Bought my 83 240D a couple months ago. Has issues, of course, but starts, runs, no smoke (very little).
The glow plug light always illuminated (on startup) until a couple days ago. Car still fires up and runs, just no glow plug indicator light on the dash.
Checked the bulb, its fine. Checked the glow plug system, it's active. Pulled a couple out and connected them while watching if they heated up. They did.
The plugs are the Bosch "pencil tip".
They look fairly new, not corroded. Turns out 2 were bad. Waiting on a new set from the "bird". We'll see if that clears things up.
So, to my original question, what turns the light on? Does it come straight from the relay on the inner fender, up front, drivers side? That also looks fairly new for a 36 year old car. Doubt it's original.
Unfortunately, didn't get any "history" with the car.
Any ideas would be greatly appreciated.