OK, curiousity got the best of me, so today I went into my garage and removed the sender from a spare tank I have. This tank was from an SV I bought used in 1975. The car sat unused from 1977 to 1988, including 7 years stored outside thru Cleveland winters. Since 1988 the tank has set hanging from the ceiling of my unattached and unheated garage. The sender came out of the tank easily, just one tap with a screwdrive to one of the locking tangs. What came out is a marvel. I showed it to my wife and she assumed it was NEW! It is made of 3 pieces of steel stamping (plus a separate cover plate), welded together, unlike the die casting in Jeff's photo. The yellow chromate (?) used to prevent corrosion has worked extemely well.
Inside is a bar that looks identical to the bar in Jeff's photo, but indeed it does have resistance wire wrapped around it. I am guessing that Jeff's sender has similar wire wound resistance wire wrapped around the bar in his unit. I measured the resistance and it varied from 13 ohms at one stop to 235 ohms at the other stop.
The most important thing I found was that the biggest contact problem is NOT where the wiper contacts the resistance wire, but in the ground contact. If I connect my ohmmeter from the connector - where the wire harness connects to the sender - and the long float arm, I get reliable and steady readings. But if I move the ohmmeter lead to the housing, where the system ground connection normally is made, no matter how solid and clean a connection I make to the housing, the readings are very erratic. This tells me that the problem is where and how the wiper makes contact to the housing. I saw a bit if gray powdery surface on both ends of the shaft of the wiper pivot and on the mating holes in the housing and cover. Not wanting to destroy the anti-corrosion coating on the steel housing I used a Q tip and some Radio Shack contact cleaner to clean the surface around the pivot holes on the housing. I think any cleaner might do as well. And then I used a piece of 600 grit sandpaper to thoroughly clean the brass pivot until it was shiny brass where it fits through the holes. If the wires and wiper themselves were gunked up or corroded, I would probably use contact cleaner on those surfaces.
After re-assembly the sender works perfectly.
Tom Hayden