Beginner Alternator Question

[C10_Blackie]

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.

Thanks - I was hoping somebody would respond so I can place the order now.

I checked the output just now at the alternator post and verified that the alternator output is low and that it is done for.

 

[dsteelejr]

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

I came across some aftermarket alternators with left and right offsets that said they were a fit when I was looking to upgrade to one wire units. They were not GM, they were aftermarket brands, but still came up in my search results as a possible fit and I almost bought the wrong one. If it wasn’t for that I wouldn’t even know what an alternator offset is.

 

[Ricko1966]

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.

Does your charging light or gauge work? That is the exciter for the alternator,a burnt out light will keep the alternator from charging. Find terminal 1 on the back of the alternator everything connected hook a 12v test light to battery+ and terminal 1 the light should come on. Start the truck the light should go out and the alternator should be charging. If exciter circuit is the issue.

 

[vintovka]

Had a original 63 amp Alt that came with the 84 GMC sierra classic chassis. Constant power problems. A 100 amp 12si cured my issues specially h heavy power draws from big Spal fan. Adding up to 8 gauge wiring and HD circuit breakers helped as well and now can participate in low speed events like parades and cruises

 

[3dbello]

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.

If you have another vehicle, remove your alternator and bring it to your auto parts store of choice and have it tested. That's what I've done in the past for my vehicles. Good luck bud!

 

[75gmck25]

For everything you want to know about GM alternators and wiring, you might want to try this site.

MadElectrical.com - Mad Enterprises

 

[79SierraClassic]

Be careful when upgrading alternator sizes. On my 79, when I upgraded to the 80 amp alt off from my 80 Caprice, I also had to change the top mounting bracket as the alt was a larger diameter and would not fit under the original bracket.

 

[GTX63]

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.

Video breaks down what wires for where and why.

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[Robert Bare]

Hi. Haven't been around in a while, and don't see it mentioned here, the "poor mans alternator upgrade"
This works for chevy small-block trucks, that have the alternator in front of the passenger valve cover.
Go get an alternator for a 91 454 c/k. The 100 or 105 amp one. Now, the alternator does not have the hole to bolt the lower spacer on, but, works fine, just slide it on, like the older trucks. Zip off the nut, swap the serpentine pulley with your v belt pulley off old alternator. Yes, v belt will line right up. You have to buy a plug for the newer alternator, and wire it to your plug($10 now,maybe), or even buy an adapter, if they still make them, so you do not have to crimp 2 terminals,lol.
Now the hard part, the upper bracket slot will be too high. Smack it once or twice downwards with a hammer, till you can start upper bolt, and it slides in the grove, for adjustment.
The new alt. will put out 100a at rpms, also will charge more at idle.
Been doing this swap for years. An easy way to get true 100a cheap. Heck, you can find these alternators on the bay for $65, no core, shipping included.
Have a good day,all.

 

[YakkoWarner]

Before junking the alternator, I'd check to make sure the exciter wire is in fact exciting the windings. From personal experience, this is a real possibility. I had mine go to a no-charge situation after replacing the distributor.

It turns out whoever owned this truck before me (who should NEVER have been allowed to touch anything involving wires) had installed an alternator with the 3 pin connector (in addition to the larger +12 connections). My truck doesn't use the other 2 connections, so the connecter had been spliced onto the single exciter wire simply by shoving the stripped end of the factory exciter wire into one end of a crimp butt connector, shoving the (hopefully correct) wire from the 3 pin connector into the other side of that butt connector, and then wrapping the whole mess to around 1/2 thickness in electrical tape. No actual crimping was done (and the other 2 wires from the 3 pin where just taped up in there as well under some form of hope that they wouldn't end up touching anything else?).

Surprisingly this worked for almost 2 years, but merely touching this the wrong way caused the exciter wire to come free of its uncrimped coupler and thus losing the charge. I was calling the parts stores looking for an alternator when I decided to do some electrical archeology into that suspect-looking blob of tape. The +12 going to the firewall junction box was also spliced to a wrong-color wire and just twisted and taped in there as well, so I ended up spending a day ripping all that apart, properly splicing in the correct color and gauge wire for the +12 going to the firewall (soldered and triple shrink-wrapped, then additionally run through a section of small heater hose for chafe protection/support), correctly splicing only the 1 needed wire from the 3 pin connector to the exciter and removing the 2 unused wires, and suddenly I have perfectly functional charging again without buying an alternator.

 

[75gmck25]

Once again, if you buy a 94 amp GM 12si with the correct clocking, it is a direct bolt-in to replace a 63 amp 10si. It is exactly the same physical size.
Depending on the year of the new alternator, the bolt on top that you tighten to keep tension on the v-belt may need to be metric to work with the new case, but no other physical changes are needed.

The one electrical change you need to make is to upsize from the stock 10 gauge charge wire (the red wire running to the threaded Bat terminal on the alternator) to an 8 gauge charge wire. The new wire should be protected by a 12 gauge fusible link on the end where it connects to battery power.
I took the simple route and went to pick and pull, and took the charge wire from a newer pickup.

 

[C10_Blackie]

Once again, if you buy a 94 amp GM 12si with the correct clocking, it is a direct bolt-in to replace a 63 amp 10si. It is exactly the same physical size.
Depending on the year of the new alternator, the bolt on top that you tighten to keep tension on the v-belt may need to be metric to work with the new case, but no other physical changes are needed.

The one electrical change you need to make is to upsize from the stock 10 gauge charge wire (the red wire running to the threaded Bat terminal on the alternator) to an 8 gauge charge wire. The new wire should be protected by a 12 gauge fusible link on the end where it connects to battery power.
I took the simple route and went to pick and pull, and took the charge wire from a newer pickup.

I took the old alternator and battery to the local auto parts store and surprisingly both tested out just fine. By luck the guy who helped me was a C10 enthusiast and I had seen his truck at a local show just a couple of weeks ago. He thought the newer 350 sbc crate engine was simply drawing more power than either the battery or the alternator were designed for.

I installed the new Powermaster 12si 100 amp distributor from JEGS along with the beefiest battery they had at the store for my application today. I’m measuring 14.7 volts across the battery and across the alternator and ground, and suddenly all the accessories like the wipers and blower are working much, much better. Whoever put the engine in just didn’t think this part of it through and I’m mystified as to how it didn’t get noticed and fixed in the intervening time before I bought the truck.

The truck drove great on a ten-mile test today and no wires melted or anything like that but I eventually will go back and follow your recommendation on the wiring. Thanks for all your help, and I even came out of this with a perfectly good alternator and battery to sell to defray my costs.

 

[C10_Blackie]

This is the truck, BTW: