glowplug coming on while driving
 

I have a 83 300d, turbo, 220,000 miles. My glow light is coming on while I am driving sometimes and it is very random. It will stay on for a few seconds to a minute and then go out. What would cause this. Oh and the car does not act any different. Thanks

The light is controlled by a solid state device in the preglow timer. Could be a malfunction there, esp if the lght does not stay on for the "normal" period of time. Or it could be a problem in the key switch and/or related wiring that is causing the timer to act like the key has been cycled off and back on.

The glow plug system from that era is an electronic timer on the left front fender ahead of the wheel well. Under the cover are 2 connectors. The forward connector goes to the glow plugs, either 4 or 5 wires depending on whether you have a 240 or a 300. The rear connector has 4 wires, it is tagged C130 in the diagrams.
Pin 1 hot with key in pos 2 or 3
Pin 2 hot with key in pos 3
Pin 3 +12 V supply to bulb in cluster
Pin 4 ground

I can think of a couple things
- Bad relay causing bulb to come on randomly
- bad ground to relay causing relay to find ground through the bulb (weird but possible), relay is grounded on G102 behind LH headlights
- another weird one, something shorted in instrument cluster causing voltage to get applied to bulb.
- and like he said, check wiring from key to pins 1 and 2 of C130, could be an intermittent causing relay to recycle randomly. However that would be really crazy and you should be noticing other things happening in the car. Per the wiring diagram, the Pin 1 source comes straight from the key and gets tapped from the top of F2, there are a ton of things off of F12 that would be cycling on and off as well (F12 feeds among other things the gauges, so your temp and fuel gauges would be bouncing around as well). Pin 2 gets tapped from the same wire going to the starter solenoid so I would think you would know about that one too...

But I would suspect the relay first.

probably the relay
 

I had this happen on two cars with different GP systems and it was the relay both times.

I had short preglow times and then, later, the light would come on randomly when driving. New relay fixed it for me.

Same problem awhile back..As others have said, a new glow plug relay fixed it!

cool thanks all! hoping this weekend I will be able to look at it. Short is possible due to the wipers sometimes turn on when I turn on the right turn signal.

found used turn signal and installed. All is good:D. Car is almost perfect again.

Does this mean that replacing the Turn Signal cured your Glow Plug Light issue?

I do not think so, but so far no more glow lite coming on during driving....I will post when and if it does it again.... oh, I did swap what looks to be relays on the driver front fender well....Do not have wiring diagram so not sure what they are for