Most of the kinds of problems you can have with a 122 gas gauge
I have had with mine and I was stumped for a while.
I have a wagon and had occasionally had water intrusion into the
spare tire well (of which the top of the tank is the floor).
This made a pool in the gas gauge well and got rust on the
wire and partially shorted the sender. Got that fixed.
The resistance wire from the sender came loose on one end.
I managed to take the thing apart and fix that.
Then I started to smell a lot of gas in the car when the tank was
full. About the same time it quit again. I replaced the screw
that you hook the wire to, re-soldered the other end of the resistance
wire to the new screw, got some plastic tubing, washers and gasoline
resistant permatex, and everything was fine for a while. Then it quit
again. I looked around for a new or used sender but everything
I checked seemed to indicate that the sender was OK, and there was power
to it also. But the gauge would not indicate. The old Army motto was
pretty slow coming into my head - RTCPMS!!*

Well, the trouble shooting guide said that if you disconnect the
wire on the back of the gauge that goes to the sender, then turn
on the ignition, it should indicate "Full". It did not.
So I got in there and disconnected the other wires (3 wires go to the
other terminal) noting that I had a hard time getting the nut off,
and that the plastic cap that is supposed to cover the end of the
screw had been melted. When I got it out, I noticed that the rubber
washer under the nut that holds the screw in place had been melted
also and the screw was loose. Also, one of the mounting screws connects
a ground strap from the gauge to the panel. The strap was corroded and
so was the screw. I managed to clean things up, replaced the melted
washers and tightened the "hot" screw (which goes through 3 connectors
plus one inside the gauge) so that it would make good internal contact,
scraped the ground strap shiny, wire brushed the mounting screws,
and connected a wire to the "hot" (+) screw. I touched this to the +
terminal of the battery and grounded the ground strap. Voilá! It
indicated full! (like having the sender wire disconnected)
Then I grounded the sender connection and the needle went to "Empty",
without passing "Go" or collecting $200. Put it back in the car and
it works like a charm.
This is just another example of the kinds of electrical problems common
on old Volvos and other European cars - they tend to develop nonconductive
corrosion films, sometimes completely transparent and invisible but
nonetheless allowing little or no electricity to pass. Finding where
the one is that is bugging you without scraping every electrical part
in the whole car is sometimes a challenge.

* RTCPMS = Read the Cotton Pickin' MANUAL, Stupid! --
George Downs, Bartlesville, Oklahoma