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Where do the ammeter feeds pick up their signals? To what/where, physically, do they connect? I've traced the yellow and red wires (654 and 655) from the cluster to the firewall. I see them come out of the firewall and into the harness, but then they are missing-in-action. Maybe they change color code along the way? By jumping the ammeter (cluster removed) to the a) battery and b) the hot side of the starter relay I can get action on the gauge when cranking, but no significant movement otherwise.


So where, originally, do these wires originate?
 

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This is a shunt type ammeter. I've had my 69 since new and I don't remember if it ever worked. The positive battery cable connects to the front starter solenoid post. One end of wire 38 connects to that starter post, and splices to three other wires. Two of them are 655 and 38A. Lets follow 38A. It goes to another splice with three other wires. Two of them are 38 and 654. 38 goes to the regulator field terminal. Wires 654 and 655 go through the firewall plug to the ammeter.
The thing works by shunting a small amount of current around wire 38A and that's what runs through the ammeter.
 

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The ammeter is series connected in a parallel shunt with one wire splicing into the alternator output wire at the harness plug, and the other wire connecting to the power distribution at the starter solenoid front, large lug. The wire gauge and length determines the amount of current to be shunted.

The Ford ammeters are notorious for not doing a heck of a lot.... especially after many years of use. A thorough cleaning sometimes brings them back to life. Rocketman can also convert your ammeter to a voltmeter while retaining its stock appearance to give a little more "insight" into charging system performance.

https://www.rccinnovations.com/index.php?show=menu-volt-all
 
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