[stackj]

Bob,

In my case, it was the alternator switch (a double rocker SPST switch). I was able to swap the wires to the unused halves of the switch, turn the old switch upside-down and eliminate the problem. Dirty contacts in any of the four switches between the field circuit breaker and the alternator can cause the problem. The switches most often "switched under load" would be the Alternator (sometimes labeled GEN) switches.

I would have replaced the switch, but was unable to locate a replacement.

The suggestions about looking through the alternator field circuit and all of its components is a good one. Most likely you have something through that area which is causing the fluctuation.

This won't pertain directly to your 73 P model, but if you look on the technical data page of this website, near the bottom, you will find a file entitled "Electrical System Schematic". This is based on the 38 amp Ford alternator system in my 67 337B, but it is laid out as a field current flow diagram and may help you understand your system a little better.

Unfortunately, I am out of town right now, so can't reference my service manual to see how similar the two systems are. I think they are pretty different, but the diagram may be helpful.

Good luck!