Ammeter Started Showing Charge At Cruising Speeds

[YO7_A66]

 My ammeter usually shows straight up while cruising. I was able to drive it a lot over the last weekend and on the last trip, I started it up and it was idling funny. I took off thinking it was just heat soaked and it seemed fine after I got moving.
 Then when I came to a stop, the ammeter was fluctuating back and fourth from straight up and the voltage gauge was bouncing back and fourth all over the guage, and the engine was running rough. It appears that it was not getting a good solid voltage output at idle in D with the brakes applied (at a stop light), and my ignition is not getting a full 12v making it idle rough.
 As soon as I pulled away from a stop, the voltage gauge went back to solid reading just under 14v, but the ammeter shows a slight charge now. Any cruising speed showed the ammeter in the charging position (maybe 1/8 - 3/16" to the right) up to 60mph. I was never like this before. Something is going out.
 I took my battery in and had it tested and it showed 1007cca (850/1020 battery rating), so it was in great shape. I took my Powermaster #7508 alternator (60a/99a) in to have it tested and they could not get a good test reading. They did not have any models in the computer for the Powermaster brand alternators. So they picked a 65a unit from 1974 and it would not complete a test.

 Since the voltage was steady from just off idle all the way up to 60mph (VR good), and the battery tested good, that leaves the alternator in question. Maybe the alternator is no longer charging at idle (950rpms). Maybe the ammeter is showing an issue with the alternator at cruising speeds (showing slight charge).

 What do you think?

 Thanks


 Note: I have to keep the in D idle rpms above 900rpms to keep a solid voltage reading. As soon as the rpms go down lower, then my voltage gauge starts to bounce. I am assuming that the alternator is not putting out enough amps at that low of rpms.

[ChallengerHK]

My first guess would be that the voltage regulator went south.

[dodj]

I would start by checking connections. Bulkhead connector, vr ground, valve cover connector, ammeter etc. Then if nothing obvious is found, swap out the vr.


It sounds like you have factory wiring. If originality is not a concern, get rid of the full flow ammeter and go to a volt meter.
 

[YO7_A66]

 My voltage regulator has never gone above the 14v reading at cruise where there is more amperage available.
 Last year, my VR went south and the voltage jumped up to just shy of 16v at idle. The high voltage was my hint. But this time, high voltage is not the issue. The issue is low voltage at idle "all of a sudden".

 Thanks

 EDIT:
 I have altered the original wiring for the higher amp alternator. I run the ammeter on purpose and will not bypass it. I do have the bulkhead bypass wires installed. My VR has an external ground wire coming off of each of the mounting bolts and going to different grounds in the engine compartment.

 Thanks

[mopar jack]

I have had good luck on alternators that fluctuate by moving the the brushes. The field connectors that go to the alternator plug into the brush holder and it is easy to just move it a little. The brushes get a wear pattern in them and sometimes they will stick and not make a good contact. The brushes are fairly easy to remove and check.