Finally got my '92 300SE back in one piece. I've got an issue with the HVAC and I'm trying to run it down...

When I reinstalled the controller unit in the dash, as soon as you start the car, the radiator fans come on at stage 1 (low speed). Disconnect the controller, fans shut off. This condition exists whether or not the controller is on or off.

Switch the controller to LO on both dials (max cooling), and you get no A/C compressor. HVAC fan runs on high speed as expected, but no A/C compressor.

There is pressure in the A/C system.

Ran a diagnostic from the controller (press Rest for > 5 seconds). ECT sensor (sensor 6) shows HI, even when car is stone cold. Value does not change as car comes up to temp (about 85c indicated). Refrigerant pressure shows 5 bar - about 72psi (sensor 7) which I think should be sufficient to allow the compressor to engage.

Disconnect one sensor at a time at the thermostat housing - no change in behavior.

From this, I make the following assumptions:
1) A/C compressor is "off line" because the controller thinks the car is overheating.
2) Fans are running (even if at low speed) for the same reason.

Here's the rub - I got the car back from my mechanic in August after he did a head-gasket replacement. The car was in pieces when I took it to him as I was in mid-stream doing a more-or-less full interior replacement. I'd gotten too busy to finish the interior work, so I let him have it to do the head-gasket.

The HVAC didn't exhibit this problem before I took it in to him.

I've seen some harness degradation at the connector to the A/C clutch. I pulled that part of the harness loose (includes all the wires to the fans, plus one wire to the coil and another unknown connector) and stripped the cover off of the compressor leg all the way back to the main harness bundle (about 12") and found no degradation at all - wires seem very pliable and insulation solid. Used some hi-temp electrical tape on the exposed wires and recovered the harness exposed and reinstalled.

I don't think it's a harness thing, at least as far as I could tell.

The harness that works the injectors, etc has already been replaced.

I do have a spare HVAC controller which I can plug in to test the problem, but I'm wondering...

If the ECT sensor is shorted out, shouldn't disconnecting the sensor allow the HVAC system to reset (no longer seeing overheating)? Or does the absence of a sensor reading translate to overheating?

I've got a little more debugging to do before I take it back to the mechanic and start with the "it wasn't doing this when I gave it to you" dance...

Any suggestions?