Best AC thread ever. (maybe I should save a copy.... :) )


At 11:05 PM 5/12/00 +0000, Jerry O wrote:
>I have hot air coming out the bottom center vent but only when the climate control is placed in the 2nd from the left slot.  I replaced the valve and I recently had a new climate control installed.  With the air on it does blow cool but not cold - the rear air is actually colder.  The Porsche dealer says it needs a new servo (don't know what this is).  The vent inside is now blocked open permanently to try to keep the air cold.  I read all the posts on air conditioning but cant figure out what to do.
>Advice please.

Adjust the Temperature Setting Flaps and Setting Motor
1) Set system to max cooling.
2) Remove the side console panels (two Phillips screws each side, pull down and back). Disconnect link to setting motor (box on left side).
3) On right side, find a short lever on top and a short lever on the bottom connected by an adjustable link. Loosen the screw in the link.
4) Press the top lever as far forward as it will go, press the bottom lever as far down as it will go, and tighten the screw with them in that position.
5) Turn setting motor link until the holes align and reinstall link.

If that doesn't help -
1) Go an auto parts store and buy a MityVac vacuum tester.
2) Remove the air cleaner assembly and test the heater control valve using the MityVac. Replace if bad ($11.28).
3) Remove the side console panels (two Phillips screws each side, pull down and back). Find the vacuum manifold. Take a digital photo or make a detailed list of where each hose attaches to its solenoid.
4) Test each vacuum actuator individually by disconnecting the hose and using the MityVac. The actuators cost less than $20 each. Some are easy to replace, some are a pain. There are discussions in the archives and on Greg's tip page and the Owner's Club tip page.

Wally Plumley
928 Specialists
At 09:52 AM 5/6/2000 , you wrote:
>Hey all!
>Yesterday, my mechanic and I serviced the AC on my 82. The good news is that the unit is healthy and working fine.  The bad news is that we can't get the heater to shut off.  Consequently the cold and hot air mixes and it comes
>out as in between. Without removing the dash, is there any thing I can do to close the heater door?  Is is a manual connection to the switch or an electronic servo?
>Any ideas or advice?
David -

A few things to check:

1. The heater control valve (just behind the passenger side of the air filter box) is an inexpensive, easily replaced part that tends to fail over time. Make sure it is receiving proper vacuum... then check to see if it is operating (closing) properly when vacuum is applied.

2. There is a micro-switch on the right side of the temperature slider of the climate control system. This switch can "stick". Try moving the slider against the far right side several times to free up the switch. If you eliminate #1 (above), then this may be the problem.

3. The fresh air circulation flap (comb flap) is also vacuum operated and may not be closing completely, allowing warm air (hot air, if combined with problem #1 above) to mix with your a/c air.

Hope that's a starting point for you that helps!

 - michael

Hi All, I just wanted to remind folks that may be experiencing air conditioner failures, that the relay on the controller board has a history of failing. Mine went out last fall and I assumed loss of refrigerant was to blame so I put off fixing it until the weather started warming up. Following tips from Dr. Bob, Wally Plumley, Steve L. and others on the list, I decided to check voltages back to the controller board. Sure enough, I found my relay had failed and wired in an $11 replacement. My air conditioner has now been restored to its previous performance with much less effort than anticipated. This is a good first check.

Best regards,


At 12:47 PM 5/11/2004, Robert Smith wrote:
>Just got the system charged with R-12, and even with the temp slider  and rear A/C dial on max cool, the system blows cool then warm, then cool again (front AND back). I suspect the anti-freeze sensor up front  -- I'm going to test it out by jumpering its attached switch and work from there...what do you think? Could it be the climate control head?
>'89 S4 5-speed (dual A/C)

Two most likely choices are the anti-freeze switch and the under-sized relay in the A/C control head. Easiest way to tell might be to hook either a test light or a voltmeter to the input of the anti-freeze switch, putting it where you can see it while driving. Drive the car and see what the light does when the cool stops. If the light goes off, the problem is either the relay or something else in the control system. If the light stays on, move it to the output of the anti-freeze switch and try again. If the light goes off, the switch is bad or the probe is loose. If the light stays on, move it to the output of the pressure switch on the side (not the fan pressure sensor) of the receiver/dryer. If the light goes off, the system is undercharged, leaking, or blocked.

