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  #1  
Old 01-28-2009, 04:01 PM
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Need help diagnosing W123 heater problem

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.

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Old 01-28-2009, 04:03 PM
79Mercy's Avatar
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Join Date: Mar 2008
Location: Florida
Posts: 4,131
what year?

I'd check the monovalve if you have one.
__________________
1985 300TD Turbo Euro-wagon
1979 280CE 225,200 miles
1985 300D Turbo 264,000 miles
1976 240D 190,000 miles
1979 300TD 220,000

GONE but not forgotten
1976 300D 195,300 miles
1983 300D Turbo 175,000 miles

http://i42.photobucket.com/albums/e3...e485-1-2-1.jpg
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Old 01-28-2009, 04:04 PM
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I'd begin by checking the monovalve. Your symptoms are consistent with a torn diaphragm.
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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13
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Old 01-28-2009, 04:08 PM
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Quote:
Originally Posted by kerry View Post
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?
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Old 01-28-2009, 04:09 PM
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Join Date: Jan 2002
Posts: 18,355
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.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13
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