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#1
10-20-2013, 08:33 PM
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1985 300D Heat Problem.
Hello all, I'm hoping you can help me solve my heat (or lack thereof) problem. I bought my 1985 300D last June w/o working A/C. My brother and I were finally able to get everything up and running and now I have little to no heat. When we first got the A/C running I could switch between blowing cold A/C and hot blowing heat, but now I get lukewarm air at best. If I start the car with the heat on it blows hot for a bit, but as soon as I start driving it cools down. In getting the A/C up and running we repaired the monovalve with a repair kit and I checked it today to make sure it was still in perfect condition. The car runs at just over 80 degrees celsius, rarely much hotter or cooler once it's been up and running. It doesn't take an abnormal amount of time to get up to 80 either. My brother and I are pretty stumped at this point and I'm hoping someone here can help.
Car Info: 1985 300D Turbodiesel Electronic/Auto Climate Control 
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#2
10-20-2013, 09:41 PM
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Welcome to the forum. Checkout this link No Heat, All of a sudden
You could be experiencing the same thing. Search for monovalve. You will find different asnswers. It could be the magnet on the monovalve, foam hose on the temp sensor, temp dial, coolant pump etc.
__________________
'85 300D Cal 280,000 miles '14 GLK 350 60000 miles
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#3
10-20-2013, 10:07 PM
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Sounds like a torn diaphragm in the monovalve to me.
__________________
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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#4
10-20-2013, 10:26 PM
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try unplugging the monovalve wire, and see if the heat works.
__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread "as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do! My drivers: 1987 190D 2.5Turbo 1987 560SL convertible 1987 190D 2.5-5SPEED!!!  1987 300TD 2005 Dodge Sprinter 2500 158"WB 1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!
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#5
10-22-2013, 09:33 AM
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Mine had poor heat when I first got it.
Turns out that the foam tube had deteriated. It goes between the small intake in the middle of the dash and the diaphragm. Was easily accessed by removing the glovebox.
__________________
85 Merc 300D - Unwinding 31 years of wear 86 VW TD Mahindra Diesel Iseki Diesel In 2007 I didn't own a diesel. Last edited by Silber Adler; 10-22-2013 at 09:47 AM.
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#6
10-22-2013, 08:04 PM
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thanks for the responses, I unplugged the monovalve with no difference. I sat in my garage for a bit with the engine running to no avail so I drove around for a bit and still no heat. If anything it got cooler again. The above mentioned foam tube was gone, but I replaced it with a length of rubber hose wrapped in foam tubing. I've tried selecting the "compressor on", "overall ventilation", and "compressor off" buttons with no difference. The only thing that really changes the temperature is turning the dial colder which, with the compressor on, blows nice and cold. unfortunately I'm in the Chicago suburbs and the highs all week are looking to be in the low 40s so A/C does me no good. I'm hoping to do a coolant flush this weekend and put some proper coolant in (running green right now) and change the thermostat just in case.
Does anyone have any other ideas?
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#7
11-02-2013, 01:42 PM
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Update:
Case closed. It was indeed the monovalve. Replaced the recently installed repair kit with a new one and we've got heat just in time for this cold weather.
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#9
11-02-2013, 02:48 PM
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I had the same problem on my 1985 300D. The mono valve magnet had failed. I had replaced the diaphragm in the mono valve but it was the magnet itself. I got lucky and found a good mono valve on a wrecked car. Problem solved.
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