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Alternator going bad?

The vehicle in question is a 1977 Chevy C10 with the 250 ci inline six and a th350. Just took a day trip around southern Arizona and stop at a steakhouse on the way out. After leaving the restaurant, i noticed my voltage dropped to 11V, which it normally runs steady just above 13v. I thought for sure i wasn’t going to make it home, but sure enough, it ran the whole hour and a half home at a steady 11v. Is this a sign of an alternator going bad? Bad gauge? Short? Seems like they usually just quit. Any knowledge or suggestions on this would be appreciated. Thank you.

The alternator has likely failed. I have driven for an hour from out of town on battery power on an old vehicle like that, a late model car will only last 20 minutes off the battery.

That voltage gauge is not designed to be accurate for diagnostic purposes, only to display good/bad. With a failed alternator and still drivable the battery voltage will usually be 11.9 to 12.3 volts.

Thanks, ill take it off and have it tested. It was an old one my buddy gave me, a 94amp as opposed to the stock 63 amp. Will post results. Also, the truck starts fine this morning, i has a pretty big battery but i dont think it would run my truck with headlights for an hour and a half and then have plenty of power to crank right over. could be wrong.

second edit; Sorry for all the edits, as i drink my morning coffee im thinking of ways to diagnose this problem. I tested the voltage with a handheld tester. I have the same 11v volts with the engine off, running and with the battery cables disconnected. I was thinking a maybe had a bad short somewhere.

A short would cause the voltage to be zero.

“same 11v volts with the engine off, running and with the battery cables disconnected”.

Not sure what this means. You measured 11 volts under three conditions, engine off, engine running and cables disconnected?

If correct, then the alternator is definitely not charging the battery, which could be bad alternator, regulator, or a bad cable. The battery is also marginal, as it should be 12.6 volts unless almost totally discharged.


Yes, i tested the voltage under those 3 conditions. I couldnt have the alternator tested, due to a faded part number sticker. I bought a new one for an 88 s10, its 105 amp, and it matches the old one pretty close(correct plug and mounting points). I’ll put it on now and test it. Thanks guys.

Don’t be surprised if the battery is bad, or at least partially damaged.

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New alternator seemed to fix it, my v belt pulley was too wide so i took the grinder to it a little. I couldnt fit the lock washer back on so i used blue locktite. Im running past 13v now, we will see if the battery will still hold a charge. Thanks everyone.