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.

__________________
80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250

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.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

The exact operation of the preglow system varies by model.

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

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

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

__________________
http://i190.photobucket.com/albums/z...0TDnoplate.jpg

Alastair AKA H.C.II South Wales, U.K. based member

W123, 1985 300TD Wagon, 256K,
-Most recent M.B. purchase, Cost-a-plenty, Gulps BioDiesel extravagantly, and I love it like an old dog.

W114, 1975 280E Custard Yellow,
-Great above decks needs chassis welding--Really will do it this year....

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.

__________________
Greg
2012 S350 BlueTEC 4Matic
2007 ML 320 CDI
2007 Leisure Travel Serenity
2006 Sprinter 432k
2005 E320 CDI
1998 SLK230 (teal)
1998 SLK230 (silver)
1996 E300D 99k, 30k on WVO
Previous:
1983 240D, on WVO
1982 300D, on WVO
1983 300CD, on WVO
1986 300SDL 237k, 25k on WVO (Deerslayer)
1991 350SDL 249k, 56k on WVO - Retired to a car spa in Phoenix
1983 380 SEC w/603 diesel, 8k on WVO
1996 E300D 351k, 177k on WVO

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

__________________
http://i190.photobucket.com/albums/z...0TDnoplate.jpg

Alastair AKA H.C.II South Wales, U.K. based member

W123, 1985 300TD Wagon, 256K,
-Most recent M.B. purchase, Cost-a-plenty, Gulps BioDiesel extravagantly, and I love it like an old dog.

W114, 1975 280E Custard Yellow,
-Great above decks needs chassis welding--Really will do it this year....

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

__________________


1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

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.

__________________
80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250

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.

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.

__________________
80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250

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.