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fixit-p · 02-09-2009, 09:34 PM

I just installed a 12 si alt. in my 69 and did some reserch on the ammeter. The red wire going from the batt to the alt junction is the ammeter shunt that is it is a fixed length and fixed gauge so therfore it has a fixed resistance and is calibrated with the ammeter. The external shunt type ammeter measures the voltage drop from one end of the shunt and the other (power on both sides) if the voltage on the alt side of the shunt is higher than the batt side the ammeter shows charge if the voltage at the batt is higher than the alt side it shows a discharge. If your ammeter is reading a slight charge after startup then drops to the center its working if it shows charge all the time than you would be over charging your batt. One mistake I made when I installed relays for my headlights was that I installed 8awg wire from the alt to the batt in doing so I changed the resistance of my shunt and my ammeter was off. someone mentioned the voltmeter from a 73-87 if it fits that would be a great upgrade.

02-09-2009, 09:34 PM	#19
fixit-p Registered User Join Date: Dec 2008 Location: Yay Area CA Posts: 2,329	Re: Battery Gauge Question I just installed a 12 si alt. in my 69 and did some reserch on the ammeter. The red wire going from the batt to the alt junction is the ammeter shunt that is it is a fixed length and fixed gauge so therfore it has a fixed resistance and is calibrated with the ammeter. The external shunt type ammeter measures the voltage drop from one end of the shunt and the other (power on both sides) if the voltage on the alt side of the shunt is higher than the batt side the ammeter shows charge if the voltage at the batt is higher than the alt side it shows a discharge. If your ammeter is reading a slight charge after startup then drops to the center its working if it shows charge all the time than you would be over charging your batt. One mistake I made when I installed relays for my headlights was that I installed 8awg wire from the alt to the batt in doing so I changed the resistance of my shunt and my ammeter was off. someone mentioned the voltmeter from a 73-87 if it fits that would be a great upgrade.