I've done a search, but didn't find anything similar to my problem. I have a 1980 300SD with about 180k miles. It runs very well. The glow plugs act strangely: they stay on for 27 seconds (timed by listening to the clock) every time. Whether it's 10*F or 100*F. I can drive it 100 miles, shut it off for 5 minutes to fill up, and wait 27 seconds.

What triggers the glow time? Is there a sensor in the head/radiator? Is it all in the relay?

Any help is appreciated.

The system functions normally.

The glow time is a function of how long you leave the key in the #2 position. When you turn the key to #3, the plugs shutdown.

If you leave the key in #2, the plugs remain on for 25-40 seconds. I've never noticed the time vary with temperature to any meaningful degree.

It's a dummy light mostly to tell you that you need to us the GP's to start the car. The relay usually stays on after the light goes out. You can hear it click off if you wait. So the plugs can be on when the light is off.
I bet you can acutally start it without waiting the whole time when the engine is hot.

The light in my 300TD stays on for the same length of time no matter what.
But the light in my 240D stays on for about 30sec. when the engine is cold. But stays on for only a few seconds when the engines hot.

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For warm starts, I don't bother waiting for the light to go out. Cold starts in cold weather I always wait.