The way Ford used this ammeter made it virtually useless as a diagnostic tool.  However, it also wouldn't be capable of starting a fire if alternator had an overload, or stop charging if the ammeter circuit opened.
With that being said, from my experience on several Ford vehicles of many years, if everything is 'normal', the gauge rarely ever noticeably moved.  As you indicated that you had a 'prolonged start', your battery may be getting near to needing to be replaced, just a guess.
Note that the ammeter is not connected to the CVU that the other gauges receive their power from, and will have no impact on one another.

The stock amp meter in the real old cars...Model A's...carried all the current from or to the battery so it really showed you something.  But back then 20 amps was about all the system needed.
On the 60's stuff Ford went to a "shunt" type ammeter.  This used a 16ga wire from the generator/alternator to and through the ammeter, then back to the battery side of the solenoid.  There was also a short piece of 10ga directly from the alternator to the solenoid/battery.  So the higher resistance "shunt" wiring through the ammeter is in parallel with the short 10ga and only carries a low representative  percentage of the current the system was sending to, or drawing from, the battery.  After 50 years of corrosion, crappy terminals and higher resistance, the shunt system shows less and less of whats happening.  Also it still is providing a solid, unprotected wire directly from the battery to the rest of the system...NOT GOOD. 
Butt (TS&T),  that's how and why it works.

If I were you, Gary, I'd make sure the connections from the alternator to the solenoid are clean and solid.  You DO NOT want that ammeter system carrying any load.

Last edited by Bullet Bob (6/06/2021 4:32 PM)

MS wrote:

Voltmeter is much better indicator of charging system function, to me. Before changing to voltmeter, headlight brightness was about the only indicator of what was going on.

I removed a few feet of wire from the yellow wire coming from the alternator and it made a world of difference in the magnitude of the ammeter readings.  Just sayin'

Last edited by John Ha (6/07/2021 12:04 PM)

I think I found the problem. The stud on the hot side of the starter solenoid where the battery/alternator (along with other stuff) attach was only snug allowing movement of the cables. Upon dismantling I found the threads on the copper stud partially stripped. Tightening the nut as much as I dare has turned the ammeter back to its lazy self. 
Need a new solenoid. Now to find a good one.

Which one?

Last edited by RV6 (6/07/2021 3:19 PM)

Some have slightly shorter studs.
Be aware of that.

The Standard Blue Streak line is a quality product.