Hi Art,

That's indeed some serious rust you have there... Why does all that stuff look so shiny?

I was going to have a closer look at my sender unit sometime soon, but your post got me so curious, I decided to do it now.

When I first inspected the top of the tank sender unit, I was greeted by a common site of dirt and rust, though the amount of rust wasn't too bad (1).
Only the return line was rather flaky, and it was what decided me to buy a new sender unit too (2).
At the time I didn't know the actual cause, so I'm glad I did.

A closer look at the old sender unit revealed there was no rust under the feedthrough (3).
But on the side of the feedthrough plastic cup there was some white stuff I could scrape off with a knife. Also on the bottom, the plastic was not nice and straight, it looked a bit brittle and broken, with that white stuff in the crevices (4).

I put a saw in the plastic and removed it to get access to the innards and the photo reveals the cause (5).
There's that white stuff again, and now I'm thinking that maybe it's some kind of corrosion product, but it's not green. I cut the wires right above the epoxy, and they all showed shiny copper strands. Also notice that the sealant has been pushed out of the pastic cup a little (6).

Here's what I think happened: the sealant and the plastic got separated over time and moisture could creep in and reach the tabs onto which the wires are soldered. It was just a matter of time until the pump connection corroded through as this seems to have gotten most of the moisture, leaving the fuel level sender signal and ground connection intact.

(1) http://i.imgur.com/hAgNa4W.jpg
(2) http://i.imgur.com/xczftlZ.jpg
(3) http://i.imgur.com/BNaUeFL.jpg
(4) http://i.imgur.com/ZDGuCM4.jpg
(5) http://i.imgur.com/5ykAP2x.jpg
(6) http://i.imgur.com/NhzvT0Y.jpg