The 1980 - 86 Ford trucks with full gauges had an ammeter that was intended to show the current into or out of the battery. In other words, whether the alternator was working, as determined by whether there was current being pulled from the battery or pushed into it.
Here's how the ammeter is wired into the system:
Here's how the ammeter is wired into the system:
As you can see, the ammeter has the battery on one side of it and the alternator and load on the other. So, if the alternator is charging the battery the ammeter should show a positive flow, and if the load is greater than the output of the alternator then the ammeter will show a negative flow. However, in my experience the ammeter isn't effective as the deviation of the needle usually isn't enough to get my attention. In fact, in one case the alternator quit but I didn't notice it on the ammeter. Ultimately I realized the headlights were getting dim, and then the ignition started failing.
Further, the ammeter/shunt combo is only set up to handle about 60 - 70 amps, max. But, if you upgrade to a 3G alternator you could easily be throwing twice that much current, which would probably fry the shunt. Yes, you could replace the shunt with one having half the resistance, which would double the capacity of the ammeter, but it would also cut the indication by half as well. So, an already ineffective gauge would be even less effective.
The solution, in my opinion, is to have Rocketman convert your ammeter to a voltmeter. Here's a link to a good how-to on modifying the wiring on these trucks to accept the voltmeter, although there are generic instructions on Rocketman's web site as well.
For documentation purposes, in case someone is wanting to devise a shunt to handle the higher current of a 3G alternator, here are the readings I got from measuring the voltage and current it took to swing four different ammeters, by themselves and not including the shunt, to full scale in each direction. As you can see, they are none to consistent nor, therefore, accurate. But, they are sensitive little rascals.
