I had an alternator before that would peg when revved. Was a bad alt with an internal short.
At first it would go the other way, but then I reversed the field and it would peg.
Sadly, the first time it pegged I hit my flashers and headed for the shoulder. Guess what 40+ volts does to incandescent bulbs?
Was a spendy weekend.
If you're going by the gauge when it happened that's 40 AMPS I'd have a real hard time believing it was 40VOLTS. The most I've seen on one that max'd out/pegged is slightly less than half that
(around 18-19v) but yeah that can fry bulbs.
Usually when I see that high of volts I find the battery has been cooked "dry" I don't know how much of that is the "chicken" or the "egg" (did the voltage go full tilt because the battery was low on water or did it get cooked out because of the charging system)
On my old wrangler I had that happen 2x over the 19 years I owned it, about 6years apart. Both times I smelt got battery acid, popped the caps and saw it was dry, a new battery fixed it each time.
On my 79 d100 I had an overcharge condition right after a motor swap. The motor had swung and smacked the VR and broke off one of the wires in the VR connector. The valve cover on the/6 smacked it, VR was right in the middle of the firewall on that one. A new VR pigtail solved that one.
