Added a jumper wire to glow plug light from glowplug
 

I am always interested in having things work as the factory designed it to.
I do not do work arounds or shortcuts or ditching factory engineering.
Unless it is a true improvement.

But I am getting a lot older now and tired of chasing down some problems.

My glow plug light went from intermittent to not coming on.
(working in warmer weather, but not cold...etc,,etc..)

I put 12 volts to the dashboard glow plug light harness wire
at the relay and got a good light (so bulb is good, and ground at the dashbaord is ok).

I jumped a small wire from glow plug harness to cabin harness.
Now I get a glow plug light that at least tells me that the glow plugs are getting 12 volts.

The relay is factory new.
Could never understand why the glow plug light was so touchy.
I have considered replacing both the harness from relay to glow plugs, and the harness from relay to cabin.
Soon as I get a chance I will replace these.

Anyway, the down side is the new method means the light does not go off
(it no longer is a glow plug timer).
But the upside is it will be a reminder to wait 30 seconds or more before starting. Longer when it gets cold.

Could be a problem with your glowplugs. The light doesn't just measure whether the plugs are just getting 12 volts. It evaluates whether they are working properly.

The exact operation of the preglow system varies by model.

Sounds to me as though the problem is at the dashboard end.:rolleyes:

The bulb does not earth at the dash end its connected to switched live supply!
--Which sounds like its disappeared, and its now the lamp is earthing through summit Else connected to that particular line:eek:

Earthing the black wire at the glow-timer should light the lamp!:)

Then why did you disable the light that tells you the status of you glow plug system? If the light does not come on, then you have a problem with one or more of your glow plugs or the relay.

OR the Positive Supply to the Lamp--At The Dash End!;)

:D Whats the chances of that compared to a bad GP or three:)

All five glow plugs tested.
 

Several here mentioned the glow plugs, and that the light can be an indicator of a bad plug.
I agree.

Earlier in the same day all 5 plugs were removed and tested using a battery charger.

I remove all 5 plugs at the first sign of a glow problem / starting problem.
It is fast and easy to do, and a sure fire way to make sure they glow.

Anyway. reamed the glow plug holes (I do not do this part frequently, only when I feel a bind). Vacuumed out the soot.

Another thing I will do when sensing any trouble is apply a volt meter at each plug and watch voltage during pre-glow and starting to make sure nothing is faulty.

So, 12 volts all checked out, brand new relay that I happened to have in the shop (not junk yard new, factory new). Five working glow plugs.
One 30 year old wiring harness and 323,000 miles.

I am intrigued by Alastairs comment about the bulb at dash end not earthing
and that earthing (grounding for us yanks) at the relay end should light bulb.
This goes contrary to my thinking and experience with exposed metal in
american cars, but I am always open to new ideas.

I have tried an added ground wire to the dash in the past to strengthen the ground signal.
My temp gauge also reads a little funny, but a mercury thermometer in the radiator
and a laser non-contact thermometer each confirm the car runs spot on 80 centigrade.

So those two harness between engine and relay, and relay to cabin carry all this.
The temp and the glow plug light signal.
After all the years I am inclined to replace them .
I have a spare instrument cluster (dashboard gauges), and tried that too.
It changed nothing in the way temp and glow plug light behave.

I really appreciate all the input. Makes me want to get back out there and solve this instead of ignore it.
The car runs great and is a daily driver.

That entire notion is also contrary to the facts, in the case of the USA-version vehicles.

PM sent. Read the full section, there are two methods for troubleshooting that specific problem depending on year.

glow plug light fixed.
 

Thank you Yak. Following the directions was easy to do and very helpful.

And my glow plug light now works as expected.

The 1980 300D that I have has two temperature sending units in the head.
One for the pre-glow system, one for the temp guage.
Later models went to a single temp sender.

The end result is I ran two new wires from the sensors to the glow plug relay.
Wires 6 and 8 on the relay in the harness from glow plugs.

The connectors on the sending unit wires were both down to one strand of copper each. Just poor, old , brittle, hard, wiring.

Using standard bullet connectors (thank goodness they are the same size)
I was able to run two pieces of wire from each sender to the realy system.

Glow plug light works like I remember. Great news.

This car is to be passed on to yet another young driver in my family
and I wanted to get everything just right as the beginning driver learns to drive a diesel .

I have made this a right of passage and have taught 6 beginning drivers in this car,
now on to the seventh and last driver.

Yak - how about sharing with the rest of us?

No secret formula. I just pointed him in the direction of section 15-710 Checking Preglow System of the FSM. That's it. No added info.