Against all logic and good advice I doggedly proceeded to install a full instrument guage panel in my '85 F150 XL 5.0 EFI that originally had idiot lights and no tach.

After mapping the circuit boards on both panels, everything seemed to be a 1:1 match up except for, of course, the ammeter. My idiot light had what I believe is called a flex circuit connecting the posts on the light.

The experts on this board reffered several times to a "shunt" that runs the ammeter located near the alternator. The problem is I have no idea what the shunt looks like and every parts person I talked to gave me a blank stare. In an inspired flash I connected the flex circuit from the old panel to the posts on the ammeter and connected the old wires.

The everything seemed to work perfectly and I broke my arm patting myself on the back. The next day the ammeter stopped working and the day after that the truck stopped working.

The battery is new, I pulled the alternator and had it tested and it was ok. I replaced the fender mounted voltage regulator and everything seemed to be working again.

Later that day, the ammeter stopped working again. So I went and exchanged my voltage regulator in case it was going out again, removed the flex circuit and disconnected the wires to the ammeter.

Am I barking up the right tree with my diagnois? Anybody have a suggestion on where to find a "shunt"? My forum search turned up some confusing statements concerning the battery won't charge if the ammeter is not hooked up, is this correct?

 

The battery may not charge, because the idiot light told the regulator to bring the alternator "online" when the key is turned to run. Since you are messing with the idiot light, you are messing with the regulator wiring.

The only place you are probably going to find a shunt is in the junkyard. It is located in some of alternator wiring. And the idiot light alternator wiring is totally different from a ammeter wired alternator. Here are the two basic circuits. The top diagram is the ammeter wired alternator.

The shunt is not shown in the diagram, but it will be located somewhere in the large output bat wire, and will be a strange looking splice with two little wires coming off either side. The two little wires run all the way into the cab and hook to the factory ammeter gauge. The factory ammeter gauge is really a very sensitive voltmeter. The shunt is really just a very very small resistor. When current flows through the shunt, the resistance causes a small voltage to be generated, which the ammeter picks up and gives a reading.

 

Thank you! Back to the junk yard!