If only the front or only the rear were going warm, I would suspect moisture inside the system - both going off is probably the compressor cutting off.

Wally Plumley
928 Specialists


First, make sure that you are connecting your meter to the pressure --switch--, and not the pressure transducer that connects to the fans/flaps controller. The switch is the lower device on the plumbing by the receiver dryer, connects electrically with a two-prong plastic with 1/4" spade female wire connectors embedded. Connectors push up from the bottom onto the spade prongs of the switch itself. Transducer is the round brass/gold colored cylinder, has the two screw/knob terminals on it, and is mounted on top of the same piping on my car. When you mention the +ve and -ve connections, it leads me to believe you are connected to the transducer, not the switch.

-- A quick and easy test you can do with your meter to see if the switch is closed: Using your meter set on Ohms (resistance), test between the two terminals of the switch itself with the wire connector pulled off. With adequate freon in the system, the meter should show continuity as very low resistance. That would be on the order of a few ohms maximum.

-- A quick and easy test you can do with your meter to see if you connected the clutch coil wire: Using your meter set on Ohms (resistance), test between the two terminals of the switch wire connector and ground with the wire connector pulled off. With the clutch coil connected, the meter should show continuity through the coil on the order of about 6 ohms. Do this test with the key off, and expect to see that continuity through only one of the two wires that connect to the switch. Other wire will show infinite resistance.

-- Voltage/current to the clutch coil passes through the relay contacts in the control head, through the freeze switch at the base of the windscreen, through the wire harness to the 14-pin connector by the jump-start terminal, through the low/high pressure switch, and then on to the clutch coil. You can test for voltage at the freeze switch terminals with the ignition on and the AC button depressed by connecting your meter to either of the wire terminals on the freeze switch, other meter connection to ground. If you see battery voltage, go to next step. If low or no voltage, think about the notorious controller relay after you check the fuse.

-- Do the same resistance test for the clutch coil as above, with key off and AC button in the non-AC position (up). Using your meter set on Ohms (resistance), test between either terminal of the freeze switch (choose one...), with the wires connected, other meter lead to ground. With the clutch coil connected and adequate pressure in the system, the meter should show continuity through the coil on the order of about 6 ohms. If the meter shows high or infinite resistance, you've narrowed it down to at least the 14-pin connector, the clutch coil/wire connector, or low gas pressure

-- The clutch coil is energized through a circuit that passes through the 14-pin connector by the jump-start terminal. This terminal is pulled apart when you pull the front engine wire harness as part of the t-belt replacement. It's possible that the connector is not fully seated if you still see no voltage at the compressor coil.

-- Many people replace the pressure switch, thinking that maybe the switch is the cause of the problem. I've never seen a bad switch.

-- The connector between the front harness and the clutch coil is a 1/4"
spade terminal inside the plastic shell. It's possible that the female end (on the clutch wire) is not seated correctly over the male part captive in the shell. It pushes up a ways inside the plastic when fully engaged, and there's a slight possibility that the female part can slide past the male part without actually engaging it. That would allow you to see full voltage all the way to the pressure switch terminals, yet not be able to pass enough current to pull the clutch in. Consider pulling and reconnecting the clutch wire where it slides in the plastic connector shell there on the front harness. I also take the precaution of using a wire tie to secure both wires and the connector itself to the front harness once the connection is made. Supports all those pieces to avoid flex/stress on the connectors and a possible future failure.

Summary: There are two electrical points you disturbed during the T-belt
job: The clutch wire connection to the harness, and the 14-pin connector by the jump-start terminal. I'd be looking there first using the tests described above. You also point to the fact that the system was underperforming before this work was done, leading to a possibility that you have already lost some of the refrigerant. Over the course of the work on the t-belt, you pushed and pulled on the suction hose to the compressor, maybe enough to bleed a little more freon from a leaky connection. Or maybe your 'old' leak finally let enough out that the pressure switch is now protecting the compressor. Get a gauge set, or test for adequate pressure using the meter method described in the first step above.

Good luck!

dr bob
'89 S4 in Glendale, Ca, USA
Just finished the whole t-belt project myself...