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GMC alternator problems

About every 6 to nine months I have to replace my alternator. The parts guy replaces it under warrenty but says it is due to a bad battery. The battery guy replaces the battery under warrenty but says it due to a poor alternator. I cannot purchase a bigger alternator as the next size up requires a differnt mounting setup. I’ve purchased the top of the line batteries each time and now from several makers (currently an interstate). Dispite all this I hear my alternator squealing again. Is there something else I should be looking at or for? Is this a common problem with this make (explaining why they went with a bigger alternator in new models?)

Remove both battery cables. On the positive cable, peel back the red rubber cover to expose the battery terminals. If a lot of corrosion is found under this red cover, replace the positive battery cable assembly.


You don’t have a big screen TV and a million watt stereo system in there do you?

Do they test the alternator to verify it is bad, or just give you a new one? Perhaps one of the mounts is bent or missing a spacer.

Yep, checked that. Terminals are clean as a whistle, good firm contact with the cable.

All OEM. Replacement radio but its plain jane.

You bet they test it. They don’t want to admit their part has quit before the time is up.

My mechanic says everything lines up.

Has the shop checked the output current of the replacement alternator to make sure it isn’t running at full output all the time? Certain wire faults can cause the alterator to keep running at full output if it thinks the battery level is low but really isn’t. The squealing may be an indication the alternator output is overloaded causing the belt to slip.

The only other thing to check is the bulkhead connector at the firewall. I’ve seen where water can leak in on older GM trucks at this connector where the pins rust. But this usually causes a problem in various elecrical circuits. But you never know!


What sympton is your truck showing you that causes you to look for a problem?

OK I’ll check

The first time around the gauges all went loopy and I thought it was a fluke. I drive until the bat died and I sat on the side of the road. Now I pay close attention anytime things under the hood sound different. First sign of problem is a squeal from the alt.

With the correct belt and a good tensioner you should be able to turn the crankshaft by turning the alternator pulley nut. If the belt slips when that nut is turned the belt is too loose and may be an incorrect belt or the tensioner could be failing. SBs of that time used several belts of similar size. You may have a belt 1 inch too long.

You must determine why that squeal is happening,it is not from the alternator siezing.

You might want to state how your alternator is failing before you start everyone to guessing the problem.

Failed front or rear shaft bearing, rotor copper winding, stator coil copper winding, slip rings, slip ring followers, diode cluster, voltage regulator or what? If a copper winding problem is it due to overheating or an insulation dielectric failure?

How often do you diagnose the specific cause of a failed alternator, Who? When the common symptom is squealing, which has resulted in repeated replacements of alternators and batteries I feel somewhat comfortable in suggesting that the belt tension might be loose. And I have rebuilt a few alternators and starters and generators but is’t been years. It’s not worth the effort anymore.