have the Bently. Sorry for the rant.

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Audi > A8/S8 > 1997 - 2003
Heating & Air Conditioning
87 - Air conditioningA/C pressure switch -F129-, functions The high/low pressure switch (located between terminals -1- and -2-) switches the compressor off (via A/C control head -E87-) when there is an excessively high or low pressure in the A/C system.
Switching pressures (in bar)
Switch open (compressor off):
Low pressure: switch opens at 1.2 bar (17.4 psi) and closes at 1.8 bar (25.2 psi)
High pressure: switch opens at 30 bar (420 psi) and closes at 14.5 bar (203 psi)
Switch closed (compressor on):
1.2 (1.8) bar to 30 (14.5) bar
The high pressure switch (between terminals -3- and -4-) switches coolant fan -V7- to speed 2.
Switching pressures (in bar):
Fan speed 2 on (switch closes) at 16 bar (232 psi)
Fan speed 2 off (switch opens) at 12.5 bar (181 psi)
Removing and installing
Remove bumper
=> Repair Manual, Body Exterior, Repair Group 63.
Disconnect harness connector.
Remove A/C pressure switch -B-.
Tightening torque: 5 Nm (44 in. lb)
Replace O-ring seal -A- (10.8 mm x 1.8 mm) => page .
To test cooling efficiency with switch removed, bridge terminals -1- and -2- on harness connector -B-.
The A/C refrigerant circuit remains closed due to the Schrader valve in the line connection.
Testing
Start engine.
Compressor switched on (AUTO mode selected)
If the compressor is activated:
Disconnect harness connector -B- from switch.
Compressor will stop (low pressure switch open).
Remove switch -A- and install harness connector -B-.
Compressor will not be activated (low pressure switch open).
If the compressor is not activated:
Disconnect harness connector -B- from switch -A- and bridge terminals -1- and -2- on harness connector -B-.
The compressor will be activated.
Notes:
If the compressor is activated when the terminals are connected together, either the pressure in the A/C system is too low (A/C system empty), or the switch is malfunctioning.
Check the pressure in the refrigerant circuit => Refrigerant R134a - Servicing
If the compressor is not activated and the measuring value block display indicates A/C pressure switch -F129- as the cut-off condition, find and repair the open circuit in the wiring between A/C pressure switch -F129- and A/C control head -E87-.

roughyear

when i jump the switch, should the pressure in the system drop (on the guage, currently reading about 65PSI) to indicate a more reasonable reading??

also, when i vag-com the AC system, should the car/engine be on to run the test??

PaulW

What I would do if I were you would be to order the Bently so you can see all the pictures, see all the measuring blocks, know all the processes.

I could PDF everything for you, and answer all the questions, and even work on the car for you....but I don't have the time.

If you don't have a wiring diagram and don't know what to jump, and don't know what the problem is....I would be very, very careful.

I would start by opening the hood and turning the car on, and have somebody change the CC from AUTO to ECON and see if the low speed electric fan comes on/off and see if you can hear the AC clutch pick up.

After that....you need vag-com on measuring blocks....check for fault codes.....do an output test of the AC clutch.....etc., etc.

PaulW

You might not have it properly attached to the system, i.e. the shrader valve is not depressed.

If you have never had to charge the system before....I would be careful.

How about trying to run the system in RECIRC mode? Sometimes the fresh air intake damper fails and doesn't allow cooling across the coil.

Your system might be completely charged.....once again, you need vag-com/Bently and troubleshoot the system.

roughyear

but that little diagram/procedure you had done with your car sure made things look real easy!!!

PaulW

See if you can hear the clutch engaging/disengaging with a change in the HVAC to ECON/AUTO.

If you do this with the car cold, the electric fan should turn off (econ) then on (auto)....once the engine is warmed up, it might run all the time.

Then also put it in RECIRC and see if you get cold air.

roughyear

and suddenly i got some cold air coming out of the vents... mainly from the passenger side, but also from the driver side (though noticebly warmer on this side)

i haven't had the AC recharged in 3yrs, i wonder if it was overcharged back then... but it was working just fine (cool-wise) until late last year (mid-november when it was still in the 80s here in maryland)

should i release more pressure from the low valve.. the air coming out of it was NOT cold at all, felt more like regular air coming from a tire hose.

PaulW

It's just freon. If you have warmer air coming out of the driver's side, it's a low freon charge.

Try it again from the port under the hood per my procedure. Hook everything up with the car off, then start the car and put the AC on LO.

Do this with the engine and AC system cold (i.e. in the morning).

It should work no problem.

Adjuster

Now that summer is on it's way, my 2001 S8 has stopped working AC wise.
It was working fine even this winter, as I noticed it would condense water on the suction tube (large line) while running the defrost setting, as it should, to help remove water from the air in the car.

But now, the clutch on the commpressor does not engage, and last fall, I knew it was low on freon, as the cooling ability was not up to what it should be. (This is my 3rd A8.)

I was going to re-charge it at that time, but did not have the right adapter for the low port fitting.

So, here we are, I have the Freon Tank adapter needed for the low port, I screwed it onto the port, and hooked up the recharge kit we have used on my Dad's S500, and a few other R134 vehicles with no problems... But nothing.

The compressor will not kick in, and I did not have time to search for the low/high pressure switch to jumper it, but it did not seem like any freon was flowing anyway.

Has anyone had issues with the adapters not depressing the Schrader valve in the low port? I remember reading something about this, and I tried removing the seal, so the adapter would screw down further, and just teflon taped the threads, no leaks and it was under pressure from the can, but again, no flow....

So, here is my question. Where is the low/high pressure switch located, I have read you jumper wires 1 and 2, and I've done that on other cars before, and used to be a AC tech many years ago when R12 was the norm, so I'm not a complete fool here.

I was hoping the pressure from the can would bring the clutch on line, but no luck. I should have re-charged it before it was so low, the clutch does not come on now.

Thanks in advance for any advice, and sorry to bring this post back from the dead, but I figure we will all benefit from having correct info, and Paul's list of the Vag-Com data is nice to have, and not in other threads that I remember.

silverd2

The pressure switch is under the driver's headlight, just accessible (tight though) up through the opening with driver's side fog light grille removed.

Many (including myself) have had to keep constant pressure against the Schrader valve, to keep a flow going...common "problem" because of the design of most adapters.

I'm sure you know this, but it's worth noting for others doing a DIY recharge:
If you're significantly low on refrigerant (even 1 small can), you need to use a recharge that includes oil charge..or add a small can of oil charge. Since the only lubrication your AC compressor ever gets is from the oil in constant circulation with the freon, when freon leaks, you lose a proportional amount of oil charge. "Wet" (oily) areas around a refrigerant leak are the only visible indications of a leak...the freon itself is totally "dry" and disappears instantly when leaking.