To me, the whole issue of fuel gauges and how they work, other than the issues of the shape/length of the float arm, is the variable resistor in the metal box. THAT's where things happen electrically! No real dark science how it might operate as it's a simple variable resistor with a particular resistance range of operation.
The float arm is attached to a "wiper contract" which rubs the various exposed segments of the resistor to vary the voltage which the fuel gauge sees. The ONLY wear point (other than the float arm pivot). Same concept as the instrument panel dimmer in a headlight switch mechanism. Of course, special concerns/design elements due to the ambient atmosphere of the fuel tank! Same principles of operation and design on current model year vehicles, too.
With time, the wiper can degrade/wear and I suspect leave "a trail" of deposits on the resistor itself. Which I've seen happen even on some fuel tank modules of "still in warranty" model year vehicles.
"Rebuilding" would, I suspect, mean a new resistor mechanism (of a particular resistance range) onto the existing sending unit? Sourcing of such an item? That could be the tricky part, for a new replacement. Obviously, there is a dedicated supply network that not just anybody could patch into. Which then means "salvage yard" for many DIY-oriented people.
Obviously, the resistance values have a range of model years they would be common for, which might narrow the field of possible vehicles. KEY thing would be how many modern vehicles use that same resistance value. Which might generate a supply network of recent OEM replacement parts/gauge sending units. In some cases, I believe the gauge units could be purchased separately from the total fuel tank module? Would not really matter the OEM brand, just the resistanace values used. This could well be "the secret" that the rebuilders use?
Perhaps some members with greater knowledge of the resistance values needed might define the resistance values vs model years information? Or how to best clean the resistor contacts from possible erosion from the wiper arm's contact or from modern fuels?
I will concede that there are some things best-not-jacked-with and fuel tank sending units could well be one of them. Leaving the work to entities whose company insurance could cover any possible problems with their products, for example. On the other hand, swapping out the resistance assembly, replicating all existing electrical connections to at least OEM standards, with quality mechanical interfaces with the float arm amd any electrical connections, might be more doable than suspected. Which would make long-term-related quality control of the parts exchange operation an issue. Which could well venture into the capabilities/orientations of those doing the field replacements rather than purchasing a new unit from a trusted vendor.
Sharing information of what has worked/not worked and from which vendor, what part number, and when purchased for which vehicle could be beneficial?
Just some thoughts. PROCEED AT YOUR OWN RISK.
CBODY67
