I had this problem ever since I've had the car.

The GP light on the instrument panel would NOT come on. I checked and replaced GP's, checked wiring, checked lamp, and I even listened for the relay to 'clunk' at the end of its cycle. All GP's had the proper voltage at the correct times but I STILL couldn't get the light to come on. I switched my relays from my 81 300SD into this one and it worked.

After switching them back to the proper vehicle I just couldn't help my self and I had to crack open the relay. After doing some circuit board tracing I came across a transistor on the board that is connected to the lamp connector at the plug. It is a PNP type transistor part number that I have is 2N4126 that may be available from an electronics catalog. If anybody needs one and cannot find it let me know, I've got at least 10(ten) more transistors here. Anyways back to what I was saying. After replacing the transistor I re-installed the relay and IT WORKED.

I hope this may help anybody that has tried everything for thier relays and glow plugs but still no light.

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1981 300SD