I have a very random overcharging issue on an 86 D250. It will come and go with no pattern, jumps up to around 17v sometimes for 5 seconds sometimes for 30 seconds. What should I start checking.
This is a completly stock electrical system except for headlights ran off a relay.
Small alternator not the 100amp optional one.
Thanks

Most likely it's the regulator but it could also be the field wire intermittently touching ground. Check the green field wire over its entire length for any frayed insulation that could touch ground, if it's OK the problem is the regulator or an internal short in the alternator.

Intermittent faults are hard to track.

I had the same problem a few months back on my volate. It ended up ruining my Red top battery. I traced it to a pinched blue wire in the column. I found some harness tape melted and I traced it from there.

pull/check the brushes/brush holders for worn out/loose. check tightness of (2) brass female connectors in the reg triangle connector. As said any wire in the blue and green field wire circuit for loose/poor connection/continuity. reg base might be making intermittent contact, removeing/reinstalling it will elim that. EDIT easiest would be to toss on another alt from one of your other vehicles (that will elim or confirm that item in minutes)