Glow plug relay board burnt

[vtmbz]

In seven years this is the first non start for this car!
Took a long trip. Came home parked the car next day no glow plug light.
Found melted 80a fuse. Changed fuse. Burnt second fuse (note to self: don’t waste a fuse until you figure out why the first one blew)
Checked continuity back to battery. Checks ok.
Opened relay box and found fried circuit board. Relay contacts look good.
I have another one but don’t want it fry it, so why do these boards go without warning?
Am I correct in thinking this has nothing to do with the gp’s?
Now what?

[Diesel911]

I have not read of anyone saying the circuit board got fried. More often it has been issues with the contact points (sometimes stuck to feather).

It would not hurt check the glow plug wires themselves for shorts.

You did not say what vehicle it is on. On my 84 300D this is where the smaller connector goes: Square Glow Plug Connector, what wires go to it.

#1 Red Wire with White Stripe, Run Start Input
#2 Violet (Purple) Connects to the Starter Solenoid Wire (On Turbo Models at the Terminal Block in front of the Battery) allows Glow Plugs to be on during cranking. (On mine a White Wire comes up from the Starter Solenoid an during cranking voltage comes up from the Solenoid and to that purple/lavender wire on the terminal block from there it goes to the glow plug relay)
#3 Blue Wire with White Stripe, Goes to the Glow Plug Light. The Voltage comes from the Relay through the Light and to a Ground.
#4 Brown Wire, Goes to ground.

Not sure what happens to the Glow Plug Relay if the Ground wire goes bad.

I bought an 80 amp resetable circuit breaker to replace the strip fuse on the Glow Plug Relay but never installed it.

You might be able to find if one of the Glow Plug Wires is shorted by doing the Ohm Meter test just as if you were checking the Glow Plugs. A short would have little resistance.

[vtmbz]

DG: I was hoping you’d catch this.
The car is the 85 190d.
The relay looks like the ones in your discussion of years ago.
I did a check of the continuity on the gp wires through the big square socket to each gp and did not find an. Obvious short. Getting to the glow plugs wiring harness is an absolute nightmare as you know. The intake box has to come off, and every time I pull it I think It will never go back without damage. I forget how it goes, but remember it’s a huge pain.

You haven’t seen a roasted circuit board, and that’s too bad for me because now I’m afraid to put my only spare gp relay in without knowing where all that power came from. Everything on this car is pretty solid electrically, and the failure was sudden, moisture is not an issue here, everything looks pretty clean and nice. I will run your diagnostics and check back.

[ah-kay]

It is unlikely the wires in the small connector frying you relay. I would do a test on the GP wires. Measure the continuity of the big plug wires to chassis. The resistance should be quite small and is difficult to tell whether it is shorted or not unless you are using a meter with very low ohm range. Anyway, I would just jam 12v to each female connector, one at a time, with a 10/20A fuse in series. If the fuse blew then you have a short. If it does not, next wire. If all wires pass the test then I would install the spare relay. Not much more you can test. GP relay is not rocket science.

[vtmbz]

Originally Posted by ah-kay

It is unlikely the wires in the small connector frying you relay. I would do a test on the GP wires. Measure the continuity of the big plug wires to chassis. The resistance should be quite small and is difficult to tell whether it is shorted or not unless you are using a meter with very low ohm range. Anyway, I would just jam 12v to each female connector, one at a time, with a 10/20A fuse in series. If the fuse blew then you have a short. If it does not, next wire. If all wires pass the test then I would install the spare relay. Not much more you can test. GP relay is not rocket science.

When you drive a 190,everything else is a rocket. But the advice is well taken.

I’m going on the theory that our long hard drive a few days before vibrated something loose and chafed a wire. I’m due for new. Motor mounts.

[cmac2012]

My original relay went bad somehow, long story, so I put in a Febi - low priced - and when I turned it on smelled burning plastic. Crap. Couldn't find scorched wires anywhere so I conclude it was in the harness. The car started the first time I used the Febi but that was it. My car was dead. Did not want to wade into replacing the harness territory so I bought Kent's simple and crude manually operated relay kit. Works great, I do miss having the glow plug warmup phase to fasten my seat belt. Oh well ...

[ah-kay]

Originally Posted by vtmbz

When you drive a 190,everything else is a rocket. But the advice is well taken.

I’m going on the theory that our long hard drive a few days before vibrated something loose and chafed a wire. I’m due for new. Motor mounts.

My 190D 5sp is like a rocket, going downhill. Anyway, you need to have a hard short to burn out your relay, chafing a wire will not cut it. You probably will see burnt wire if that is the case. If it passes the 12v test then I would just install the spare relay.

[vtmbz]

Finally got back to this. Removed intake manifold to inspect wiring harness and found #1 glow plug was snapped in half or burnt in half between the connection point and the gp body. Why that is I can’t imagine, but I suspect that would be enough to fry the breaker. There were no loose connections or bolts since I use a lot of lock tite blue. Those plugs are only a few years old, and less than 5000 miles. I’ll run a few tests later and see if that was the problem or a symptom.

[dieselbenz1]

Someone could have overtightened the electrical nut which twisted the conductor inside the insulation. This would alter the resistance and could cause it to pop. I had seen similar before.

[vtmbz]

Exactly what happened, but three years ago. I lost a glow plug on a long trip. Maybe the extended drive times is the culprit. So changed all four.
Three years later: another long trip. Glow plug failure. The harness looked over tightened, resulting in a cracked insulator. You called it.

[filp]

Interesting conclusion. I cracked a glow plug by overtightening the nut once.
Also, I got a spare 617 relay in case any of y'all need it.

[BillGrissom]

So a short to ground apparently blew the GP fuse. That is hard to measure with an ohmmeter since the resistance of the glow-plugs is very low at room temperature (increases as they heat). Not good if the relay's PCB was damaged since it is the fuse's job to avoid any damage. Perhaps it just got a little soot covering and is fine and just needs a wash. Soap and water doesn't hurt if you dry all the water before powering.

[vtmbz]

In the end, the circuit board looked all black and nasty but when I reinstalled it with the gp harness restored properly, it was fine. It looked horrible but was never damaged, just old....

I tried to put up a photo but the website doesn’t like my iPad. Thinking about it more, if the relay from the 80a fuse isn’t burnt, probably no power makes it to the board? Anyway, good for another seven years of trouble free driving, unlike my Volvo XC90, which breaks down daily.