Yes. That is the aux water pump plug. If you want, you may keep it unplugged. This will not hurt anything. You just will not have the hottest heat in the world at idle. I live in NE Louisiana and it really never gets that cold here. This morning it was 29 degrees. But what I am getting at is that I kept the aux water pump unplugged for quite a while after I installed my new CCU because that pump is pretty pricey and I noticed no measureable heat loss in our climate here. Later in the spring when I changed my coolant, I then replaced the pump since I had the coolant drained anyway.
I drove December thru April with the pump unplugged with no ill effect.

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John
1986 300sdl
1985 380se

OK - so do you concur that I have a seized Aux pump?

Also, how much coolant do you lose during Aux water pump replacement - can I just refill though the overflow reservoir afterwards, or is it more involved?!

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1987 Mercedes 300SDL; SOLD
1985 Mercedes 300D; SOLD
2006 Honda Pilot - wife's ride; 122K;
1995 Toyota Land Cruiser - 3X locked; 182K

I do not know how much you will lose. I replaced my pump when I did a coolant flush, so I had no coolant in the system. Then I refilled my coolant in the normal fashion by filling the radiator, and adding through the coolant reservoir. I do remember that it took a while at idle for the temp to come up and the thermostat to open though.
I see that you are in Colorado. It gets just a little bit colder there than here in NE Louisiana. Did you buy a pump already? They are pricey. By the way, when I did this I removed the entire assembly, monovalve, aux pump and hoses and took it to the workbench and worked on it there. I also replaced my monovalve insert because it was just as old as my aux pump. My hoses looked just fine though.

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John
1986 300sdl
1985 380se

No, I didn't buy the pump yet... wanna be sure it's bad. If you hook up 12V accross a good one should you hear it running?! Mine is dead silent... does that mean it needs replacement for sure?

__________________
1987 Mercedes 300SDL; SOLD
1985 Mercedes 300D; SOLD
2006 Honda Pilot - wife's ride; 122K;
1995 Toyota Land Cruiser - 3X locked; 182K

Your pump should run with 12V across it. I would say it is dead if you hear or feel nothing with voltage hooked up to it. You can actually feel it run. It really is not loud at all.

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John
1986 300sdl
1985 380se

UPDATE: I went out last night to the garage to test the pump once again - I turned the music down... and rigged up the battery with neat short leads and good connection and tried it again. This time I could hear and feel it running.

I think I'll leave it alone for now. I'll probably leave it disconnected for initial testing with the new/reconditioned CCU that's coming in mail later this week.

__________________
1987 Mercedes 300SDL; SOLD
1985 Mercedes 300D; SOLD
2006 Honda Pilot - wife's ride; 122K;
1995 Toyota Land Cruiser - 3X locked; 182K

I installed the newly arrived reman CCU unit in my car this morning. Had to run some errands so after it warmed up I started running some tests. It seems that it's all well now. I have both Heat and A/C and for the first time the temperature dial seems to make a difference in the amount of heat the car put out into the cabin. It's a bit sluggish to respond to the dial changes - perhaps the cabin temp sensor is to blame but I can live with it the way it is for now.

Thx for all the help guys. One problem down, many more to go... 'til next time...

James

__________________
1987 Mercedes 300SDL; SOLD
1985 Mercedes 300D; SOLD
2006 Honda Pilot - wife's ride; 122K;
1995 Toyota Land Cruiser - 3X locked; 182K

I find this typical.

Set the dial at 20 and leave it alone. If it's really cold, you may need to push it to 21 or 22..............but massive movements of the dial are contrary to proper function and the system doesn't behave well when you do.

Remember, if the heater core is warm and the system determines that the vehicle needs to be cooler, there is a bit of a delay before all the heat dissipates from the heater core. The system doesn't switch back to "cool" mode if the ambient temperature is cold.

Likewise, if the system determines that more heat is required, it opens the monovalve gradually and the discharge air temperature climbs slightly. It won't blast the heat to quickly bring the temperature up because of the control problems that will be generated by an extreme overshoot.

If you want to heat the cabin quickly with maximum heat, just switch to defrost and raise the cabin temperature by 20 degrees within a matter of a minute or so...........however, the overshoot issue will then come into play.

OK guys I'm having an issue similar to some of you and i'd like to see if anyone has had my issues. My vehicle will produce heat (extremely well) until you are driving or accelerating and it then turns cold. Everyone on here seems to point fingers at the mono valve so I disconnected it leaving it in what i believe to be the "open" setting. I can hear the sampler motor running with the ignition on and it will hold a small piece of tissue paper to the intake grate so I assume that is not the issue. It still has the same issue. Thinking it may be a vacuum related issue I tried the heat with the vehicle in park and held at 2500-3000rpm's and the heat stays working as it should. Is there a blend door that allows fresh air inside? I have a hard time believing its the mono valve since I do have heat & I doubt its the aux. water pump as heat when idling or low rpm usage is not the issue. I bought this car in AZ and plan to drive it back to the midwest I am going to need heat! please help!!!

The issue is not valve position but valve condition. With a torn diaphragm, even a wide open monovalve can't flow enough coolant through the heater core to warm the cabin. Does heating performance at speed improve in recirculate mode?

Sixto
MB-less