Is the dashboard light for the glow plugs
ALWAYS supposed to come on when you turn the
key? Is there a temperature sensor that keeps
the dashboard light off if the cylinders
are "warm enough"?

My pre-heat light comes on when it's cold
but not always after it's been running or
even an hour after it has run or if it is
around 55 degrees or warmer.

I've had the car for 3 weeks. It has done this
since I've had it. I just changed the glow plugs
to no effect. Also changed the 80A glow plug
fuse to no avail either.

thank you,

- Jeff

1982 w123 240D

The relay determines the engine temp by glow plug current draw & that is how the relay knows how long to keep the light on. The glow system always works even if the lamp doesn't come on.

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Retired Moderator

Could the temperature sensor (for the relay?)
be out of range? I believe there is a sensor
on the block near the plugs.

Could that be why the light does not come on
when it is warm (sensor out of range)?

Could it also mean that the plugs are not
warming up enough when it is colder?

I'm also not clear on whether there is a
direct connection between the plugs "glowing"
and the dashlight illuminating. From your reply,
it sounds like there is not a connection but I
would like to understand it better.

Jeff / '82 240D

If I understand your description correctly, it sounds like everything is working normally. As MBDoc said, there is no sensor that determines this. The current draw on the number one glow plug is measured by the glow plug relay to determine when the lamp is turned off indicating that it is okay to start.

If the car is starting good, you don't have anything to worry about. If it ain't broke don't fix it.

Good luck,

Yeah, I know. I've broken a lot of not broken
things.

It's just that this is a "new" winter
car for me (upstate, NY) and I want to have
all the systems functioning correctly (battery,
plugs, etc.) to give me all the advantages I
can get. I've never had a diesel so it may be
a bit of fear of the unknown.

Jeff