Generator Light on - 59 GMC 1/2 ton w V8

[bevman]

I have a "new" (rebuilt) generator and a new regulator and the red gen light is on. Goes off when idling, but comes on about half-strength when driving. New belt has 3/8" of deflection. I took it to my mechanic and he said that I needed a new regulator so I got one. I did spark the regulator with a jumper wire, as directed, to polarize.That didn't fix the problem. This started just after I had the engine pulled to replace an undersized piston (long story). Any ideas? - Michael