Hello all,

My 85 300SD is having a heating issue of course on the day it is suppose to snow here in new york.

When I first start my car and the setting is on economy mode, the car will heat according to that setting. For example, if I start with max heat or close to max heat the car will adjust to that temperature and heat the entire drive, but if I adjust the wheel during my drive to reduce the heat, it will just go to cold air (the cars a/c does not work).

If I stop at the store for a couple of minutes and then restart my car with the heat at maximum setting it will go back to blowing high heat? Again, if I adjust the thumbwheel it will loose heat and any other adjustments are ignored, even if I put the setting back to max heat.

I am not sure how the system works, but it seems to me like the monovalve sees the initial voltage and sets that, but if the thumbwheel is moved it goes right to the cold setting signal.

Is this adjustment a resistor? If so can this be bad or do you think the monovalve is going bad?

Is there a way to the signal to the valve?

Any help is greatly appreciated!

Apparently, my gas guage quicks working once it drops below the 1/4 tank mark, but one thing at a time.

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1985 300SD 200K

Over here in the Uk my 90 300d does the same and I experience random heat at various times unless on a constant cruise speed where after time I can get the heat where I want it.

The reason mine does this is because the electric coolant pump does not work / is seized etc,so rolling to a stop in traffic see's the heat vanish as the engine begins to idle.

When the heat isn't working, remove the electrical connector at the monovalve. Then report the results.

If the two prong connector next to the battery is the monovalve, then I unplugged the connector and nothing happened to the heat when I did this?

It remained cold.

Thanks,

Keith

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1985 300SD 200K

You should have experienced full heat. Since you did not, you must have a coolant flow issue: bad monovalve, clogged heater core, etc.

Since the car does not seem to overheat, regular temp approx. 90 degC max, does that mean I should check the monovalve first?

I can get heat for over 30 minutes so I would think that the heater core is not clogged, but is there a way to tell?

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1985 300SD 200K

Always check the monovalve diaphragm for a small tear. It's a 10 minute procedure and 90% of the time, that is your issue. Look at it carefully, however............the tear might not be immediately obvious to you.

Also............don't drop the screws..................

I had the same issue on my 300d. I wrecked it before I could repair it but everyone said the problem points to the monovalve! Looked like a easy fix and the part was not too expensive either! Good luck! Let us know if that fixes your problem!

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2006 Jetta TDI DSG 320k miles
1997 Ford F150 325k miles 4.2L V6 "Work Truck"
2008 Tundra 225k miles 5.7L
1982 240D.....sold
1984 300D...Totaled OUCH!
1985 300D Turbo 222k miles "Dos" sold to 79Mercy
1986 300SDL 98K miles "The Beater"....sold
1987 190E 2.3 16v Euro spec 115K miles....sold

I need to do this.

Will I need to clamp off the coolant hoses? I don't have a good mental map of how the monovalve is constructed.

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1981 Mercedes 300TD, 1994 Honda Civic Del Sol
http://mefi.us/images/fuelly/smallsig-us/67195.pnghttp://mefi.us/images/fuelly/smallsig-us/103885.png

Nope.......you'll lose a small amount of coolant. The valve is at the very top of the system.

The monovalve is a hollow cylinder that encloses a center plunger. The center plunger is operated by a small electromagnet to stop the coolant flow when energized.

When you remove the cover, the plunger and the cover will come out together. Pay attention to the wave washer and round washers that are in the stack. They need to go back in the same orientation when you replace the center plunger.

Got it, thanks. Very helpful.

I seem to have the "weak heat at high RPM" symptom, but my CCU also is a little erratic, so I want to visually verify a tear before I spend $45 on the rebuild kit.

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1981 Mercedes 300TD, 1994 Honda Civic Del Sol
http://mefi.us/images/fuelly/smallsig-us/67195.pnghttp://mefi.us/images/fuelly/smallsig-us/103885.png

As TF mentioned, you can disconnect the electrical connector from the monovalve. This isolates the monovalve from the CCU.

If you still don't have heat, it's 99% on the monovalve diaphragm.

If you get full heat, it's the CCU or a temperature sensor.


This saves you the trouble of taking out the monovalve...........finding a tear..........reinstalling it............ordering a new one...........and doing it again when the new one arrives.

You know that's not how it works!!!

Yes, I do........

Ah, good point. Maybe I'll try that on the way home.

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1981 Mercedes 300TD, 1994 Honda Civic Del Sol
http://mefi.us/images/fuelly/smallsig-us/67195.pnghttp://mefi.us/images/fuelly/smallsig-us/103885.png