You've changed the gauge cluster so the problem isn't likely in a ground at the cluster. Pull the ground plug off the back of the cluster and check the continuity between the ground in that plug and the group of brown wired mounted just above your steering column. If you have continuity there then pull the sending wire off the sending unit at the fuel tank and check that ground.

I'm really confused that you think your car is a DSL [sic] but later, within the same post change your mind to thinking it's a SE. An SDL, as properly designated is the longer body w126 with a diesel engine. I'm not sure MB made an SDL in 1979.

Nevertheless, I'd say your problem is at the sending unit. Either a ground there or the sending unit needs to be removed and cleaned.

Have you considered which fuel to use or read the designation on the trunk of the car? I think you are confusing most of the good folks on this forum either by intent or from inexperience. Either way we need to know which engine is in this car if you expect the more seasoned veterans here to try and help you. I'm not trying to be harsh but you gotta figure out some of this stuff if you expect much help.

Does it have spark plugs or can you take a photo or draw a picture of the engine, anything?

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