1993 GMC G3500 w/6.2l: Alternator output bouncing

[Splat]

My alternator went south on my 1993 G3500 Vandura w/6.2l last September. I replaced it with a higher amperage alternator, though I forget what it is. Also replaced all the belts. Honestly, I cannot remember if the armature shop changed the pulley on this higher-output alternator or gave me whatever was already on there. I just noticed one of the rib squares(for lack of a better term) is cracked off. The belt looks 100% fine otherwise and is tight enough that I can't move it. The alternator worked fine since installing it last September but now she seems to bouncing. Sometimes she'll be charging fine at idle and speeds, but then she'll drop down to 10V-11V and usually stay there, but sometimes she'll go back up again. Idle or at speed, doesn't matter. I've checked all connections, replaced the fusible link, batteries were bought new December (charge is good), everything appears OK. Can a rectifier die slowly on an alternator or is it all or nothing? Any other ideas welcome and really appreciated.

[john8662]

I recently had my alternator on my '95 6.5 quit charging and would start to charge again, then fail. Turned out the alternator was bad because the case on the alternator had cracked.

I'd get the alternator checked out, which'll require removal.

J

[Splat]

Thanks for replying. Well, it appears the belt was the culprit. I thought I had went back and retightened it after the break-in period.... maybe not. I tightened it yesterday and all is back to normal. We'll see what happens. If it starts bouncing again I'm definitely gonna rip the alt out.

[bmiszuk]

I had the same problem a couple months ago. Volts sometimes would be low, other times it was normal. It turned out the belt had gotten a little loose and was slipping, though it made no noise. Tightened it up and the problem went away.