fuel-level sensor clean - W201
This has to be about the easiest repair on this car, since I did it wih one hand tied behind my back. Well, sprained, anyway. Symptom was erratic gauge reading, but only when the level got to about 1/4 tank. When the level is low enough, the erratic symptom goes away again.
Access was by just removing the back carpeting in the trunk removing the pop-out clips. The sensor connector unplugged, and it was recommended to remove the battery ground (I didn't). The retaining nut unscrewed with a large-opening channelocks, and the sensor lifted right out - pausing partway to drain. The tank was almost empty, so there was little smell and no dripping.
With the sensor removed, I cound see a split-slot nut on the bottom of a shaft prodruding from the center of the bottom cap. It was covered with some sealant that was crumbling off. Inspection showed it should be ok to remove this nut, and it was. I used C-clip removal pliers, but any small, narrow-tip long-nosed pliers should do. The tube with end-cap then slips right off.
Inside there is a bottom piece bolted on the shaft, with electrical contacts and support for two very thin wires running from top to bottom - 180 degrees apart. A float slides up and down on the central shaft, and electrical contacts on it short the two wires together. One wire is a zero resistance, and the other is resistive - about 75 ohms end-to-end. A copper plate on the float bottom shorts two contacts on the bottom piece that connect to the no-resustance wire and the central shaft. The top end of the shaft is the 'reserve' trigger output.
I didn't find any visible problem with the components, but I thoroughly cleaned all with q-tips and rubbing alcohol. I reassembled, dabbed the bottom nut with Loctite 290, cleaned the o-rings and mating surfaces on the tank and securing nut, and reattached. Hopefully, gasoline is not a solvent for the 290, but I don't think the nut will come off anyway. I'll know if this worked once I use up the new tank of gas.