Hello everyone, I haven't been on the forum in awhile. But I have been searching for some answers with my heat problem.
I have an 85 300SD with 196k on it. Since the fall I have no heat while driving. Every once in awhile I have heat coming out of the defrost. But only at idle. As soon as the car moves the heat disapears. It doesn't take a long time to heat up and the temp needle is where it should be.
I've read a lil bit about the monovalve. I am hesitant to take anything apart without knowing a lil bit more. Any help would be greatly appreciated.
Thank-you

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1985 Mercedes Benz 300SD, TMU
2015 Chrysler 200S

Two things that typically fail. One, the monovalve like you have found. It sticks shut and will not allow the coolant to flow. The other is the auxiliary water pump the bearings are exposed to the atmosphere and everything it holds (salt) and it usually siezes up redenering you limited or no coolant to the heater core.

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2003 Dodge Ram 2500 Cummins - Georend fully built trans, Banks Big Hoss Bundle, Smarty Tuner, 5" turbo back exhaust. 500 HP and 1100 ft/lbs to the ground.
1979 Mercedes-Benz 300CD - Lost to a four car pile up on 275, Thank You Michigan Drivers.
1986 Mercedes-Benz 300D - Sold, Euro non-turbo car.

I think typically with a seized auxilliary water pump, you get little heat at idle but adequate heat at high rpm's. My money's on a failed monovalve diaphragm. Simple 2 minute replacement. Disconnect, remove two screws, replace.

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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

I bypassed both my aux. pump and monovalve with a section of 3/4" heater hose, with a 1/2" gate valve in between so I can regulate it. I got everything to do the job in the plumbing dept at Home Depot and it works great. All the heat you could want and none of the complications.

In the summer you just close the valve completely.

You run the bypass hose from what would normally be the aux. pump inlet all the way to the heater core connection at the firewall.

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'81 300CD - 180K, cannot be killed by any conventional means
'99 Ford Escort - good MPG

Thank-you for the replies. I really appreciate it. I plan on taking a look at the monovalve later today. Hope the 4 screws come out easily.(LOL)
Just curious as to where the aux water pump is?

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1985 Mercedes Benz 300SD, TMU
2015 Chrysler 200S

This is what should pull out of the valve housing:
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=2AT0J1QHK2AZ0QDX21&year=1983&make=MB&model=300-SD-002&category=R&part=Mono+Valve+Repair+Kit

If you find that the rubber diaphragm is torn, installing the repair kit is very likely to restore you heater to normal operation.

The pump is buried between the two firewalls just to the inside of the battery.

Remove the battery and remove the section of the forward firewall (two screws) to gain access to it. They typically seize after 20 years..........unfortunately, when they do, they take out the CCU in the process.

By disconnecting the electrical to my monovalve (just unplug it), I was/am able to get full heat all the time.

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1989 300E 144K

Hey guys,
I took the monovalve off and have found that the boot does look torn along with some corrison around the gaskets. So I think I'll be ordering the rebuild kit. Thank you for all of your help and hopefully that will fix the heat issues. I really want to have heat this weekend for an upcoming trip.

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1985 Mercedes Benz 300SD, TMU
2015 Chrysler 200S