I have a 1985 300SD that usually runs very good. And all
of a sudden, wouldn't start. Like before a few times before, have
just changed the glow plugs and everything was great. This time
still don't start. I see where there is a glow plug relay.
Have never changed it. Also notice that the light on the dash
don't light like it used to, when turning the key on. Also
where is the glow plug relay??? In the electrical box or????
Also what part of the electrical box, as I don't have a manual.
Thanks in advance. And any other hints, if this isn't the problem..

Niko,

The GP relay is on the inside of the left front fender.

Inside there is an 80 amp fuse. Check this fuse first. Also, check if the GP relay clicks on and stays for about 45 seconds when the key is turned to the run position.

If the relay does not clickon, R&R the smaller of the 2 connectors on the relay about 10 times to clean off any corrosion on the contacts.

P E H

Thanks P.E. That was right on the money. Found the
80 amp fuse link burned into. Will replace AM tomorrow.
Would anything special cause that fuse to go like that?????
Thanks again.

Niko,

Fuse could be burned out due to a shorted GP. You might do a resistance test on all your GP before you replace the fuse. They should read about 700 milli ohm (0.7 ohm) each. If you find one that measures less, that is probably the bad GP. Usually when GP go bad, they have a much higher resistance which won't blow the fuse, that GP just won't get hot. But is possible you have a shorted one.

Best and easiest way to check GP resistance it to remove the larger connector on the GP relay and plug a banana plug into one of the holes. There are 8 holes, only 5 goto GP. You can use the ohmeter to trace which holes goto which GP by plugging one lead into a connector hole and touch other lead to a GP terminal. When you read 0, you have to the GP that is connected that hole in connector. Write down the hole number/GP number for future reference. When measuring the GP resistance, connect one lead of ohmeter to a good ground and plug other lead into the connector hole. This will measure the resistance of that GP only.

If you replace any GP, put the new one in cylinder #1 and replace the bad GP with the GP from cylinder #1. Supposedly the GP circuit does some measurements on GP #1 and its best to have a new GP in there.

You don't have to replace any GP other the ones that test bad. But I would buy a spare and carry it in glove compartment.

If you don't have an electronic multimeter, I would suggest you buy one. One trip saved to a MB repair shop will pay for the meter. They are handy for all kinds of testing and diagnosing. I always carry my meter with me when I go on a trip.

P E H

PE Thanks for the info on the GP's. I had already replaced
all GP's the day before. But that is good info to have. I have
a couple of Fluke digital meters, but from now on I will take
one on any long trip's. Also I couldn't find a 80 AMP fuse,
used a 65AMP, until I find one. Will that hurt anything for
a short time???? Thanks.

Niko,

65 amp fuse won't hurt anything but the fuse. It will burn out if more than 65 amps is drawn by the GP.

I hope you kept the old GP. You may need them in the future sometime. BTW why did you change all the GP?

P E H

Its been about 3 years since I changed them last, and they were autolite's, which I had not heard much good about them. And
most of all I found Bosch's GP's at the local Auto Zone for 7.99ea
which I though was a great price. Also saved the old one's for future use.

Niko,

I asked at Autozone about GP and they wanted about $25 each. I told them I could get then for much less on the internet. I bought 2 from Fastlane that were about $12 each.

Even if I had Autolights, I wouldn't replace them until they failed. I have found that GP last for many years. I still have some of the original GP in the engine in my 300SD I got 13 years and 135,000 miles ago. If it ain't broke, I don't fix it.

If a GP goes bad its not a show stopper, the engine will just miss fire for a short time after starting and then run fine. The blinking GP light will usually warn of a bad GP and it can be easily changed almost anywhere.

P E H

After that fuse has been in there a while, the screws loose some of there tension. I was told that this loose screw condition will cause some arching and with that arch comes heat which can contribute to the fuse burning. I guss the screw looses tension because of electrolysis on the fuse itself. I check the screw tension on my cars from time to time.

Great tip Mickey Mouse. Will check the screws every once
in a while. That may have been what happened. But the
GP's were about 3 years old and one or two may have been
drawing a little more voltage, than normal. But its fixed and
running for less that 45.00. Not the usual 250.00 or 300.00
from a repair shop...........