One of the items on my "to do" list is to get the fuel gauge working. I've been doing lots of reading here on Mud, and I learned a lot - namely, that gauges and senders are not at all compatible across certain years. The earlier gauges (69-72ish) will not work with a later sender. Since I'd installed a later tank, I figured that I most likely had a later sender. Luckily my crusty original tank was still lying around so I pulled the sender out of it, thinking I could simply reinstall it in my newer tank.
Crusty old tank with older-style sender:
This is what the older-style sender looks like on the underside:
Anyway, long story short: the older senders will NOT fit in a newer tank without modification. The mounting holes are the same, but the hole on the later tanks has a thin metal lip that interferes with the collar on the underside of the sending unit. You can see the differences in the following photos:
Older tank:
Newer tank:
So, I now have the correct sender ready to install, but it won't fit into the tank. After mulling my options, I chose probably the stupidest route, which was to enlarge the mounting hole on the later tank with a half-round file. This actually worked pretty well! I put a magnet on the back side of the file to catch any filings that might otherwise have fallen into the tank. After a few minutes of work, I was able to fit the older-style sender into the hole. I installed it, hooked up the wires, and...the fuel gauge still didn't work - it went straight to the "full" mark.
So, I pulled the sender out and bench-tested the resistance, and it was nowhere near what is specified in the manual. It reads about 30 ohms across the spectrum, whereas it should range from 2.1 - 7 ohms. So, it looks like I went to a lot of trouble to come full circle to where I probably should have started: I'll likely need to reinstall the later sender and replace the fuel gauge in the cluster with a later-type gauge.
I do have one question, though: notwithstanding the out-of-spec resistance on the sender, I am wondering it anyone can confirm that there should be 12v at the yellow/red wire that runs from the gauge to the sender? I was not able to read any voltage on that wire.