2 1974 Plymouth Satellites.Neither gas gauge works.Getting a pulse with my test light at fuel gauge.Checked the ground at the tank.It is good.Is the wire going to the gas tank a ground wire which varies the resistence thru the sending unit or am I wrong here.Any ideas would be appreciated on how to troubleshoot this problem.

I believe that wire to the sending unit is like a 5 volt reference. If you hook a test light up to it, you should see that same 'pulsing' at the gauge. The sending unit changes the resistance depending on the position of the float arm.

If you disconnect it from the sending unit, take a jumper wire and ground it...the gauge should go right to full. If it does, then the sending unit is most likely at fault. If the gauge doesn't move the problem lies from that connector forward.

I like to remove the sending unit and with that wire hooked up and the sending unit grounded with a jumper wire, physically move the float arm up and down and see if the gauge reacts.

If your gonna ground the wire to the sender do it with the ignition turned off, then briefly turn the key on & watch the gauge... As the needle swing toward full turn the key back off cause grounding the lead is a dead short & will smoke the gauge if left that way to long... If you want too go to Radio Shack & grab a few resistors then you can do a better job of testing.. 10 ohms is full & 75 ohms should read empty...

i don't know how many times i have had a non-working gas gauge on these cars,checked and had voltage at the sender,grounded and went to full,then pulled the sender to find the float gone!!!! and i don't mean off of the arm,and rolling around in the tank,i mean vanished like a fart in a whirlwind. if the float is missing,and you don't want to wait to get a replacement shipped from somewhere, go to your local ford dealership's parts department,and ask for a float for late 60's-early 70's mustang. for some stupid reason,they keep them on hand,or have them the next day. they are the identical float that chrysler used,and run about $3.00.

Later Ford vans used the same size float.They arent brass but work perfect.I had a few I pulled when I changed the fuel pumps on injected Ford vans.On early Mopars I either find the floats gone like said or filled with gas.Rocky

If they are standard and not the rallye guages it is often the circuit board under the nuts that hold the gauge in place are corroded.

A sure sign is if you ground the sender and get nothing but rubbing the stud/nuts with it grounded gets the needle to rise.

If that's your problem. Clean your nuts & circuit board with some fine sandpaper. While your in there do the whole instrument cluster. It should solve a load of problems.