I have a problem with my heater in my 1984 300SD. With the climate control knob at max I push the defrost button, or the front vent button, or any settings to make it blow, it blows cold air. OK that’s bad. But it some times blows hot air. It will blow hot air when the car is at idle (when it’s warming up in the morning, or at a stop light for a few min) it will blow hot air, but as soon as the car starts moving it blows cold air again. I don't understand why. Any one knows what the problem could be?

thanks

Nik

Sounds just like a monovalve:

http://www.peachparts.com/shopforum/showthread.php?p=1026761#post1026761

See the links on my entry on the attached thread.

Also look at this:

http://www.peachparts.com/shopforum/showthread.php?p=1023864#post1023864

If it is the monovalve then you need to get a replacement center piece: Bosch part number 1 147 213 007. I got mine at Olympic Auto Parts for ~$60, but you can get it on line here for less if you want to wait for delivery (I didn't). The dealer wants about $100, but some will say you need the whole valve for over $330!

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Peach Parts W124.128 User Group.

80 280SL
85 300SD
87 300TD
92 300D 2.5 Turbo
92 300TE 4Matic

Thanks a bunch. I'll order that soon and let you know if it was the problem.

i posted this probelm on some other MB forums... with no responce. thanks for the quick responce.

Nik

You're at the right place now.

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Peach Parts W124.128 User Group.

80 280SL
85 300SD
87 300TD
92 300D 2.5 Turbo
92 300TE 4Matic

This doesn't really sound like a monovalve problem to me. Why would a bad monovalve provide heat at idle, but not when driving? I guess it could be sticking closed sometimes, but that sounds unlikely. One way to find out would be to unplug the monovalve and verify that you get maximum heat. If you get max heat with it unplugged it may be the control unit acting up. I would give that a try before ordering any parts.

Depending on the size of the hole in the diaphram the problem may only occur at higher engine rpm because the water pump is working harder. At lower rpm the water pump is working slower and not causing the hot coolant to by-pass the heat exchanger. Eventually, once the hole is big enough the heat will fail entirely.

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Peach Parts W124.128 User Group.

80 280SL
85 300SD
87 300TD
92 300D 2.5 Turbo
92 300TE 4Matic

Sounds possible, I would at least take it a apart and look before ordering new parts.

Just had this problem/symptoms. A new monovalve rebuild kit fixed SPARKY THE DIESEL right up. Strange as how it sounds, it also fixed the tach problem I was having.....no tach at idle with air con/heater blower on.

Was about $28.00 cost and freight off e-bay total. Was willing to try a junker valve before invested in a good one from our sponsor.

Regards

Run-em
1983 300 S D- aka- SPARKY THE DIESEL

I just changed my monovalve and it was easy. You can find a really great "How To" at:
http://mbdieseldiy.tripod.com/monovalve.htm

My MB is a 1990 560 SEC and I wasn't getting any heat. Works great now.

BSA

Just curious about a notion mentioned earlier...when un-plugged, the mono-valve will default to...either open or closed (assumption). Anyone know which way? I ask because while I love my low budget SD and just love the hell out of wrenching-while-driving as I've been doing for the last 700 miles...well...it's getting right cold here in Va and my climate control is suspect...as is just about everything right now heat-wise. It's getting difficult to hit stereo pre-sets with mittens on...darn face plate keeps falling off.

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1983 300SD... 269,000 miles, nearly 2,500 on my B-100, Faded Grey, Ugly in an elegant sort of way...Duh-Benz


If any of this has been a blasphemy to you, then good, because it's been a blast for me to...A.Whitney Brown

The monovalve should go full open with the power disconnected (assuming it is not stuck closed). If you unplug it, you should get maximum flow to the heater core.

the car blows hot at idle but any time I push the throttle the heater water pump stops and does not come back on till I unplug it and plug it in again. I checked the mono valve and it is good and is open. Im not sure what controls the water pump and why it shuts off? Anyone had this problem?

new after market monovalves have a weak spring. the dynamic suction created by water flow will pull the valve closed. Hence, heat at low speed, no heat at high speed

Heat at idle and reduced heat or none at speed is the classic monovalve symptom. To diagnose, remove monovalve and pull up on the center ans determine whether the diaphragm is torn.

I grab all I find at the yard and throw them in my $5 pile of misc parts that the yard neither knows or cares what they are. Door bumpers are another item as are the plastic parts for the seat switches. Also useful are relays and copper fuses and any switch including headlight or cruise control.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

I have a similar problem but on my car I finally get heat after driving on the freeway for 15 to 20 miles. After I stop for more than a few minutes it goes cold again.

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1983 300SD
200000miles