Just a little background. This is a 76 F100. The battery is brand new and tested good, holds a charge no problem. I've had the alternator and voltage regulator tested, both new, and they both checked out good. The truck has an ammeter gauge, no idea if it works right, but as I understand it, it has a shunt to keep the alternator charging Even if the ammeter fails.

I've read that the shunt can either be on the rear of the instrument cluster, or inline with the alt charge wiring. I don't see it inline with the alt charge wiring, but there is a little box on the back of the instrument cluster. Where's the best place to start here? Replacing the shunt, or possibly all the wiring going to the alternator? Are both the other 2 wires supposed to have 12v key on power? Or only once the engine starts running and turning the alt? Not sure where to go from here, and I don't wanna just throw parts at it. Would rather not go GM one wire alt either...

 

Well, I just found that if I jumper the orange wire on the back of the alternator straight to a key on power source with the engine running, then it starts charging...What exactly does this mean?

 

"Little box" on the back of the instrument cluster (attaches to the printed circuit board) is the ICVR = Instrument Cluster Voltage Regulator

Affects the fuel and temp gauges and the oil pressure gauge (if present). Has nothing to do with the amp gauge.

With a fully charged battery and working alternator, the needle of the amp gauge hardly moves past center. Some people use a magnifying glass to see its position.

Turn off the engine, turn on the headlamps, wait 5 minutes, then restart the engine. Amp gauge needle should move a bit more past center.

 

Check the wiring from the voltage regulator to the alternator for a loose connector, cut or break. Also check the alternator output wire from the alternator to the starter solenoid Pos. battery cable terminal.

The ammeter shunt will not affect the output alternator.

 

I got her figured out, and it's now charging at 13.65V on a new 75A Alt. The orange wire on the back of the alt was broken about halfway down the harness, a new length of 12ga wire solved the problem. That was actually the main reason that I got this truck for free..the PO couldn't figure that one out. Thanks for the tips and help guys.

 

13.6 is pretty low. Is that at idle? The battery state of charge, and temperature factors, but that's basically a float charge.

Measure the charge voltage directly at the battery posts with all the headlights and accessories on, heater blower etc, and around 1500 to 2000 RPM. This loads up the alternator and spools it up to a reasonable speed so it can put out some juice. 13.8 to 14.2 would be in the zone.

 

Any luck

 

fyi

The shunt actually "shunts off" most of the current to the gauge. It couldn't handle full current. Most of them don't work anymore. I found a voltage readout on my radar detector give me all the info I need. 14v is close to the max you should see.

 

Yes, it did start charging at 14.2 after a few drives around the block. That was of course measured at the battery. All seems fine now, thanks.