I've gotten most everything in the rear hatch of my project w124 300TD working. I checked all the wires in the harness at the hinge and found that the large brown ground wire had broken. Once I fixed that most everything started working (close assist, third brake light, license plate lights).

The only thing that still isn't working is the rear wiper. I found that the wiper relay in the tailgate was missing so I purchased a new one. It took me a couple of tries to get the right relay because my car uses a 6 pin wiper relay not the 8 pin relay that most w124 wagons use. I think the relay is the same as the relay for the rear wiper in my w123 300TD. They are the same size and # of pins.

Anyway, once I got the new relay in I still was not getting any response from the motor when turning it on. This evening I pulled the motor for testing.

The plug for the motor has 5 pins: 1 and 3 on the left side and 2,4,6 on the right side. On my bench the motor ran when I applied 12v ground to pin 3 and 12v positive to pin 2. I think this means the motor might still be serviceable (I also opened it up and cleaned out the hardened grease).

I then plugged the motor back into the car, turned the car on and pressed the different rear wiper settings. Nothing.

For shiggles I pulled the rear wiper relay out of my w123 300TD and put into the w124. This time the motor ran for a few seconds when I pushed the interval setting on the dashboard button. Then it stopped and no more.

I put the new relay I bought for the w124 into my w123 and the rear wiper worked as expected, so the new relay appears to have been good. Not sure why it would temporarily work with the old relay pulled from w123.

Any advice on what I should test next? Could the problem be with dash buttons? Or do I possibly still have the wrong relay? Or is my motor just on the fritz and almost dead?

page 187 of this wiring diagram has the the rear wiper wiring:


https://www.startekinfo.com/StarTek/outside/12264/Program/ETM/ETM.pdf




zoom in to see the terminal numbers etc...


many wires go through the connector X8 (top of the roof near tailgate behind headliner)

I went ahead and repacked the motor with some fresh grease and put everything back together, not expecting it to work. However it now works on as expected on the interval setting and when you push the wash button. It still doesn't work when on the full-on setting, so I'm thinking maybe the button is bad.

At any rate having the wiper work at the interval setting is more than sufficient for now, I'm just not sure what I did to get it to start working

The rear wiper should also engage if the front wiper is on and the transmission is in reverse.

Probably not much help to diagnose your symptoms.

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