I just did my '91 Fuel sending unit. It's a potentiometer style and a crappily designed one at that.

You have the basic cylinder-shaped housing that is split into two pieces. BE CAREFUL separating it. Inside the cylinder are two rods, wich are just fit into two holes on the bottom of the cylinder and come out to be two terminals on the top of the cylinder with wires soldered to them, with ceramic coatings with wire windings on the ceramic, spaced from close to farther apart from top to bottom. Ok there's the potentiometer part.

The retarded part is that there are two rods and not one and the design of the float, but I digress....

The chewing tobacco can-shaped float has an oval space in the middle into which the two rods fit in and the float rides up and down upon. There is a little clip which holds the chinsy little contact set up. The contact piece is basicaly a rectangular back piece with 2 thin arms extending from either side with almost spalula end-shaped rectangular contacts on the end of them. The little thin arms actually cross over one another in an "X" and the contact pads ride on the outside edge of each rod. If you were looking at the float with contacts in and rods in place, with the contacts facing upwards (it's mounted contact clip side-down when actually in the cylinder), the contact pads would ride at approximately the 4o'clock and 7o'clock on the rods. Theoretically, it's a good idea, but put it into practice for a hundred or so thousand and you begin to get real world problems. First off, the system relies on the fact that the spring tension in the little contact clippy thing remains constant so as to ride the windings properly. Second, the contact pads wear and actually develop grooves where they ride on the rods, affecting the conductivity. Third, we're dealing with a contact piece that, when jostled just right, can have a contact arm/pad slip from the outside of the rod to the inside, thus creating a zero reading on the guage--and this usually happens when the pump is changed, just due to the twising and jiggling needed to get the pump out---which is what happened to my unit from the awesome "mechanics" who did the work when the previous owner had the pump replaced. Fourth, it's just not designed to be serviced.

When taking the two halves of the cylinder apart, there are three tabs, roughly one long tab at 12o'clock and two tabs half the first one's length at 4 and 7 o'clock. It's very difficult to get those separated without somewhat bending each half a bit. When the bending occurs, the little rods inside bend, and now the float won't "float" the entire length or will get jammed up at some point. I know this from doing it. I was an ass and now hope to save you from the same fate. Once the rods are bent, it's a painful process to get them straight, but I managed to (at least so I thought). I guess my advice would be to actually cut off the outer clip-stop on the lower (yeah, I think it's the lower half, the longer top half fits into) part so as to more easily separate the two smaller clips on the opposite sides. This way you can use a bit of compression on the small clips and not have to worry about the big clip then becoming more "clipped in". It's just plastic and it's brittle, at least mine was at 162K miles. When you re-assemble just put a dab of expoxy on the area bit that you cut, bonding both halves together.

Don't unsolder the wires from the terminals, there's enough slack to cut the wires an inch or so up and then use butt connectors to re-attach to the sending unit.

I would suggest using the "String & Wedge" Technique used in this post: http://www.brickboard.com/FAQ/700-900/FuelTankSenderReplacement.htm
I didn't use it and it may be to blame for the fact that my guage reads just under 7/8 of a tank when full....I don't know....but the guage drops normally, making me think it's gettings topped about 3/4 of the way up the cylinder. But hey, I'M NOT DOING IT AGAIN!!!!!! and I don't have that horrible orange/red light glaring at me anymore, so, I can live with it. Heck, I lived with my '84 245GL not having a working guage for 10 years.

Good luck, and I doubt it's "crud" that's causing your guage to not read correctly. My advice would be to go to the local pick and pull and get a used sending unit (who cares what the pump and actual assembly looks like), and test the sender with a volt-ohm-meter to see if it registered full continuity when the float slides to the top, and next to zero or no continuity, when the float is at the bottom. If it does, then unclip it from it's assembly, and use it on your existing one. You won't get the gradual reading with your meter that you see on the fuel guage because there is (I think) a 68ohm resistor used in the car, coupled with whatever capacitors if any that are on the back of the cluster. I read an article somewhere on testing this unit and it involved this resistor and just too much work to just test the darned thing when the "all or nothing" readings you can get from a regular $5 radio shack meter will work just fine as well.

Once you've got your donor unit in, if you choose to go that way, then rebuild your own and save yourself the time and the immense aggravation in repairing the thing. And, if you live in New England like me, all the time cramped up in the trunk in 30 degree weather.

On the sending unit seal, I would follow the installation tips in that same FAQ article I mentioned above. I found I prefer using PB blaster to lube the seal instead of Vaseline just due to the fact that the PB Blaster really helps to revitalize the seal much better, so you don't have to get a new one, despite the fact that the seal fins may be a tiny bit cracked or ripped.d

I also used Permatex "Indian Head Gasket Shellac Compound" when re-assembling the Banjo fitting (which my '91 has...). A thin, thin coat on ALL sides of all sealing surfaces will help you to avoid the, "oh, crap, the banjo fitting's leaking" problem like we've all experienced if we've ever done a 240 fuel pump and/or filter replacement on the side-by-side undercarraige setups. I also used the Shellac to coat the part of the sending unit where the wires go into it, as mine was not in good shape. I also used it on the butt connectors...it seems to seal well, and does not affect electrical conductivity.

That's my 18 cents worth.