I have posted several threads about my preglow problems (sorry for the repetition), but I have FINALLY localized the problem (I think). My preglow light comes on only 50% of the time. GPs are all one month old. When the light does not come on there is no power getting past the relay, based on a basic current test on the first plug. No power on any of the plug heads.

Here's the weird part: if I leave the key in the power-on position, the relay makes a switching sound, as it should, to turn off the power to the plugs (to save em from burn, @ 40 seconds). THEN if I reset the key and turn power on, the glow plug light comes on. It's like the relay resets itself. Any body heard of this?

Can I fix this within the relay or do I need to pony up for new one?

Thanks,

RBC

rblookc,

Did check the 80 amp fuse in the relay. They have been known to get a hairline crack and work intermittantly.

Check that first.

P E H

I believe the clicking of the relay (done by its control line) is independent of the main power feeder wire that provides glow plug power. So the relay can click and activate for all its worth, but if there is no power on the contacts that are switched into the glow plugs - no glow.

Knowing this, the most likely cause of a problem is that 80-amp fuse that PEH mentions. It is under the cover of the glow plug relay box. Just a strip of metal between two phillips-head screw terminals. Go ahead and take it all the way out to examine it.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles

Thanks for the response.

I swapped the 80 amper out first with a new one. I was hoping that would be it too.

I just dont want to pull the trigger on a relay and have it be a shorting GP or something. The power into the relay is good based on a current check of both sides of the 80 amp and the other point there. But no current at the plugs (past the relay?), as measured at the GP electrical connector or on the plug nuts, when it's acting up (I do get current at the plugs when the preglow lamp lights up whenever it damn well feels like it).

Pretty good chance it's the relay?

Thanks ...

rblookc,

I had the same problem you have. Here's what fixed it:

R&R the connector on the end of the GP cable where it plugs into the GP relay about 10 times. That should wear thru any corrosion on the terminals and the GP should work.

Let me know if that works.

P E H

Yes please let me know if it works since my car has been acting the same way recently.

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Orland Park, IL
1985 300SD 215,000 miles

rblookc and Bill,

I just remember it was the other connector on the GP relay, the one next to the GP cable connector, the smaller of the 2 connectors on the GP relay that R&R 10 times it fixed the problem.

My dash GP light is still intermittent but the GP work all the time and the timing of them is OK. That's good enough for me on the old beater MB.


P E H

So, I have changed the 80 amper, R&R'd the connector plugs and replaced all the injector overflow hoses (were leaking; of course unrelated but you never know in terms of karma) and the glow light is doing alot better. Haven't had a bad one yet. Will let you know what happens. I am hopin. Thanks for the input thus far.