I have a ford 851 with the 12volt system. I just replaced the amp guage because the old one was rusted out. Also the last several times I tried to start it, I had to put the charger on it for a while. I pulled the single wire from the alternator and with a 12V test light and the tractor running I got nothing from the alternator. My question is----does the RPMs need to be high for the alternator to put out amps ???. I've pulled it from the tractor so I could get it replaced, however maybe I'm missing something. Just after I replaced the amp guage I ran it up pretty good and the guage showed nothing. With it idleing, the amp guage would drop slinhtly when the lights were turned on. Any help would be great!!!