Converting a Vintage Tractor From 6v to 12v 

I had an old Chevy PU. I installed a 0-100 ammeter in series with the 135a alternator output lead. I installed a shunt that would cause the ammeter to read 100 when the alternator is putting out 135 amps (that would be 100% of the alternator's maximum output). That would tell me that the alternator is outputing current to the battery and load (lights, ignition and other electrical accessories). I also installed a voltmeter to measure the battery voltage to monitor the battery's state of charge. If the ammeter read 0 while driving, I'd know that the alternator failed and I'd be running on battery power. Yes, I could have bought a 150-0-150 ammeter but I used what I had on hand.