i have a puzzle that i cannot figure out.  maybe because i dont know how to troubleshoot my fuel gauges.  i have a 1972 cessna 172L, the fuel gauges are paired with either the oil pressure gauge or oil temp gauge.

i recently had an over voltage which blew up the regulator and over voltage sensor, thus frying the transponder and navcom and assoicated CDI.  this occurred after a garmin 420 install.  i dont think the 420 install is the cause its, since the mechanics believe the alternator shielding wire <dont know what this is> broke and spiked the system. all has been fixed. my crazy luck.

when the master is turned on, the left fuel gauge works and goes to half tanks, the right gauge stays on Empty until i repeatadly tap on the face of the gauge and then jumps past Full <pegged>.  my memory is blank on when this problem started, i think after the voltage spike. morecrazy luck.  but i have a hard time thinking the spike would damage the gauge since its on a breaker. when the master is turned off, both gauges slowly go back to Empty.

so my puzzle is, how to i test the gauge before i have someone pull it and send it out for overhaul? 

how would i know if its the sending unit?  first i would have to find the sending unit. 

is the OHM range 30 - 240 for these gauges?  30 being full and around 100 being half tanksish?

on the back of the gauges, there are two wires.  i suppose one is to power and the other is to the sender unit.

thanks

kyle