Why won't the light come on!?

When I got the car about 8 months ago it wouldn't start "nicely" so I checked all the GPs, and with spares from my parts car I cobbled together five good glow plugs. Now the car starts like a dream. But my GP light on the dash never came on. Thought I had a bad bulb. Pulled the cluster and moved the brake bulb to the GP, and GP to the brake, and the bulbs checked good - still no GP light. Put 12V on pin #3 on the GP relay control connector and the GP light in the dash comes on. So I have a good bulb, a good cluster, and good wiring. What I don't have is a good indication that my GP system is working. The only thing left is the GP relay control unit. I swapped that out with my car parts spare and still no light. Admittedly they have different part numbers, but the wiring is the same so I would guess that they are interchangeable. When I pull up a parts supply website, the relay has the same P/N for both cars.

So I guess my question is, is it common for the current sensing portion of the relay to go bad, providing no indication of glow plug functionality? If one GP is marginal does the light stay out? What gives?

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)

If #1 is out you get no light........but, it starts reasonably OK. If you start it without any pedal, it will give the typical 4 cylinder miss for about 10 seconds until the last one catches (#1).

Change #1 and see if the light returns to proper function.

You'll never guess what the problem was.

Oh wait, you already did!

When I went to remove the wire from #1, the internal assembly of the GP moved with the securing nut, and the nut wouldn't loosen. Had to grab the wire and hold it to loosen the nut. Replaced the GP and the light comes on.

I bow down in admiration and gratitude.

After some searching I found other threads that describe the operation of the GP system, and mentions of the FSM. Dammit, wisht I'd looked there first. At least I could have tested my system before needing to ask. I knew I'd seen the information somewhere before.

Thanks again.

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)

Good job.

Help is what we do here.

I was bypassing my ACC Servo...and now my GP dash light will not come on and car is hard to start. 79 300D. I upgraded the plugs, and removed the curvy plug to plug connectors, with individual wires to each plug, from dieselgiant. I did not upgrade the relay, but did put in a new fuse.

Car starts, but I have to hold the ignition on for about 8 seconds, and it starts ROUGH.

Should I troubleshoot as if plug 1 is bad? Is plug one closest to the firewall, or farthest, if you are standing in front of the engine looking in?

Thanks,
Marty.

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Marty Watts - Concord, NH
1979 300D NA - Maple Yellow - Palomino MB Tex

My guess is that the system is not functioning at all. The modification has resulted in the loss of voltage between the relay and the plugs.

Check the plugs for 12V when the relay is engaged.

#1 is closest to the radiator.

No your relay is different since it used to run a series system. Check for voltage at the plugs. If no voltage, check your strip fuse.

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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

Apparently, DJ manages to sell a kit to work with the existing relay. It's a bit of a mystery to me..........as well.

I put the pencil plugs in my former 77 300d without replacing the relay. As long as the plugs are getting 12v it works fine. Glow time may be a little longer than on most pencil plug relays but no big deal as far as I was concerned.

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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

You mentioned, above, that his relay is different..........???

He has a series relay on a parallel system. As far as the relay is concerned, it is delivering 12 volts to the glowplugs and doesn't know they're not series plugs. However, since the series relay has no way of checking each glow plug, the light won't indicate problems in the plugs.

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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

Will the light still illuminate for a few seconds prior to starting.............as would normally be the situation??

Yes, light should illuminate using the series relay with the parallel plugs.

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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