Heater Control fix?
 

I have a 1985 300td and the heater has begun to work intermittently. When driving on the highway it will blow hot air for a while and then randomly start blowing frigid air.
I took out the monovalve and looked at it yesterday. The black material at the tip appeared to be torn. I'd love to replace it, but it's Christmas day (merry Christmas) and I'm leaving on a car trip in a few days. I'd really like to have heat before I go!
I was wondering, since the monovalve is a control valve, is there a way to rig the thing so that it constantly sends the coolant to the blower? Would that be idiotic?
I won't need AC for a while and if that could get me until I return from my trip, I would be so happy.
Anyway, let me know your opinions.

I have nearly the same problem with mine. For full heat just unplug the wire at the mono valve. Then just switch on the heat and turn it off when your face turns red:D

Now as to why you have heat for a while and then is stops because it want's to let me know. It seams that if the mono valve works it works. Especially when you remove the power source and get full heat. That's telling me that the controller thinks the care is too warm inside. It seems all the climate controllers are brain dead, or at least confused. I have replaced the sensor tube already without success. Can anyone punch in here and us where to locate the temp sensor?

There are two temp sensors, one in the water jacket on the engine and another in the air box under the passenger side of the dash. The sensor in the water jacket tells the ACC that the coolant is hot enough to warm the car. The one in the air box helps ACC regulate temperature in the passenger compartment.

The mono valve defaults to open, meaning, if it has no power it will allow coolant to flow through. So, to turn it on all the time, just unplug the wires. If it is clogged somehow, you can simply remove it and make a jumper from pipe or hose. Some people have even inserted a mechanical valve at this point.

When you have time, there's a repair kit that you can buy for the mono valve. Dealer price is about $75, less via FastLane (supports this forum) and other online Mercedes parts suppliers.

Jeremy

That typically won't work when the monovalve diaphragm is torn. The heat still won't work at higher engine rpm's.

So what your saying is that if the mono valve is torn the heat will work at low rpm but when you get to highway speeds the heat will stop?

I'm very interested in this forum
 

My 80 300SD is having similar symptoms.
While running high engine RPM's heat is outrageous :flamethro. With lower RPM's blows ICE cold (outside temp is -5F :earmuffs:).
I already have the automatic valve replaced with a manual valve, so that water always flows to the heater core.
I have heard about an auxilary water pump, but not sure if mine has it.

I also notice that my temp gauge reads lower at low RPM's, so I'm thinking it's got to be water pump / thermostat related.

Does anybody else have any great :bulb: ?

That's correct.

That doesn't seem to apply for mine, since my valve is replaced with a manual valve.

Newguy
 

The aux pump should be next to the servo control unit which is located on the right side of the engine compartment next to the firewall. If the servo unit is completely disconnected, the aux pump may not be in the circuit or may be removed completely. It could also be bad. You also don't have a monovalve.

Charlie

I see, said the blind man.
 

I see. So the aux pump is an integral part of the servo unit then. NOW it makes sense!

Thanks Charlie. I went and looked, sure enough, when I bypassed the servo I also bypassed the aux pump. I will try to get that back in the circuit tomorrow during lunch, when I can pull it into a garage bay. It's way too cold to do it otherwise.
Thanks again, I will post when it is fixed.