No. If that grounding was bad, your starter wouldn't crank.

There are several grounding spots within the engine compartment. If you follow any brown wire (I believe Mercedes uses that colour for ground wires across all models), it'll lead to a grounding point attached to the chassis. On my car, they look like an upside down bolt.

I found one under the windshield washer fluid container. It looked good, but it was causing issues, so I fixed it.
I had another one under the coolant overflow tank. This one was rusted, and was causing electrical nightmares like no horn, no right side headlight, etc.

I hope that makes sense.

Tudor

I really want to say it's the relay itself.....I remember reading is that the 80s w123 didn't have a temp sensor in the head for the glow plug relay, it was built in to the relay itself....I think this is the op's issues....the relay is pretty cheap compared to the one on my car.....though I agree, dirty rusty grounds can cause a heck of an issue with things...

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Messed with it for a while and never figured it out. Installed a push button type relay and really like it. So problem solved on that issue. Thank you everyone for the help and advice.

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198? 300D ???K - Future DD
1989 300TE 285K - traded in '12
1984 300SD 186K - Sold in 08
1978 300SD 330K - Died in 05