Beginner Alternator Question

I bought a 1981 C10 not too long ago with no real history on it other than it had a crate 350 motor put into it at some point, supposedly 20K miles ago. There's a sticker on the battery that says "5/22", so I'm assuming that's the date of manufacture.

The battery died once recently and I charged it back up. Thought maybe I had left something on while parked but now I wonder. Seemed to not crank strongly but has started up ever since the last charge.

Finally got around to testing it today. Battery read 11.9 volts with the engine off and then dropped to 11.5 volts when I started it up and the voltage did not vary when I increased the RPMs.

So this should be telling me that the alternator is on its way out, right? That's the first question.

Second question is if that is the case is there a replacement I can source locally that people on here tend to prefer or will any old thing from my friendly local auto parts store do?

Thanks.

12.6V should be healthy battery voltage. When the alternator is charging you should see well over 13V. You can check the feed at the rear post & see if you are getting 12V & do the same on the signal for the voltage regulator. Be certain to establish very good ground paths as well. Engine to chassis, battery to engine & engine to cab/body.

HS

If you’re not getting over 13V with the engine on then you’re not charging.

Check the voltage from the alternator output wire. There’s also a two wire connector that plugs into the alternator. A larger gauge red wire that should be constant hot and smaller gauge brown or white wire (may have changed color across the years). The smaller wire is for the excitor and should have power with key on.

Some parts stores, like O’riellys, will test your alternator if you remove it and take it to them.

If it is bad you can get a remanned alternator fairly cheap from any chain part store. I stopped buying rebuilt/remanned stuff bc the quality is terrible anymore. It’ll do in a pinch, but I got tired of them shitting the bed prematurely. For things like starters and alternators I only buy new from Summit. I upgraded from a stock 63 amp alternator to a 100 amp one wire alternator on two of my squares (‘73 and ‘80). Very happy with them and never looked back.

dsteelejr said:

If you’re not getting over 13V with the engine on then you’re not charging.

Check the voltage from the alternator output wire. There’s also a two wire connector that plugs into the alternator. A larger gauge red wire that should be constant hot and smaller gauge brown or white wire (may have changed color across the years). The smaller wire is for the excitor and should have power with key on.

Some parts stores, like O’riellys, will test your alternator if you remove it and take it to them.

If it is bad you can get a remanned alternator fairly cheap from any chain part store. I stopped buying rebuilt/remanned stuff bc the quality is terrible anymore. It’ll do in a pinch, but I got tired of them shitting the bed prematurely. For things like starters and alternators I only buy new from Summit. I upgraded from a stock 63 amp alternator to a 100 amp one wire alternator on two of my squares (‘73 and ‘80). Very happy with them and never looked back.

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Got it, thanks. I wanted to check alternator output directly but wasn’t sure how. Will check out Summit as well.

dsteelejr said:

If you’re not getting over 13V with the engine on then you’re not charging.

Check the voltage from the alternator output wire. There’s also a two wire connector that plugs into the alternator. A larger gauge red wire that should be constant hot and smaller gauge brown or white wire (may have changed color across the years). The smaller wire is for the excitor and should have power with key on.

Some parts stores, like O’riellys, will test your alternator if you remove it and take it to them.

If it is bad you can get a remanned alternator fairly cheap from any chain part store. I stopped buying rebuilt/remanned stuff bc the quality is terrible anymore. It’ll do in a pinch, but I got tired of them shitting the bed prematurely. For things like starters and alternators I only buy new from Summit. I upgraded from a stock 63 amp alternator to a 100 amp one wire alternator on two of my squares (‘73 and ‘80). Very happy with them and never looked back.

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Thanks for the tips. I’ll check the alternator tomorrow.

GM originally used a 10si alternator in many of these trucks, and it maxed out at 63 amps. You can substitute a 12si ( starting in ‘83) that has a standard pulley (not serpentine) and get up to 94 amps. Very easy upgrade.

75gmck25 said:

GM originally used a 10si alternator in many of these trucks, and it maxed out at 63 amps. You can substitute a 12si ( starting in ‘83) that has a standard pulley (not serpentine) and get up to 94 amps. Very easy upgrade.

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I'm also looking at a 100 amp alternator from LMC that looks like a clean swap.

C10_Blackie said:

I'm also looking at a 100 amp alternator from LMC that looks like a clean swap.

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I like @75gmck25 idea of going with a 12SI. My ‘88 K5 uses one. Externally they are the same and use the same factory plug as your current alternator but with the higher amperage. No modifications needed. GM upgraded to these in the mid 80s with more and more electric components like power windows increasing the power demand.

Just a side note, from my experience LMC tends to be a bit more pricey that other places. Shop around. I prefer Summit, but even Jegs has a bigger selection than LMC and Summit has free shipping on orders $109 or more, LMC charges shipping on everything no matter how much you spend. Summit’s customer service has been better than LMC’s too. That’s just my opinion, I could be wrong.

