1990 300SEL Auxillary Water Pump Test?

[outofspec]

Hello,
Having issues with my heat. New monovalve and still no help, thinking it may be the auxiliary pump. Got a coupe questions, hoping someone can help.

How to test?
Understand the plug is somewhere by the battery, need help locating?
Is it a big job?
They want big money for the Genuine MB part; has anybody used an aftermarket with success?

Appreciate any help and direction.

[Diseasel300]

The plug for the aux pump is right next to the battery. Look down next to it and you'll see a plug sort of clipped to the steel bracket that the hood switch is connected to. That is the plug for the Aux pump.

To test it, set the heat to MAX on Defrost and manually set the fan to low. Turn the key to RUN but don't start the engine. You should be able to feel the Aux pump and feel it vibrating if it's working. If it isn't, separate the plug and see if you have 12V present on the control end of the plug. If you do, the pump is probably seized or failed.

I'd still expect you to get some heat even if the Aux pump is bad. Did you install a genuine MB Monovalve cartridge? The aftermarket and MTC ones are known to be weak and "suck closed" when you actually want heat.

[outofspec]

Thanks Diseasel300!
Kind of a long story. Last winter, heat failed. I had a break in the weather, so I installed a MTC monovavle repair kit. You are correct, the MTC is junk. Still without heat, and another project tying up my shed (workspace), I was forced to bring the car to an indy repair place. The indy changed out the monovalve repair kit, with a new MB, and also told me that the magnet, that actuates the plunger was bad, so he replaced that with a known good (allegedly) used one. Heat worked OK for a while, but soon was not working, more than working. Occasionally, it would work, as if it was dependent on what button I had engaged on the push button unit. In the cases where it did work, it would decide to quite supplying heat after a few minutes, and I was unable to regulate temperature. AT one point I tried disconnecting the monovalve wires, but that was no help either. I wanted to rule out the Aux Pump. Also wondering if it might be a temp sensor either cabin or ambient air temp. AC and vents all working as they should.
Thanks for your help!

[Diseasel300]

With the wires pulled off the monovalve coil, the valve should be wide open or full heat, regardless of what the ACC system is doing. If you have no heat, you could have a stuck monovalve (especially if it's the MTC brand), a problem with the heater core, or could be a problem with circulation. I'd expect you to still have SOME heat even if the Aux pump had failed.

[outofspec]

Thanks D300.
I will conduct your tests, and verify that the monovalve is genuine MB.