Hi all,

The a/c isn't working so I thought it might be nice to at least get some ambient air blowing into the cabin (live in the Bay Area so we're not baking). Alas, it's not to be. No matter which button I press or where I set the temp wheel, the fan roars into action, pumping hot air into the cabin.

Could it be the ceiling sensor? The climate "control" unit? Or do I own Christine, the car from hell?

1987 W124 turbodiesel sedan

I had the very same symptoms in my 1985 300D few years back - it would blow hot air no matter what I did. Here are the things I did (according to my notes from that experience):

1) I found the Mono Valve defective and I replaced it first - still no change.

2) I removed the glove box and found a disintegrated rubber hose to the cabin heat sensor, which I fixed with a piece of new insulation foam (the one you wrap around pipes). Still no change.

3) I determined that the cabin sensor was defective - it was an open circuit no matter what temp; I replaced the temp sensor with brand new one.

4) I temporarily placed a jumper on the KLIMA Relay (you'll need to search the forum to see if that applies to your model) - anyway, I later determined that this was not the major issue on mine and so I removed the jumper at the end.

5) I had a bad connection on the plug going to the compressor - which I had to reseat to get a good connection.

6) With a help of a friend who does HVAC we evacuated the system, flushed the system and recharged with freon and this time it worked!

You may have different problems - there is an AC troubleshooting sticky (I believe) that is worth looking at but that's how I fixed mine just before I sold it... I hate AC and I never use it, but it's good for the resale value.

Good luck -
James

__________________
1987 Mercedes 300SDL; SOLD
1985 Mercedes 300D; SOLD
2006 Honda Pilot - wife's ride; 122K;
1995 Toyota Land Cruiser - 3X locked; 182K

wait till winter, it'll feel much better then.

Until then, clamp or plug a heater hose.

Checking the monovalve electrical connector would be worthwhile.

Does the problem persist when the temp control is in the "MIN" detent position?

There are 2 wires to the monovalve. One gets contsant 12V when the engine's running, the other gets ground depending on what the ACC unit decides - no ground for full heat, steady ground for full cold, pulsed ground for modulated heat. Later ACC units have a fused circuit in case the monovalve or auxliary pump shorts. Early ACC units go down when the monovalve or auxiliary pump shorts. When the ACC unit goes down, the blower shouldn't function.

How far are you from SJC? We need AC down here.

Sixto
87 300D^2

"Climate control blows hot air aggressively no matter what"

Somehow, don't know how but you have a politician in your dash!

__________________
1991 F250 super-cab 7.3 IDI. (rebuilt by me) Banks Sidewinder turbo, hydroboost brakes, new IP and injectors.
2003 S430 - 107K
1983 300SD - Tanoshii - mostly restored ~400K+.
1983 300SD - Good interior. Engine finally tamed ~250K.
Monark Nozzle Install Video - http://tinyurl.com/ptd2tge

You may have this issue.
Pelican Technical Article: Mercedes Benz - Cleaning Heater Control Valve



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__________________
ASE Master Mechanic
asemastermechanic@juno.com

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
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1985 300SD
1983 300D
1984 190D
2003 Volvo V70
2002 Honda Civic

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