LMC has a great variety, but their prices are on the high side, and shipping adds quite a bit. For common hard parts I use Rockauto (usually cheaper) if I have time to wait, or go to the local parts store.

Alternators are referenced by a universal identifier called a Lester number. The Lester number for a 94 amp 12si with a v-pulley is 7294-3. This is a 10-15 minute install, and all the brackets and electrical plugs from the old alternator will match. It also has the same “clocking” as your original 10si that is mounted high on the passenger side.

I’ll echo what others have said, a 12si is a good upgrade for these trucks, both of mine run them.

When I got one for the 79, I went with a Powermaster as at the time (may still be that way) a reman from the local auto parts stores was almost the same in price as the new one from Powermaster (via summit).

I have had pretty good luck at the local pick n pull getting alternators. We don’t always have a ton of older stuff come through, but when they do it is not that uncommon for them to have a fairly fresh reman alternator. If timed right on a half off day, can get a good 12si or 10si for about $7.

I've been looking at the "gen" light illuminated for several years while driving my '72 Camaro. Occasionally I would take my meter and check to make certain that the alternator was charging even though I had that constant light. Well, a couple weeks ago I drove it and the light remained on while the car was switched off. I unplugged the regulator and the light went off. It was definitely a parasitic drain and most likely due to a defective diode in the unit.
So, most of you are going to point out that the '72 WAS externally regulated originally. I pulled the signal wires from the external regulator and rerouted through the harness back in the early 90's when I first built the car. I used a 63 amp unit with internal regulator and all was fine until the mid 90's when it started overcharging and seemed like it lit up the countryside that night. Thankfully I got home before it could fry any components and blow all the lights.
I bought a reman unit from the local parts house and have been driving ever since. The gen light started somewhere around 2010 but it still charged so I kept riding.
I went through my envelope of receipts and found where the alternator was purchased in 1996. My SIL laughed and told me that I had bought the alternator and installed before he was even born! I was able to take the unit and receipt and get another unit for free. The price has gone from $26 in '96 to $94 in '24. Wow what a jump in cost.
Well, the alternator was not just a plug-n-play unit as I run a factory Ford pulley (slightly larger diameter and deeper groove) and then I had to drill the wide mount anchor to 7/16 because I have a 396 Big Iron in the ole girl.
So, when any of you purchase a replacement and notice a thin sleeve that may be in the anchor, you know that the alternator came from a big block application and that sleeve is there to accommodate the space difference for a small block application.

HS

When several different people say the same thing, I know to listen. I'm going to do a quick test of the alternator itself after my next meeting - I have to believe it's going to be the alternator and not some sort of new wiring or ground issue. Meanwhile I'll learn how to quickly source a 12si alternator locally.

Thanks to everybody for the great advice.

75gmck25 said:

LMC has a great variety, but their prices are on the high side, and shipping adds quite a bit. For common hard parts I use Rockauto (usually cheaper) if I have time to wait, or go to the local parts store.

Alternators are referenced by a universal identifier called a Lester number. The Lester number for a 94 amp 12si with a v-pulley is 7294-3. This is a 10-15 minute install, and all the brackets and electrical plugs from the old alternator will match. It also has the same “clocking” as your original 10si that is mounted high on the passenger side.

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Thanks for that! Is this my guy?

C10_Blackie said:

Thanks for that! Is this my guy?

Powermaster 7294: GM 12si-Style Alternator Natural - JEGS

Alternator; 10si-12si; 100 Amp; Natural; One Wire;

www.jegs.com

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I know you were replying to @75gmck25 and I don’t mean to step on his toes, but yes. 12si is 94ish amps. You’d know if it’s a 10si by the lower amp rating, usually 63 for a stock one. Also, when you shop alternators be sure to make sure the offset is correct. If you were replacing a stock alternator with another one correct for your year model it wouldn’t be an issue, but now that you’re venturing into upgrades it’s something to keep in mind. The offset is how the mounting bolt holes are lined up. That is a center offset alternator, which I believe is correct for your truck, because the bolt holes line up down the center. I’ve attached an example of a left offset so you know what to look for. There are right offsets too. Also, it would be advisable to add an extra wire, 10 AWG, from the alternator output directly to the positive battery post. Your battery will be charging faster at a higher amperage and the stock wiring was designed for 63 amp alternators. It would take some of the load off the old wiring on your truck and reduce the risk of frying wires or components.

I’m not sure I’ve seen anything but a center offset mounting ears on Gm alternators.

If you go to a parts store, tell them is for an 84 Z28 Camaro with 305, that should produce a 12si with correct clocking of the terminals.

Regular production 12si alternators can be spotted as the fan is different than the 10si. For aftermarket some of the 12si units are equipped as so to appear like the 10si.

12si on left, 10si on right