W123 aux water pump fuse install w/pix - PeachParts Mercedes-Benz Forum

Jeremy5848 · #1 07-05-2006, 12:49 AM

It has been stated on this forum that the auxiliary electric water pump, which helps the Automatic Climate Control (ACC) by increasing the flow of hot water to the heater core, can freeze after many years of service. A frozen electric motor will draw more current than one that is spinning; this one has been known to fry the ACC controller. Unusually for Mercedes, this pump is not fused.

After removing and inspecting my ACC controller, and seeing that it had been previously damaged, I decided that it would be a really good idea for me to check out the pump, make sure it worked, and add a fuse.

As far as I know, the aux pump is in the same place in all W123s, low down on the right (passenger) side of the engine bay, on a bracket attached to the inner fender. It is not difficult of access except for the 1985 California version, in which the air filter housing sits over the pump and makes it difficult to see, much less touch. I removed the filter housing -- it was filthy dirty anyway and I wanted to clean it up.

With the air cleaner out of the way, I first stuffed a shop rag in the mouth of the turbo, so nothing would get in it. The aux water pump can be seen in the first two pictures. The loose plug is the pump's electrical connector. I disconnected it so I could test the pump and pull the other end up where I could get into its cable.

I first put an ohmmeter on the plug; it measured about 8 Ohms. This sounded reasonable so I connected 12 volts from the battery to the plug. That was OK -- I could hear the motor running. I wasn't sure which pin was positive and which was negative (everything down there is black with soot) so I tried both ways. The motor ran both ways so I knew it was OK. My ammeter told me that the motor was drawing about 0.9 Amp at 12.6 Volts. A fuse of about 1.5 to 2.0 Amps should be sufficient.

The pump's cable runs up by the radiator's overflow tank and joins a larger cable. Once I had pulled the cable up so a few inches was free, I sliced open the covering with an Xacto knife and pulled out the wires. You can see in the next picture that the wires were color coded. This made the wiring job much easier -- in German wiring, brown is always ground, so the other wire had to be the "hot" wire.

I pulled a fuse holder from my junk box and spliced it into the positive (red/black/violet) wire. After taping up the cut in the cable, I routed the cable back to where it came from, put the plug and socket back together, and put a 2 Amp fuse in the holder. With the wires back in place, I laid the fuseholder on its pigtail next to the overflow tank where it would be out of the way but easy to get at if the fuse ever blew. I do need to remember to carry spare fuses as they aren't the same as the German fuses.

Then I cleaned up the air cleaner (worst part of the job) and put everything back together, being careful to remove the rag from the turbo. With the car back in one piece, I started the engine and checked it out.

This is an easy and inexpensive job and one that every W123 owner should do.

Jeremy