I have a 1976 300d. I noticed lately that the pre-glow light would not come on when I turned the ignition on. When I move the key to try to start it would flash and then come on. Now the light won't come on at all. Now I can't start the car at all. Any ideas on what the problem could be? Could the relay be bad? Glow plugs bad? I'm lost on what to do.

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1972 220d gone lived a good life
1972 280se gone but not forgotten
1978 300D the best i ever had
1977 350se speed racer gone
1995 honda accord lx coupe victim of the growing family
1988 300e gone but not forgotten

I don't know much about this stuff. If it happened to my SD I'd...

Check the GP relay box to see it the fuse is blown. If it is there's probably an issue with a GP grounding due to a swollen tip touching something. If it's not the fuse then check the wire connections in the box to make sure they are tight and clean.

The bulb is suspect though I've never seen one burnt. Removing and reinstalling the bulb holder will likely remedy a contract issue between the cluster and the bulb holder.

The last thing I'd investigate would be the wiring between the GP relay and the bulb. Check all grounds first.

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84 300SD
85 380SE
83 528e
95 318ic

Read the owners manual on how the flow plug light works. It behaves differently for different model years. Generally, a flashing glow light indicates one or more burned out glow plugs. Have you tested your glow plugs? There are many ways to do it, from very easy (with a clamp on ammeter) to a lot of work (take them all out and glow each plug individually).

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

You can do a rough estimate of the glow plugs' heating ability by measuring the resistance of each GP and compare to a known good one. The GP really needs to be removed, hooked to a battery and be made to glow (or not) to really test it.

I think Diesel Giant has a pictorial on testing the GP wiring. You should also check the fuse which may be located in the relay box. I don't know on your car.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

OP has the series wired glow system.

One plug fails, or one bad connection - no glow at all.

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2014 ML350 Bluetec (wife's DD)
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both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Sorry OP. Forgot to mention, I have the newer style GPs'. Its advisable to upgrade to them. You will be genuinely pleased with the starting performance.

Your current relay will work just fine with the newer style GPs' - it will just stay on longer.

In case you are considering upgrading, the GPs' you need are called - "Bosch 80035/0250201044 Duraterm Glow Plug"

Especially if you own an older Mercedes diesel with the relay under the dash near the brake arm. You should have a spare glow plug relay.

Cheap if you have a pick or pull car locally that is Mercedes diesel and older than 1981 probably. You can check the years for applications. I suspect the same relay from maybe 75 till 80. Verification is easily done on a parts listing site.

I have had a couple of these old relays just age and use fail. You can make a manual system with an old ford fender mounted starter relay as well. Did a quick parts check. It quickly looks like the original glow plug relay is a 1968 to 1976 specific part.

Some of us highly recommend this solution....

• reduces complexity
• increases reliability
• glow plugs last longer
• controllable after glow

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Current Stable

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Hello,

When you turn to the key to where you would be glowing the glow plugs, measure the voltage at the #5 glow plug (black wire). If the voltage is 12v then you have a bad glow plug somewhere (check each plug and resistor bar), if it's roughly 10.5v then the plugs are ok, if it's 0v its either the relay or the power feed to the relay is bad.

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Marc-André

W115 1975 300D "Geraldine"
W115 1976 300D parts/tutorial car

Anyone have a link to a DIY article on this?

I looked under the dash and the relay is good. As soon as I unplugged it and plugged it back in the light came on. The wires are loose. One of the wires was extremely hot. Could this be the cause of the problem. What do I need to do in this case? Do I just need to make sure the connections are not fried.

__________________
1972 220d gone lived a good life
1972 280se gone but not forgotten
1978 300D the best i ever had
1977 350se speed racer gone
1995 honda accord lx coupe victim of the growing family
1988 300e gone but not forgotten

I would try unplugging and plugging the relay under the dash. Hopefully your relay is still good but has mild corrosion or dirt - which can easily be cleaned. The other option could be that the relay is bad.

All this is assuming that you have checked your GP fuse. They are cheap and you should keep a few handy. Try a new fuse first.

Sometimes my GP light doesnt come on, I lightly tap the bottom of my dash, which triggers the relay. This is probably an indication that my relay might be going bad; however they are expensive and some folks have claimed that new relays are also finicky. So I m choosing to maintain mine and run it as long as possible.

Good luck

Thanks! So on an old series glow plug car, would you run the big hot wire straight to the number five glow plug?

Yes to the fifth glow plug where the current wire is attached. Actually using the strip fuse holder is the best place to install the feeder wire from the new relay. Rather than the number five glow plug. You want to retain the strip fuse in the system anyways.

Got it. That makes sense. Thanks!