123 Glow Plug Relay Repair

[Turbo300Mercede]

The temperature just dropped here and one of the "project" cars did not want to crank this morning, early 80's 123 Wagon. I tried the normal things, pump the hand pump on the injection pump, extended wait time for the glow plugs-although the light was not coming on, I could hear the relay click. None of this worked. So I broke out the multi-meter. 4 of the 5 glow plugs were good at .7 to 1 ohm. One was open, obviously a bad one. Good continuity between the pins at the glow plug relay to each glow plug, 12V to the relay, 12V to each side of the large flat fuse in the relay. No voltage to the glow plugs while the relay was latched. So, the relay is not passing voltage through it even though I can hear it click on and click off.

Un-hooked the battery, 10 mm socket to remove the main power to the relay, pulled both pin and socket connectors off the relay and a 8 mm socket to removed the relay from the fender.

Take it into the garage and 4 small phillips head screws is all it takes to pull the cover off.

[Turbo300Mercede]

123 Glow Plug Relay:

[Turbo300Mercede]

Here you can see the 5 pins that connect to the harness that supplies power to each glow plug. The other pins go to a smaller connector.

[Turbo300Mercede]

Here you can see that the contacts are weathered and corroded.

[Turbo300Mercede]

Another close up of the contacts. I tried running 600 grit sandpaper through them while assembled, but the pitting was too deep.

I removed the two contacts. They are attached together and all it takes is removing the small spring at the rear. See the other pictures for the spring.

Once the contacts were free, I cradled the relay in my palm and pressed the contacts against the top of my work bench, where I had 280 grit sandpaper at the edge of the bench. I worked the contacts left to right across the sand paper.

The larger one had such deep pitting that I filed it with a fine file, then went back to polishing with the left to right motion on the bench and sandpaper.

[Turbo300Mercede]

This picture was after I had tried sanding the contacts while they were assembled.
All I accomplished was to just clean away some of the larger corrosion debris.

The picture shows that the contacts remain attached with the braided copper wire.

[Turbo300Mercede]

Here I am pointing out that the larger contact has an oxidized film on it that I suspect is non-conductive.

Also note the rust on the solenoid. It has built up some and may prevent the contacts from coming together as the contact set pulls up to the top of the solenoid when power is applied.

I later removed that rust, but did not bother to try to polish it, just to remove the debris so the contacts could come together.

[Turbo300Mercede]

Here you can see the filed / sanded / polished contacts on the relay body and the contact set.

In a similar manner as I polished / sanded the contact set, I held the relay in my hand and moved it left and right across my work bench edge, with the contacts down fairly snug against 280 grit sandpaper. I used the better part of a full sheet of sandpaper, folding it / or tearing it as I worn off the abrasive. These contacts were severely pitted.

[Turbo300Mercede]

Here I am pointing out the spring that holds the contacts in place and keeps the relay "open" when no voltage is applied.

Note the formed copper tabs / lugs and the square cutout in the contact set. The square cutouts fit on the tabs / lugs. The spring keeps everything in place.

I used the long pick to easily stretch the spring back over its pin.

Once the relay was re-installed onto the car, even with only 4 glow plugs working, the engine started immediately. Best start it has ever had in the two years it has been sitting in my garage. Mission accomplished, no cash outlay other than one sheet of sandpaper and an hours time.
And the glow plug light now illuminates on the dash and there is a much louder audible click from the engine compartment when the glow plug relay engages.

I also carefully cleaned the circuit board to remove solder rosen and contamination. Rubbing alcohol usually works well for this with a stiff nylon brush. I finished up with brake cleaner and careful application of shop air. Don't want to blow debris into places it does not need to be.

[Diesel911]

Cool.

[vwnate1]

Very nice .

I often take apart old switches and relays to clean and burnish the contacts, it always seems to help a lot .

You're pictures are nice and help DIY'ers to know what to do ~ this relay was junk so if you'd been unable to fix it or broken it, no harm, no foul .

[Turbo300Mercede]

6 year check back in, the relay is still working.

[vstech]

Nice...
Maybe this should be a preemptive service on the relays...

[Diesel911]

Originally Posted by Turbo300Mercede

The temperature just dropped here and one of the "project" cars did not want to crank this morning, early 80's 123 Wagon. I tried the normal things, pump the hand pump on the injection pump, extended wait time for the glow plugs-although the light was not coming on, I could hear the relay click. None of this worked. So I broke out the multi-meter. 4 of the 5 glow plugs were good at .7 to 1 ohm. One was open, obviously a bad one. Good continuity between the pins at the glow plug relay to each glow plug, 12V to the relay, 12V to each side of the large flat fuse in the relay. No voltage to the glow plugs while the relay was latched. So, the relay is not passing voltage through it even though I can hear it click on and click off.

Un-hooked the battery, 10 mm socket to remove the main power to the relay, pulled both pin and socket connectors off the relay and a 8 mm socket to removed the relay from the fender.

Take it into the garage and 4 small phillips head screws is all it takes to pull the cover off.

Old thread. But serves as a reminder when you post issues to describe it correctly. The engine not cranking and an engine cranking but not starting are different descriptions to different issues.

[vwnate1]

THANK YOU for the update ! .