I'm having a problem with my Climate Control in my 1987 300D Turbo. The climate control blows hot air only when the wheel is turned to max hot, and at this point it is really HOT. At any other position its the coldest air possible that begins to blow. So I have to switch it on and off every few minutes. I decided that I will buy a used pushbutton unit, so I interchanged it with the unit from my 1990 300D 2.5 Turbo, to see it any other unit will work, or if it has to be the same model #. This worked great, I can regulate the heat and enjoy my ride.

If anyone can tell me if these units are interchangable, I don't wana buy a unit and find out in the summer that the a/c does not function becase of a different layout. Can anyone direct me to a Electronic Layout of these control units. I noticed that my '87 connection do not have a wire for pin #1 & pin #11 on the Right side plug in comparison with my '90 which does have one.

Second issue is in both vehicle's the hot air is much cooler when when I'm stopped at a red light. As soon as I begin drving, the air begins to blow significantly hotter. Someone told me this is some sort of Auxillary Pump??

ALL the help is appreciated. Thanks.

Number 1 problem is the heater (mono) valve..

Also the in-car temp sensor must have air DRAWN thru for it to work properly.

In the summer the fresh air door must close completely for the A/C to cool properly.

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The climate control in my 91 2.5 turbo has been acting flakey for a couple of months and I finally took some time to troubleshoot it.

Guess what I found?

1) Bad monovalve
2) Bad blower motor for the air sampler system on the cabin temp sensor.

MBDOC nails it again.

Couple of quick observations for people troubleshooting these systems in the future.

1) My car had no heat output when the engine was idling - I thought this was a classic symptom of auxillary water pump failure so I just replaced mine without even checking it. If your car produces heat at any engine RPM range below 1500-1600 RPM your aux water pump is working. My monovalve was sticking in the closed position - I could see the climate control unit had dropped power to the monovalve but I was not getting any heat. Tapping the monovale with a screwdriver would free the plunger and result in instant heat. I'm betting alot of aux water pumps have been replaced when the real problem was the monovalve. Check your aux pump BEFORE assuming its bad.

2) If you place a one cm2 piece of paper over the grill in the overhead console where the cabin temp sensor is located the paper should "stick" on the grill. This is because the air sampler motor should be pulling enough air through the inlet to hold the paper in place - if your paper falls off the grill you have a problem with the air sampler motor or it's plumbing.

I'd forgotten how well the climate control system works (when it's operating properly) - my car now holds the preset temperature +/- 1 degree of the setting on the temp wheel.

I would do a simple check of the sample air system first.

Cut a piece of paper 1 centimeter square and hold it up at the little grille near the rear view mirror while the ignition switch is on. If the paper is not held in place on its own, the problem is probably the sample blower. It is located in different places throughout the 124 series, but on yours it is probably to the right of the glovebox. By removing the glovebox you will be able to reach it and check it.

Good luck,

Thanks, I will check my mono valve. In the mean time I took apart the Climate Control from the 87' 300D and found a "fried" section near a few solder points. It is safe to conclude that was my problem with regulating the temp.

Since my '90 300D 2.5 woked in the '87 is this interchangable totaly....will this affect any sensors??? Does anyone have access to an Electric Diiagram for these unit, or even where they can be purchased.

I could not find a model # on the '87 climate control, but after doing some research, I found that the ones they sell for the '87 300D is # 124 830 18 85. Can I insert and use the:
# 124 830 08 85, which is for: 1986 300E & 1987 260E & 300E.
OR can I use the:
# 124 830 33 85 which is for: 1988 - 1995 260E, 300E, 300CE, 300TE 300E 2.8, 300E 3.2, E320.

Fianally my current Climate Control is in Celcius, will I be able to use one that is in Farenheight?....Is it just the sticker? The one from my '90 is in farneheight and worked fine. I don't wana find out that in the summer the A/C aspect of it is messed.

I am not sure what my outside temperature display is in the instrument cluster, whether Celcius OR Farenheight since it is burned out. I am interested in purchasing a used working one and would like to know if mine is indeed Cecius then will one that diplayed Farenheight in the previous car work in my '87???

Thanks to EVERYONE!!!