In my 1982 300 TDT the sender has resistance wire looped around a
shaft. When the tank is full, a float raises a contact up to the top
of the shaft, so the electricity has to go through all the looped
resistance wire. When the tank heads towards empty, near the bottom
there are two contacts that come together. This turns on the "Low
Fuel" light. As the float goes down with the fuel level, less and
less of the resistance wire is in the circuit because the contact on
the resistance wire slides down the shaft as the fuel level drops.
On my sender unit the resistance wire wore through, and the sender
unit quit working at about 325,000 miles and 25 years on the car.
That resistance wire is really hard to find, and the original stuff is
in metric units, which is not the way wire like that is stocked here
in the U.S. If you take an ohmmeter and a length of the wire, you can
estimate the resistance per millimeter. That is the property of the
wire that's important, and that is how it is sold.
The problem I ran into was that nobody seems to sell resistance wire
retail in the U.S. anymore, even on line. They want you to buy it in
100 meter rolls, which costs more than at new sender. I finally hit
upon a good guy at one of the wire places who just sent me 4 meters of
wire that was pretty close to what the original was for free because
his computer system was not set up to charge for less than 100 meters
I rewound the sender and put it in the tank and it works pretty good.
I had to slightly compromise on the resistance per millimeter because
the wire the guy sold was in Imperial units, so it was close, but not
exactly the same. With the original, when full, the needle on the
fuel gauge would go a little above the "full" mark on the gauge. With
the rebuilt sensor, the needle goes exactly to the "full" mark when
the tank is full.
The "Low Fuel" light, which does not depend on the resistance wire,
works exactly like it did before.