ACC no heat after 20 minutes on '82 300D
 

My older daughter, a Cornell graduate student will be home for a visit today with her Orange 1982 300D (turbodiesel). The heat will work for about 20 - 30 minutes then shut off. The last time she was home I drove the car on the freeway and experienced the same problem. Around town and on the highway it ran okay for about 20/25 minutes then the fan stopped and the car started cooling down. Apart from that it does some other odd things.

I know this has been discussed ad nausium on this forum but I've never understood the ACC on MBs well enough to systematically trouble shoot problems. Does it sound like the main control unit needs to be replaced? I don't know if the aux pump is working. A year or so ago it stopped heating at highway speed and I replaced the monovalve and that fixed that.

Will it start providing heat again if you bump up the temperature wheel significantly?

If so, then you probably have a decomposed tube behind the globe box that goes to the temperature sensor in the top middle of the dash. That tube was made of foam and just falls away after years of age. The tube's purpose is to allow a small amount of air to be drawn through the temperature sensor, to sense the cabin's temperature level.

When the tube drops away, then what you sense is the temperature inside the dash, where all the heating ducts are located - much warmer than the cabin. So the ACC thinks you are nice and toasty and throws in cold air.

So, if you get heat by turning up the temp wheel all the way, let me know and I'll give you more details on the foam tube replacement. (Basically, if you remove the glove box you should see where it goes.... and perhaps the remains of a foam tube.)

Ken300D

Ken

Thanks for the advice about the pickup tube, but no, heat does not come on even when dialed up to max heat. No pushing of buttons or adjusting the dial will result in heat ... only cold air.

Robby

You still need to check the tube. Remove the glove box and you will find hard tube about 5/8" dia. that goes toward the center of the car. There will most likely be no connection between it and the sensor tube that is below the little square grill at the top center of the dash. Go to Home Depot and get some foam water pipe insulation tubing. Fit the tubing over these two tubes to connect them.

Have a great day,

Almost all of the time that problem comes from a HYDRO-locked mono heat valve, even if it is almost new! start there.

Larry & MB Doc

Thanks, I'll check the items you mentioned when I get home today.

Robby

Ilse just mentioned to me that the fan shuts off and nothing comes out.

With the button pushed that is next to the defrost, she starts the car, it warms up, the fan comes on with warm air. 20 to 30 minutes later she notices it is cold in the car, the fan has shut off. Turn the car off and back on doesn't help.

Robby

There was NO foam tube between the two plastic/rubber tubes. I replaced the missing section and We'll see what happens. There was nothing left of the foam tube. I was surprised how easy it was to effect the repair. 10 to 15 minutes total time to remove the glove box, put in a piece of tubing and replace the glove box.

I had some fireproof fuel line tubing on the shelf in my garage that the ID was the same as the OD of the sensor tube so I used it.

Robby

hate to say it but mine gave me the same problems and it was the mono valve. If i hit the right button the heat would come on for while then would get cold afer about 20 mins.
m

Re the monovalve ... last night as I was working on the car I could hear/feel it rapidly clicking. Also when I had the glove box out it appeared the the temp sensor for the AC evaporator had been jumpered around as there are no wires connected to it. I have a new evap temp sensor somewhere in the garage. I'll work on these two items tonight.

Thanks

Well, the tube needed replacing anyway. It could be the monovalve, but time will tell.

I would suggest getting some foam tubing and replacing what you put on it. The conductivity of the foam tube is virtually zero and will work much better.

Good job!