Quick-Check
Here's a quick check to just verify if your mechanic is at least close:

1. Observe the front of the compressor carefully while the engine is not running. Notice the front side, the area where the belt runs, and backing of the belt disk. From there on its just the compressor. Here is a magnetic coupler that engages when you push the AC button.

2. Look for the sight glass on top of the dryer. Its a small 10mm round glass, just under the left-side bracket of the AC radiator. You will see nothing, its just plain dark inside. Maybe use a flashlight to see what it looks like at off-state.

3. Now start the engine en watch the things turn at the compressor.
Notice that the center shaft does not spin yet.

4. Now push the AC button. The center shaft should start turning also, caused by the coupler locking the belt-disk with the shaft.

5. If you look inside the sight-glass of the dryer again you should be able to see a milky like bubbling fluid. This is caused by the compressor pressure.

Now: if you compressor does not run, nothing will happen. The center-shaft of the compressor does not spin. Then you have to have the system diagnosed. If the compressor does run, but you see no bubbles whatsoever, the compressor might be defective. If you see bubbles in the drier (R12 series) but get no cool air, you are up for further diagnosis.

This is not 100% accurate.... but it gives a good feeling in 2-minutes of what might be wrong.
Good luck.
Theo
1988 928s4 cherry red
The Netherlands
http://www.jenniskens.eu

 

Checking a non-working the AC system

  1. Power:
    1. check fuse #6 (5amps) (from í90 onwards fuse #37, which is 7.5amps)
    2. check 12v power at both sides of fuse #6, check fuse #17 if no power (fresh air blower power)
    3. AC button light should go on when pressed
  2. Clutch:
    1. You should be able to hear the engaging compressor clutch when the AC button is pressed.
    2. Check if the compressor is running. You should be able to see the front disk running with the clutch engaged. You should be able to turn it freely when the engine is off.
    3. If the clutch does not engage, check the power to the clutch. Cables exit the compressor at the right side just behind the pulley. One wire is bolted to the clutch chassis (ground) and the other wire moves up to a connector close to the belt tensioner, but is located close to the compressor or even behind the belt cover. From there onwards it moves up to just next to the 12v remote start point. Multi plus pin #9, or at the electric air valve. The electric air valve is wired parallel to the clutch. Again: putting +12v on this wire should make the clutch click.
    4. check frost switch at evaporator (12v at both sides, switch closed, purple/yellow wires). It should open at 0C or -1C.
    5. low-pressure switch check (lower at dryer) (12v at both sides, switch closed, black wires) It interrupts the compressor is the pressure is below 2.2 bar or above 27 bar. The high pressure switch (upper one at dryer) is linked to the air-flaps and blower system, and will close to ground at some point when the AC is running.
  3. Cool air:
    1. Check if the warm water valve is closing correctly. It is vacuum operated, and a leaking membrane will not make it close. Also sometimes the internal seal leaks, which causes a bad cut-off. Typically  the lower left side of the dash-grill feeds warm air even when the AC is on.
    2. Check for small bubbles at the sight glass on top of the dryer. A milk like fluid should be visible. Itís a bit hard to see since its located under a bracket of the radiator.
    3. Check the outside temp sensor in the wheel well at the driver side. It should be 933 ohms at 0C, 588ohms at 20C, and 457ohms at 30C. The system will respond to non-regulating heating (19-30C) if a break occurs, and will be fixed half-way warm if a short circuit occurs.
    4. Check internal sensor in dash (should be 1900 ohms at 30C and 2470ohms at 20C). The blower must work! The system will respond to non-regulating heating (19-30C) if a break occurs, and will be in cold position if a short circuit occurs.
    5. Test operation on the AC control panel. Take the 4pin inside temp-sensor plug, and measure between socket and pin1 (yellow): lever at 18C: 760 +/- 100 ohms, 30C: 1760 +/-100 ohms.