[dedub]

Hello again, got another question.

I bought an 80 El Camino and I'm suspicious of the alternator. It's a standard alternator you would find on any 350 small block, meaning I don't think it's a high amp one. At first I thought the volt gauge was off because of age but a volt meter shows the battery at 12.5 v while running, the most it's read was just over 13v. If my memory serves me it should be around 14.2v. I don't remember if these cars came with voltage regulators but I don't think they did. Auto Zone ran a diag on the battery and said it is good to go. At times it is hard to start which leads me to think the alternator isn't charging it properly. My question is, ... is there a way I could check this myself to be sure? Is there a backyard mechanic way to test an alternator before I go buy a new one?
Thanks
Darrin

[T-bucket23]

Originally Posted by dedub

Hello again, got another question.

I bought an 80 El Camino and I'm suspicious of the alternator. It's a standard alternator you would find on any 350 small block, meaning I don't think it's a high amp one. At first I thought the volt gauge was off because of age but a volt meter shows the battery at 12.5 v while running, the most it's read was just over 13v. If my memory serves me it should be around 14.2v. I don't remember if these cars came with voltage regulators but I don't think they did. Auto Zone ran a diag on the battery and said it is good to go. At times it is hard to start which leads me to think the alternator isn't charging it properly. My question is, ... is there a way I could check this myself to be sure? Is there a backyard mechanic way to test an alternator before I go buy a new one?
Thanks
Darrin

The voltage is to low. Check to be sure the belt is tight and all connections are clean and tight. You can take the alternator off and have it tested. Autozone and some of the other parts stores will do this for free. Without a good AMP gauge there is no way to tell if it is working or not. The lack of voltage is a good sign that it is not working but you need to see what the amps are doing. If it is a 10SI they are fairly easy to fix with standard hand tools and a rebuild kit. Usually arrounf 25 bucks or so

Chet

[dedub]

Thanks Chet,
Two more questions...
Will auto zone do it with the alternator off?
What does 10SI mean?

Thanks again
Darrin

[russlaferrera]

Start the vehicle, turn all electrical items on (lights, fan etc.) alt. should carry the load. If not, time to pull it off and check it on the tester.

[malc]

Check out this website for some electrical explanations on alternators and vehicle electrical systems.

[dedub]

Great link Malc.

Thanks

[75gmck25]

I had similar readings on my '75 350, and when I turned on the headlights and heater blower, it dropped to about 11 volts with the engine running at 3k RPM. I couldn't drive at night for any length of time because the alternator could not keep up.

The GM 10si alternator should have been standard in 1980, and they came in versions with up to 63 amps. If you had factory AC, it was probably a 63 amp alternator. The 10si was later replaced with the 12si version, which had output of up to about 94 amps.

The 12si can be a direct replacement for the 10si, as long as you get the V belt pulley (not the serpentine belt version), and upgrade the charge wire from 10 gauge to 8 gauge (including a 12 gauge fusible link).

I'm using a 1985 vintage 12si on my '75 engine, and it charges very well. You need to find a 12si with the same "clocking" as your current alternator. Clocking refers to the position of the power plugs relative to the mounting brackets. If clocking is wrong, the wires might not reach, or the plug might end up down next to the valve cover where you don't have clearance.

I used the data from the web link at Mad Electrical, but some of the parts numbers didn't quite match those at the parts store. I had to have the auto parts place pull a couple of alternators before I found one with the V-belt pulley and the right clocking.

Bruce

[dedub]

75gmck25,
Thanks for the info. After doing *LOTS* of research on alternators with the provided links I found out I did have a 12SI 94 amp. Still suspicious of it I swapped it out with a 10SI 63 amp and low and behold.... 14.2 volts varying with my idle. I just happen to have this other alt. laying around, lucky me. So, off to Auto Zone with the 12SI and the guy tells me its fine?????? Then driving home I notice that even the 10SI starts dropping to mid 13's. Could I have two bad alternators? Could it be the heat? Who knows, I'm going to keep the 10si in until something catastrophic happens and go from there.
Do you know of a place that sells rebuild kits for these things?

Darrin

[75gmck25]

Rebuild kits should be available from most full service auto parts stores. Summit Racing also sells the Powermaster rebuild kit online.

My 12si charges at about 14 volts when I first start the truck, and then drops to mid 13's after the battery is fully charged. Even with all the lights on and the blower motor on high, it never drops below about 13.5 when the engine is running, even at idle.

Bruce

[dedub]

Good info, thanks Bruce.
That's probably what's going on with mine also.

Darrin

[T-bucket23]

Originally Posted by dedub

75gmck25,
Thanks for the info. After doing *LOTS* of research on alternators with the provided links I found out I did have a 12SI 94 amp. Still suspicious of it I swapped it out with a 10SI 63 amp and low and behold.... 14.2 volts varying with my idle. I just happen to have this other alt. laying around, lucky me. So, off to Auto Zone with the 12SI and the guy tells me its fine?????? Then driving home I notice that even the 10SI starts dropping to mid 13's. Could I have two bad alternators? Could it be the heat? Who knows, I'm going to keep the 10si in until something catastrophic happens and go from there.
Do you know of a place that sells rebuild kits for these things?

Darrin

mid 13's is fine. The voltage will vary with load. With a battery that requires charging the voltage will tend to be lower as the current requirement goes up. As the battery charges the voltage will go up. Depending on the health of the rest of your electrical system and where your sense wire is connected it is possible to have some fluctuation.

Chet