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Old 02-21-2009, 12:45 AM
Registered User
Join Date: Nov 2004
Location: LA - the state
Posts: 146
Glow plug dash light and plugs...

The '83 300 D, with 167,000 miles, recently had no glow plug dash (idiot) light illumination, even after replacing the bulb.

All plugs are original. Testing with the ohm meter at the plug relay showed plug nos. 3 and 4 were toast. I replaced them as well as the strip fuse relay. Still, no glow plug light illumination. Plug no. 4 does show .1 ohm lower on the meter than the other plugs. Would that a pose a problem? I twisted the wire insulation off a little as the plug broke during removal.

The car has always started easily, and in fact I just did a 600 mile round trip with the non-functioning no. 4 still installed.

I plan to find another dash bulb and replace it as soon as I get one. After that, should I go ahead and replace the rest of the plugs, or just learn to live without the light?

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Old 02-21-2009, 04:00 AM
Registered User
Join Date: May 2004
Location: Flowood Mississippi
Posts: 438
Glow plugs

Greetings from a fellow Louisianaian....
Change them all...long as your getting down in it might as well know they are all fresh....
The ohmmeter tests are not very good at establishing true condition of the plugs...
Best way is to bench test them so you can see if they light and how quick....
Ohmmeters just do not push enough current to evaluate condition....
The meter will just tell you if they have continuity or if they are open....
I just use extra heavy jumper clips and a fresh 12 volt car battery or high amp charger....
You can check them one at a time if they are parallel type or rig up the series connector bars to test the old series style plugs on the bench....

(When you see them glow...You know)..
1978 Yellow 300D (The Mustard Toad)
1980 Blue 240D (The Iron Toad)
1989 Grey Mitsu.4WD Mighty Max Pickup (Needs a Diesel transplant bad)

(Open the pod bay doors HAL)
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Old 02-21-2009, 11:29 AM
Registered User
Join Date: May 2001
Location: Michigan
Posts: 1,373
Agree with previous post.

If my memory serves me, the number one plug is often the problem related to the light.

I gave up years ago and simply bench tested them. There is no rhyme or reason to the crazy things. One may last 5 months and another 10 years.

What really, really helps however is to determine which ones to bench test. For that simple exercise, I pull the cap off on the relay, put the multi-meter into each hole (red) while grounding the black on the engine block. In that way, I often only have to bench test one plug; at the most two.


Diesel Don
'84 300DT 298k (Aubrey's)
'99.5 Jetta TDI IV 251k (Julie's)
'97 Jetta TDI 127k (Amber's)
'97 Jetta TDI 186k (Matt's)
'96 Passat TDI 237k (Don's
'84 300D 211k Mint (Arne- Undergoing Greasecar Conversion)

'82 240D 229k (Matt's - Converted-300DT w/ 4 speed
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Old 02-21-2009, 11:40 AM
Registered User
Join Date: Jan 2002
Posts: 18,350
I don't recall a single post on this forum describing a bad glow plug light and I've been on here 8 years. Anyone else recall a bad light?
The light doesn't work if there is a difference in the amount of current drawn between plug #1 and the others. There are two coils in the relay that conduct this comparison when the glow plugs are energized.

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
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