98 C280 gas gage erratic, now what?
 

Hello all... the wife's gas gage is on the fritz. '98 C280, 140kmiles. Key on, engine on or off, the gas gage needle pegs to the left against the center of the temp gage. At the same time, the low fuel light comes on. Sometimes the gage needle will wander erratically, but the low fuel light stays mostly on. Or turns off, no correlation to needle behavior.

Gage started this behavior after I disconnected the battery overnight. Coincidence or ****to luck?

Did some searching on this forum and found that the tank is a saddle type with siphon between two sides, pump and filler on right. There are two senders in series, perhaps one or both are bad. And they're not cheap.

How do I access the senders, or their connection to ohm-test them? I removed the trunk floor panel between the rear wheels, and see three two-wire connectors mounted in the plastic that sits on body metal. Which to of these three are to the senders, and what should the sender ohms reading be if the tank is full or nearly so.

Wife will have to get along with the tripmeter rather than gas gage in the meantime.

All suggestions welcomed; this forum seems a wealth of info.

Yes I've heard of this before.

It happened to me when my car was new (1998 C230) and it fixed itself after going through a few tanks of gas.

I have a Haynes manual, when I get a chance tonight I'll review the sender unit section and post if no one responds.

I think the original senders were bad. The original owner of mine said it got changed under warranty. I don't know if his symptom was the same though. Maybe it was just inaccurate.

Some excerpts from the haynes manual that is euro spec but it helps for US spec cars

Disconnect battery negative terminal

There are two fuel gauge sender units, each one located at the lower front of each half of the fuel tank.

Unbolt and remove the covers from the bottom front of each half yank.

Sender units have drain plugs (tanks must be near empty) and drain any remaining fuel.

Sender unit removal, The retaining rings must now be loosened and removed.
MB techs use a special tool which engages the holes in the ring, possible to fabricate a home-made version.

Removal tips: recover special shaped gaskets, Note the location of stud and grove to ensure correct refitting. When removing left hand sender unit, disconnect the internal pipes leading to each half of tank.

My dealer changed the sender unit under warranty. They waited for a while before changing it out, first saying they were waiting for an updated sender unit. This, of course, was years ago, when the car was relatively new. When the "updated" unit was installed....same problem. Somehow, though, the problem disappeared on it's own over a period of time. No empirical evidence, but I think it fixed itself when I switched to Shell gas. Again, I have no reasonable idea why this would eliminate the problem. I use all name brands (but mostly the major brands) of fuel, Shell happened to be the station that was convenient at the time.