civitar wrote:

I have a 66 Galaxie.  My fuel gauge always reads full.  I have a pulsating circuit at the sending unit wire when the wire is not connected. But the second I connect it to the sending unit the live circuit is not live anymore. I replaced the sending unit with an aftermarket one.  What gives?  Any ideas?

what do you mean by "pulsating circuit" ??  

This is the basic operation of a fuel or temp gage in a 60s era Ford:

12V is supplied from a switched source
12V gets cut down to 5V by the voltage regulator
regulator connects to the Gauge (which basically measures voltage)
gauge then connects to the sending unit (which basically restricts voltage)

A sending unit is nothing more than a variable resister, in the case of a temp gauge the hotter it gets the less resistance, in the case of a fuel sending unit the fuller the tank the less resistance.  These gauges are nothing more then voltage checkers, full or hot would be 5V.  Several things can cause a gauge to peg:  to much voltage, a short in the wire, or a bad sending unit.  The best way to solve an electrical issue is to test it in sections.  First make sure there is only 5v coming out of the regulator, if its more than 5 than you have probably found your issue.  Next turn the key to ACC and disconnect the wire at the fuel tank sending unit.  if the gauge reads anything other than empty than there is a short in the wire.  if all those things check out then the sending unit is probably to blame, remove it and hook a meter to to check ohms (one lead to the body of the unit and the other to the wire stud).  as you move the arm the ohm reading should change, if it does not the unit is bad.