What have i done to my fuel gauge? My car i believe is a 300 DSL which for some reason the PO changed the instrument cluster on.

He put in one from a gasoline engine (a 280 SE i think) and apart from the tachmeter not working, the internal lights underwent the mother of all shade tree mechaine repairs. Disaster.

They stopped working so i pulled it out to see the mess. But now i have a mess.

I went to the junkyard and got a really clean instrument panel, also from a gasoline car (no glow plug window and a red line on the tacho). I connected it and found that the fuel gauge didn't even move BUT the low level fuel warning light did stay on (!).

(Previously, the low level fuel light didn't come on and the fuel guage would wonder between 1/3 and 2/3 and jump back to one third if i hit the top of the dash)

Hmm...i thought. Let's put back my old unit and see. Weirdly now my old unit is bahvin strangly. The fuel gauge now jumps right across to full and even after taking the key out stays there for about three minutes before coming back.

What have i done?

I checked the pins on the back of the plug and scrubbed them with a SS brush, as i thought perhaps this was a poor earth.

Are gasoline w126 instrument panels compatible with diesels? That might be a good question to start with, straight off the bat.

And can i improve the earth of the instrument panel itself? Where could i connect a ground wire to it?

thanks a lot for all advice

MJ in Morocco

UPDATE: I've also noticed that the clock doesn't work as well, so im certain there's a ground/earth issue.

UPDATE: According to this site https://en.vindecoder.pl/WDB12603612025321
the car started life as a 500 SE, which might explain the choke on the dash just tucked away on the left side of the steering column. HOWEVER

According to this VIN site, it's a 300 SD https://uk.vin-info.com/order-reports/WDBCB20C3FA122852

Who do you go with?