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Daniel H
01-03-2006, 12:59 AM
Ok, where do I start. This is 82 300cd. I have heat, but only when the temp wheel is turned on full blast. Even if I un-click it off max heat, it'll cool right off to cold air. Also, I have heat coming out of the center vents, which, from what I was reading in the owner's manual, is incorect, Can anybody point me in the right direction of where to start? Are these two problems related?
Thank you,
Daniel H

01-09-2006, 05:38 PM
start with a through inspection of the vacuum system

do the vacuum door locks work ? and do they hold vacuum and unlock
after car sits for a few hours ?

a mess of vacuum lines inside the dashboard for flap diaphrams -

no easy way or common thread when the car is this old - sorry

A poor performing vacuum system will also give you poor braking because of poor vacuum brake booster performance - and - eventually
the engine shut-off/key switch will fail to shut the motor off,
AND the auto trans shifting will also be 'off'.

Daniel H
01-11-2006, 09:15 PM
Ok, the vacuum system is tight, I replaced the pump, and the door locks work, even if the vehicle sits. The servos in the climate control seem to work fine, all the vents open and shut properly, the only problem is, I only have heat when its on max heat and the center vents stay open when its on heat as well, The ac system works fine, what do you think about maybe the temp wheel on the push buttons, or maybe mono-valve? or maybe the temp control relay? I'll do some reading in my factory manuals and start there, any more advice is really appreciated,
Daniel H

01-12-2006, 12:51 PM
well - good place to start - read the M-B bible and study how the system works, whats open and closed during cooling and heating, check the temp wheel resistance readings, incar temp sensor resistence values, yes, the MONO Valve not working/sealing is common and a start to gettinig the entire system right. IT's been almost 20 years since my M-B dealer line duties and even when your on top of your technicians game - the M-B manuals are the place to start. It sometimes is a component by component diagnosis. Understanding how each button function operates, and what things actuate - at what temp
settings. AND, add to the problem the time that these cars have been on the road and any 'improper' repairs or "by-pass molestations"
have occured - making any repair a challenge.

goo luck and report your findings or clues as you go.


Daniel H
01-12-2006, 10:23 PM
ok, I spent several hours today going through the different test procedures per mb service manual. I narrowed the problem down to the temperature relay. I replaced it and bam!!!! now the heat works great, as far as the center vents go, I think that a certain amount of air-flow will occur through the center vents, if you read the owner's manual carefully, it says that there is some air flow through the center vents when the button is on EC. Thats all I can think of, I tested all the servos and switch over valves, heater box sensor, and air temp sensor,and oh yeah, the mono-valve. I think its normal that air flows through the center vents.
Thanks for all your help,
Daniel H

01-13-2006, 10:15 AM
..congrats ! you sound like a mature, patient, closet technician.
what's your day (former) job?

Daniel H
01-16-2006, 07:38 PM
Thats kinda funny, because when I'm not at college, I'm turning wrenches at a small independent MB shop.........hehe
Daniel H

01-17-2006, 11:41 AM
.......... good for you. A good technician is alway welcome these days at BMW and Mercedes-Benz dealers. Factory training is very rewarding.

06-19-2006, 02:43 PM
Where is the Temperature Relay located?

I have a problem on my 1979 AND 1980 Mercedes 200SD. The heat always trickles out of the vents even when there is I have the climate control off.
My AC also doesn't work but I think it is the AC compressor.

Can someone help with what causes heat to always come out.

I do notice if I put the heat on defrost and shut off heat, then heat will trickle out the dash. If I put it on Low heat and then shut off, then the heat will trickle out down by my feet.

Can someone Help?

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