1984 300D Fuel Sender Repair

[BillGrissom]

I read several older posts about cleaning the fuel sender, so got the bright idea to remove it in my 1984 while perfecting the car for sale (fleet too large, don't need two 300D). I recall the gage always showed 3/4 when full, but mix up with my 1985. Refer to earlier posts for removal and disassembly. I used a 45 mm socket & short extension (HF Pittsburg 3/4" drive) to unscrew from tank (below med kit tray, rear package shelf). Small needle-nose pliers (or snap-ring pliers) easily unscrews the bottom round nut (2 recessed slots in nylon holder) to remove the outer aluminum tube (strangely a problem for some posters).

There was some black gunk inside on the lower plastic disk but otherwise fairly clean inside. I carefully cleaned the Nichrome wires (2) and copper wire (1) with a toothbrush and ethanol. Might have messed it up since after re-installing, the gage needle jumped around when driving after a full tank. Don't recall that before. So went back in and found 2 problems.

The jumpy needle was because the copper pickup finger on one side of the float which touches the NiCr wire was on the wrong side of the wire, to no longer clamp it. I might have knocked it loose with the toothbrush. That finger is sort-of spring-loaded, so I just pulled the wire out slightly and pushed the finger under to the correct side (V side should touch wire, photo). It then read smoothly as the float moved. Should be 1.6 ohm full to 56.9 ohm empty between pins "T" to "G" and mine was close (subtract ohm reading with DMM leads together).

A bigger problem was that with float bottomed-out, should get a connection between pins "W" and "G", but nothing. That turns on the dash low-fuel warning lamp. I first cleaned black coating off the copper plate on bottom of float which touches the two sharp pins (silver coated?) at bottom plastic spider. One pin should connect to the center rod (to "G" terminal), but no connection. A bit hard to probe since touching copper parts required digging in the probe to get thru surface coating (dull copper). Eventually, I found the copper bar run to the pin was cracked at the 90 deg turn (bottom of photo). I fixed by "simply" soldering tinned-copper mesh (solder sucker) between that pin and square brass plate which presses against center rod. Harder than it appears with my fat fingers and basic iron and not purty but worked and gage reads just slight of "F" now after driving 50 miles after a fill-up.

Don't know if I'll drive that car thru a tank before it sells to totally verify, but "should work". Last time people reported buying a new sender here (ca 2008), they stated $200, so worth your time to repair. Seems the same basic fuel sender was used in other Euro cars like BMW. The float is stamped "VDO", a company which makes many automotive gages and sensors.