Need help diagnosing W123 heater problem - PeachParts Mercedes-Benz Forum

IdahoMauleMan · #1 01-28-2009, 05:01 PM

I have some ideas. But could use more.

I was driving across northern Nevada in very cold temperatures this morning with the climate control set on 24. The heater was working nicely, churning out warm air to counteract the natural drafts inherent in the old girl.

Then suddenly the air turned cold. The engine temp needle rose a little, suggesting that warm air wasn't being diverted away from the radiator.

I tried every combination of temp setting, vent button and fan combination to bring back the warm air. No dice. So I drove for a few more miles as the car got colder and colder until I couldn't stand it any more (it was still dark out). Then I pulled over, stopped the car, got my boots and coat out of the trunk and bundled up.

When I started the car, warm air came back out. The climate control was set at about 24 again. Great news! But after about 10 minutes or so, it moved to cold air again. Permanently, just like before. Rats.

Then I stopped for gas. When I got back in and started it up, same thing. Warm air for about 10 minutes then cold. I deliberately stopped again and restarted. Same result.

Then I had an idea. I stopped the car again. Then I put the climate control to MAX. Started it up. Warm air. For good. It didn't go cold for the next 90 minutes. Until I turned the temp control down from MAX to a temp in the 20's. Then it went cold again after about 1 minute. Turning it back to MAX doesn't help. Once it goes cold, it stays cold.

I turned off the car and repeated the MAX setting trick. Same result. Stayed warm until I moved the temp knob off MAX. Tried to move back to MAX and it still stayed cold.

If I kept the control on MAX, and then pressed the 'OFF' button, waited a few minutes, then turned the heat back on, it seemed to work a little for a bit, but then went cold. The only way I could get continuous, warm air was if I turned the car off, put the temp on MAX, turned it on (any vent/fan combo seems to work) and then leave it there.

So in summary, the climate control can't seem to 'hunt' for a certain temperature any more. But it can blast out MAX heat. And only MAX heat. If you go off MAX heat, it can't come back.

Does this sound like a climate control issue? Or maybe a thermostat issue in the engine? Any ideas? Thanks.

79Mercy · #2 01-28-2009, 05:03 PM

what year?

I'd check the monovalve if you have one.

kerry · #3 01-28-2009, 05:04 PM

I'd begin by checking the monovalve. Your symptoms are consistent with a torn diaphragm.

IdahoMauleMan · #4 01-28-2009, 05:08 PM

Quote:

Originally Posted by kerry

I'd begin by checking the monovalve. Your symptoms are consistent with a torn diaphragm.

Thanks. It's a 1982 300D. Is the monovalve the thing under the passenger kick panel?

kerry · #5 01-28-2009, 05:09 PM

On the firewall in the engine compartment right side with heater hoses going into it and an electrical connnection on top. Search on it and you'll find pictures.