Technical School me on GM alternators

I’m putting vintage air on my wagon and the compressor bracket necessitates a GM alternator.

I need about 100amps,

I don’t know anything about GM alternators,
What do I need,
1 wire? 3 wire?
is there one I can buy at the local parts store?

thanks for the education!

 

The is basically three types of alternators working on how many amps output and what RPM of the
"" alternator speed"
Most Older Gm between 35 & 65 amp (example)
@ 1,000 rpms around 15 amps
Above 3,000 rpms 65 amp ish
This is Rpm's of alternator Not engine
Rpms , Example
Some alternator are 3 stage ,
3 different amperage output
3,000 35amp , then 6,000 65 amp
Then 12,000 100 amp
One place You can read about this at
Power masters website.
The more amps alternator puts out the bigger awg wire size required
From alternator back to battery

 

If the car has an idiot light, you might want a 3 wire, so the light will work.

The old design 10SI alternator was used from 1969 into the mid 80s, and is my "go to" for most retrofits. I prefer the standard design (used in cars, not tractors) 3 wire style, for one thing it's still available in stock at local parts stores, and for another is the cheapest alternator there is, usually. and it's pretty easy to adapt to lots of different engines.

The current rating...hmmm....how do you know how many amps you need? It's usually a guess, based on what accessories you have. A big electric fan will drive up the current requirement.

 

thanks!

Current is a guess, but I had a 60amp alternator and it couldn’t keep up at idle with the electric fan and headlights on. I switched to a 95amp alternator that I had and my charging problems went away.
I’m estimating that with AC, electric fan and headlights 100amp would be sufficient give or take a little.

 

lots to read here about 1 vs 3 wire, 10si vs 12si, remote sensing, etc. and even has part numbers to make things easier

MadElectrical.com - Electrical Tech

-rick

 

@AldeanFan
My self I ran into a problem alternator
Not keeping up , My issue was because I needed a small compact size ,
100 amp Denso , My electric fan itself
Pulled 35 ish amps .
@ engine rpm's of 1,000
& alt speed of 2,500 ish the alt only put out 36 amps , plus the extra 20 amps @ night my electrical needs ( 55 amp), this was only a issue @ engine Idle with a
2-1/4 alt pulley & Fan On ,
Batt would drain in 10 mints @ Idle from 14v to
12.2 v with Yellow top witch higher reserve amp per hour then Red top
Above 1,500 engine Rpms no issue.
I installed a 2 stage alt ,
65 amps below 3,000 rpm's of Alt speed
Above the 3,000 Alt rpm's speed 150 amps & needed to use # 4 awg charge wire

 

that's another reason I love mechanical fans....

 

which alternator did you end up with?

-rick

 

I used a single wire 100 amp GM alternator with my heat & air in our wagon. HRP



Sometimes you just gotta use a few GM parts. HRP

 

I had issues with the fan and headlights at idle. I went to newer relays...I had an old relay set up for the headlights. made a difference.

I use a one wire 100 amp at idle

 

Denso 2 stage Alternator , 2 functions ..
Not meaning Stage 2
Its 150 amp above 3,000 Rpms of Alt
Speed , Below 3,000 Rpms its 65 amps .
All alternators have a Spec of
Amps out put @ what Alternator Rpms speed, Most are 2 stage Most are based
On 3,000 Rpm' The split between 1st and 2nd stage.
For Example if not understanding the way I write , 100 amp Alternator
Is above the 3,000 ish Rpm's 2nd stage
The 1st stage Below 3,000 could be as low as 20 amp out put

 

I remember being told to get one for an 80's car with a rear window defroster element to get the highest amp output. Don't know if this is fact or not.

Gary

 

Thanks for all the great advice.
I’ve decided on an AC Delco 335-1094
It’s a 94amp 12si that came on some 80’s cars.
I’ll post an update when it’s installed and working.

 

I understand sizing for worst case situation, but are you really going to be driving in hot weather (A/C, fan) with the lights on? If so, might as well add that sound system and wipers. FLA does crazy stuff.

 

last summer we got caught in the rain at the beach and stuck in traffic. The windshield was completely fogged up and it was over 30°C out, so I had the headlights and radiator fan on and would have had the defrost on if I’d had it.

the cost difference between a 60amp and a 90 amp alternator is $20 so I’ll definitely go bigger.

 

Here’s hat I went with,
It’s supposed to be a 78amp 12si

 

@AldeanFan
Just asking did it come with Spec / data tag what amps it has @ engine Idle
Below 3,000 rpms of Alternator Rpms
Then Amps above witch should be
94 ish amps

 

unfortunately no, it didn’t come with do***entation.
I’ll report back on how it works once installed and the car is back running.

 

If you have Access to a clamp around
Amp meter Ac & Dc compatible
Nice extra tool to have

 

My favorite is a 12SI 3 wire

 

Just a related comment. You can find 10SI and 12SI rebuild kits for cheap on ebay. New bearings, brushes, voltage regulator for about $20. Example 10SI and 12SI kit here:
https://www.ebay.com/itm/190618856210
You can get kits with more parts if required for more $.
Get a core 10SI or 12SI for $5 at the swap meet, along with one of these $20 kits, and you have a new alternator as good as buying a rebuilt one at the auto parts store. Very easy to rebuild at home.

 

Nice thing about those 10SI, and 12SI, is that you can even find a spare anytime/nywhere. I knoww I have a couple myself, available to someone who needed it in pinch.
And, as mentioned above, simple and cheap to rebuild. We repaired one in a Meier's parking lot years ago on the way home from Nats North, the kits were on the pegboard!

 

this is great advice!

 

that’s exactly what I want,
I road trip this car so I need to be able to do roadside repairs that don’t require exotic parts.

 

My kids could repair a 10 or 12 SI . Just make as one wonder why when they have it figured out why change it to non rebuildable trash ?

 

I know you bought a 12si (that's a bigger case then the 10si).

Just some reading material.
https://www.qualitypowerauto.com/it...put-Alternator-internal-voltage-regulator.htm

https://www.powermasterperformance.com/alternators/gm/?alternator_case_style=55

 

What is this about rebuilding a GM alternator? Do you mean to say I will need to rebuild mine some day. Bummer! 13 years and 30,000+ miles now. AND IT IS ONE WIRE!

Ben

 

You might have to,,,,someday. Just know it's inexpensive and simple, if needed.

 

I replaced the voltage regulator in my 10SI.
It took longer to get the alternator off the car than to do the part replacement.