That the gauges change readings when you turn on the lights directly indicates a ground path failure .

Remove the gauge binnacle and take the time to ascertain the ground paths, at least one will have a screw or other fastener in it, use this to add a brown wire from the binnacle proper to a good ground point on the body .

Once you've done this and have the binnacle re installed, remove the tank sender using s 46MM socket of large pair of Water Pump Pliers, turn the key on , re connect the sender and invert it whilst you watch the dash gauge .

It should go from empty to full, kif not you'll likely have to carefully and gently take the fuel sender apart and clean it well using soft brushes like paint brushes ~ it's very easy to break the two delicate fuel sender wires , I am sure there's plenty of threads concerning sender cleaning ~ I've saved almost every one I took apart .

Keep us posted and try to remember : you're not a Mechanic so it may take you a bit more time and effort but you will get there .

I can dig your car being a "B.I.T.S.A", I used to build them, and Motos too, you're cleaning up some one else's lazy / sloppy job to don't let anyone rile you .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

Hi
Thanks for that. Really appreciated. So, are you saying that the earthing problem is on the circuit board itself, or in the 15 pin socket? i wasn't entirely sure by your message.

I have already checked the 8 pin connector on the back of the middle section and found the earth. I checked it had continuity with the sterring column so now im searching for other bad earths and think maybe its the circuit board itself.

The really weird thing is this has happened while/during I removed it (which is particulary soul destroying). Im wondering whether the earth connector point, which a screw goes through, on the circuit board itself which connects to the metal strip is poor or needs to be tightened.

Is one of those 15 pins an earth? Or perhaps more importantly, is it possible that one of them is being used erroniously as an earth?

The other weird thing is if i connect the screw hole connector on the board to an earth, then the gauges go mental. Why would they do that? Surely that would make everything cool, no?

thanks for your patiance and input.

MJ

Yes, one of the pins is supposed to be earth .

Yes, circuit boards often get "opens" in the circuitry making repairs necessary .

This is where taking some time to trace the ground paths with a TEST LIGHT (never a meter because it often gives false positives) and battery to discern exactly where they go, they meander all over the place .

FWIW, if you ever find the number 31 that's a direct to ground circuit and DIN codes say it should be BROWN .

I have no idea what the junkyard situation in Beruit is but I have old Chevrolet and GMC buddies who are constantly sending me junkyard images of old trucks so I imagine there are Mercedes out there somewhere .

Even the most mangled ones will usually surrender good harness bits, bulb sockets, proper hardware, on and on.....

It's nice to be rich and just cut checks at the Mercedes dealership but most of us still here now are average working Joe's who have to account for every penny yet still want to drive a nice old 'Benz .

Most German gauges are designed to work on 12 volts directly but in general it's bad practice to apply straight 12 VDC to a gauge head or sender .

Do the rest and every time you find a contact, polish it using metal polish or cleaner typ wax, never use sandpaper, wire brushes, sharp edges and so on ~ electrical contacts are delicate and only pass minescule amounts of current so they're easy to damage .

Take your time here .

What you learn now will be very useful when (if) you go back to Old Blighty and decide to up fix a Morris Minor like your Mum had....

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better