glow plug light comes on, but never goes out
 

It's an '80 300sd - it starts, glow plug light comes on when I turn the key, goes out after I move to 'start', but sitting in 'ON' position, it stays on.....I've been led to believe it should go out in approx 10 seconds, as the plugs heat up....

What's up with it? whack the relay with a hammer?

TIA,

Matt

More like 30 seconds, not 10.

If you have a volt/Ohm Meter check the Glow Plugs. The Diesel Giant site has a pictorial under Glow Plug Repair.

If your Glow Plug Relay is connected to a Temp Sensor in the Cylinder Head try disconnecting the Wire and see if the Light goes out. I am not sure what year the temp sensor was put inside of the Relay but have read some of the older one had the temp sensor in the Cylinder Head.

Is the Relay itself working OK.

The procedure for pre glowing goes like this:

1. Insert key
2. Turn key to 2nd position
3. Watch glow plug indicator light and wait until it turns off
4. Crank

Number 3 is very important. Are you just waiting 10 or so seconds and cranking with the light still on? You should be cranking when the light turns off.

Even though it is warm here in CA when I continue to Preglow after the light goes out; I keep the key in the #2 position and wait until I her the Glow Plug Relay click off.
That way I get the approximately 10 seconds the light stays on before it turns off and additional 20 seconds of glowing.
I do not have to do the above to get started but I do any way.
I would definitely do it if I lived somewhere where it was cold. Infect I would Preglow more than one time before trying to starting if I lived somewhere were it was cold.