thermostat temp. question/ heater problem - PeachParts Mercedes-Benz Forum

Wasuchi · #1 11-12-2002, 11:39 AM

What temperature thermostat should a 1985 300D have? The one in it is 80 degrees but it seems like it stays open too long or something because at higher speeds the heater doesn't blow hot air, at lower speeds (30 mph) it blows hot air just fine. The temp gauge does come up to around 80 though and it runs ok. Maybe the thermostat isn't the problem but thats the only logical conclusion that I've come up with. I can't see that anything else could be speed related. Thanks

dieseldaddy · #2 11-12-2002, 11:42 AM

I am by far not the expert in this area, but I can offer that I have heard/read that the symptoms you are experiencing may be a bad monovalve.
Try a search on that...and I am sure others will help you out here too.

84300DT · #3 11-12-2002, 06:02 PM

thermostat and/or more likely monovalve.

my car would not heat up (barely got to the 80 deg mark)
replaced thermostat (bosch) and all ok.

engatwork · #4 11-12-2002, 06:07 PM

My 300D did the same thing when the monovalve was bad. It is a good diyer type of job. Do a search on monovalve and you should find some pics in one of the threads.

#5 11-12-2002, 06:37 PM

I have been experiencing this same thing. I removed the monovalve and it opened and closed with the temp wheel so I assumed that it works, however I only have warm air for a few minutes. I have to turn off the blower for about five minutes and them it blows warm again. Could the monovalve be bad & still operate?

Wasuchi · #6 11-12-2002, 07:03 PM

Thanks for the help, I'm betting it's the monovalve. I pulled the thermostat out to see if had something stuck in it, it was fine..it's only about 9 months old anyway. I guess I'll check the monovalve out when I get a chance. Thanks again.

lrg · #7 11-12-2002, 07:05 PM

Yes. What usually fails on the monovalve is a rubber diaphram inside. The valve mechanism still works mechanically but the torn diaphram won't let it transfer enough hot coolant to the heater matrix for it to heat up properly. You probably only need the monovalve insert that contains a new rubber diaphram (you can get one on Fastlane). Just unscrew the four screws at the top of the monovalve, remove the top, withdraw and replace the insert and reassemble. My California '85 has a heat shield in front of the monovalve that necessitated my actually removing the whole valve to get at it. I believe the 49 state cars can be done without removing the valve, Even with that it was a 20 minute job. Don't overtighten the screws when you reassemble it or you may crack the valve housing and need a whole new monovalve.

engatwork · #8 11-12-2002, 07:10 PM

The monovalve should last longer than 9 months easily. Check the aux pump located inside the wheel well on the passenger side in front of the turbo. You should be able to put 12 vdc to it and it should run. Also, you will want to confirm the climate control module is sending the 12 volts there when the wheel is on max hot.