I have been looking for all the information I can on the auxiliary water pump. This is what I have found:

*If the pump does not function when hooked up directly to the battery with jumper cables than it is not working.

*If the pump is not working it should be disconnected from the electrical wiring to prevent it from using the automatic climate control push button module for a fuse. It will fry the module in some cases.

*If the pump does fry the ACC module, it is possible to carefully resolder the module.

*If it is working, than a 1 amp inline fuse should be installed in the hot wire going to the pump to protect the ACC pushbutton module.

*If it is not working or if you have to disconnect it, you will still have heat at high RPM's, but not at idle. The heat in the passenger footwell will also be better than the drivers side, and that is a symptom of a malfunctioning aux. water pump.

* Some people have disconnected it for several years because it is not working. The reason being that it is not that essential in warmer climates.

*If it is leaking, it is possible to replace the O-rings. An internal o-ring leak will eventually ruin the pump.

*If it is not working, some people claim they have put new brushes in, and this repaired it. But if the motor is bad, there is no hope.

*If you have a climate control servo, you do not have a monovalve, and vice versa.

*The aux. water pump is widely used, and has a high failure rate.

*Has anyone successfully repaired one? I intend to take mine apart, and have a look at it. I am quite sure the majority of people buy a new one, for between $79.00-250.00plus.

*There is even more to the pump actually being sent a signal to tell it to turn on. It is supposed to pump while the thermostat is closed, and when additional heat is required in the cabin. If the pump is not being sent a signal (i.e. 12 volts), or is sent a signal continually that is another problem.

Please feel free to correct, amend any or all of this information. Whatever can be added would be greatly appreciated.

On different cars some of the things listed are true & others aren't. On the 1979 car there is a small temp switch in the air intake that supplies power to the pump. There have been 2 different temp ratings for this switch. Replacement pumps are inductive instead of direct shaft drive. Most newer pumps aren't repairable & water doesn't invade the motor, due to inductive drive. I am surmising that you have a weak heater, esp at idle!

Thanks M.B. Doc,
This is all information I gleened from the Mercedes Mailing list. I know my pump is siezed, and wondered if anyone had ever repaired one from a '79. It is the old style.

I do not know where the air intake is? I would like to check the switch. If someone could post directions to the switch, I would appreciate it. Thanks! 1979 300SD

If there is power & ground to the pump when the incomming air is below 75Fdeg, then the switch is good. Its on the heater box above the blower. Under dash panel, & or glove box removal is required. It's about the size of a dime & has 2 slide on male spade connectors that wires are plugged onto.

I have a 79 300D for parts and still have the aux. motor in there. I can check its operation, if you are interested. It is about 139k old. -Larry

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09 Jetta TDI
1985 300D