I have a 1985 300D, Euro version, non turbo. I just had the transmission replaced. Drove it home and parked it for a few days. Now it won't start. The glow plug light on the dash comes on, but never goes off. I thought something might have gotten loose, but all the wiring looks tight.

Would the light not go off if one or more of the glow plugs are bad, or is it more likely the relay is bad?

Any help would be appreciated.

When you turn the key to the glow position, you should be able to hear the relay click on then eventually click off after the cycle is complete. This is not directly linked to the function of the dashboard light (relay remains on after the light goes off).

Have you checked inside the relay that the strip fuse is intact? These can even develop hairline cracks that are hard to see by eye.

With a multimeter or tester light, check that you are getting power to the relay at the bolted wire under the cover (that is 12v into the relay). There should be power there at all times. Then if the fuse looks intact, check for power on each side.

If all this checks out, then have someone turn the key to the glow position while you tap lightly on the side of the relay body. When they get old they sometimes stick.

Other than that, I guess the obvious question would be, how old are the glowplugs?

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

I'll check that out. Too darn hot here to do it today. The Glow plugs are a few years old, but the car does not get that much use. Probably not more than 10-15k miles since I replaced them.

When you say only a limited number of miles, that has to be taken relative to the number of glow cycles the GPs have seen (10-15k of many starts for short trips adds up).

If they are not Bosch plugs, they are probably worn out. There is basically Bosch and everyone else, exception Beru and NGK but they are hard to find for these cars.

Also check that the connections to the GPs are secure, in particular at the first GP.

I suspect your relay is bad but have never read of the issue you are having so that's just opinion.

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

I think the relay is the problem. I'll wack it tomorrow. and see what it does,