The only replacement sending unit I have found so far was for the seat back tank, I also have the main under the bed tank. Since neither has ever read anything close to correct for being off empty and the seat back tank is easy to get to, I ordered one from Dennis Carpenter parts, C1TZ-9275-K. I checked the ohm reading on the unit I received and read 10 ohms full and about 75 ohms empty, really close to the part specifications of Full: 9-11 ohms Empty: 70-75 ohms. which I read should be what was needed for my vintage of truck.

I hooked the new sender to the wiring before doing an installation and moved the lever; empty looked good, but full only caused the gauge to right at the 1/2 full, Lowering the lever, it did appear to start registering change somewhere around 1/2 empty from full, until them it just read the same. Even that could be a win, if I knew I had a least 1/2 a tank and it started reading accurately below that, it would be workable for my needs. I had checked the fuel gauge out by grounding out the center wire for the sending unit and that causes the fuel gauge to read full. so why doesn't the sending unit that meets the spec's do the same.

Either the spec is really 0-75 ohms, or there could be some resistance in the circuit. If the spec is 10 ohms for full, even if grounded out, should there be call it 10 ohms of resistance in the wiring, the gauge would read full. Add in the sending unit with it's minimum of 10 ohms and now it would be at maybe 20 ohms and maybe you get to 3/4 of a tank. That doesn't seem to make sense for reading just a half a tank, regadrless I pulled the ground wire for the sending unit and sanded the ground connection pieces to be bright, the lug(s) were oxidized. I think that helped a little but not a lot.

Having read in another thread that the sending unit works off of pulsed instrument voltage, I bought a test light. The thread indicated the light should go flash.flash.flash, not flash...flash...flash. Mine actually seemed a little variable, sometimes faster, sometimes a little slower. I pulled connector to the voltage regulator by the radiator (which is where I think the instrument voltage comes from) which looked like it could benefit from some clean up, so I did. Plugged it back in, started the truck and checked the regular output for the battery, 14.2V, that's good, doesn't tell me about the instrument voltage but at least I seemingly didn't kill anything. The sending unit still didn't go to full but after the ground and regulator connection cleaning it did seem to improve a little but not a lot. That is as far as I've gotten, it' livable but it would be nice to know why it won't go to full. The sending unit was incredibly simple to change, the only minor thing was the ground wire tab on the new unit slightly is less think than the original, so I had to tighten the spade lug opening for the connection (just a little).

 

Your "IVR" is on the back of your instrument cluster
The pulsating voltage is slow IMO
Flashes about once a second IIRR

 

Thanks for the clarification on the IVR location manic.. My pulses seemed variable; faster was close to 1/2 second, slower close to a second. The change in rate didn't seem to make any difference in the gauge reading, so for at least the present I wrote it off as normal.