How Do You REALLY Tell An Alternator's AMPS?

[uglytruk]

Among many things that I collect, I've got loadsa' GM alternators from the 70's and '80's. All built in regulator style.

I always wonder what makes an alternator 100 amps vs 50 or 75. Someone somewhere told me it's the windings in the rotor. I have yet to see a rotor that's "full".

I'm aware of the diff "codes" assigned to the variety of similar but slightly diff GM alts. any "pro" care to answer???

What 'bout all the "100 amp" chrome alts for sale on eBay? Are they really 100amps? Can one check the watts to convert ta amps?

 

[thatK30guy]

How Do You REALLY Tell An Alternator's AMPS?

You go up to it and ask it nicely.

 

[dremu]

Among many things that I collect, I've got loadsa' GM alternators from the 70's and '80's. All built in regulator style.

I always wonder what makes an alternator 100 amps vs 50 or 75. Someone somewhere told me it's the windings in the rotor. I have yet to see a rotor that's "full".

I'm aware of the diff "codes" assigned to the variety of similar but slightly diff GM alts. any "pro" care to answer???

What 'bout all the "100 amp" chrome alts for sale on eBay? Are they really 100amps? Can one check the watts to convert ta amps?

Well, power (watts) = among other things, voltage multiplied by current, i.e. watts = volts x amps.

In an engineering textbook, anyway =)) I know, I had them in school.

In the real world, the alternator will only put out as much as is needed, up to its limit, and both the output and limit are a function of the voltage which is partially a function of the regulator and partially the drain. Oh yeah and the alternator speed.

Umm, that means if you put a (good, digital) volt meter on your electrical system, with the ignition off it'll be like 12.5V or so, with the alternator on at idle it'll go up to 13.8 or 14, and if you race the engine so the alternator is putting out more oomph, you could see 14.5V. YVMV (your voltage may vary =))

ANYWAY, there are alternator load testers, that the rebuilders use when they give you a spec sheet with the ones at the parts stores.

I would think your best bet would be a big honking ammeter (like 150A or so max scale) in line with your alternator, a way to spin in (mounting it on an engine, or a test stand like the parts place use for their testers) and a nice big carbon pile aka resistor aka battery tester.

Or you could see if your local parts house will test them for you -- their test stand should have all that =))

-- A

 

[uglytruk]

FLAPS has a tester. I bought several cores a coupla' years back, cut offs, and wonder now if there was an AMP gauge. Maybe I'll pass by...
BTW, some of those cut-offs were bad. Go figure, but at $5 each, no problemo.
There were 2-3 big ol' 100 AMP monsters, ALA Cadillac... the ones you'd find ona' Military. Takes a special bracket ta clear the heater nipple...

 

[BlazerBud]

Google "Weldernator" that should keep you occupied for a while or unleash a new monster within. I assume you have some of the same quirks I do.

An alternator can make much more power than they are tuned for in a car.

 

[Blue85]

The only way to know is to test them into a big load and measure. If you find a setup to do that, make notes about the current vs. speed of the alternator. You should be able to use the ratio of pulley diameters (in vehicle) to match alt speed to engine rpm.

What you want is a lot of current at low speed. Many of the aftermarket "high current" alternators require high speed to get rated current (some are worse than stock at low speed). If you are winching or something, do you want to keep the engine at 3000rpm the whole time? Idle is the limiting factor, so IMHO the current output at idle is the most important number.

 

[dyeager535]

Well, the easy way if they haven't been monkeyed with (unlikely at the age most are now) is to look on the case. GM stamps them with the amp rating.

However, in my experience, most rebuilders grind those numbers off, so you have no idea on most of them that I've seen. Obviously to them it doesn't matter, as an example, you can fit guts from a 78 amp unit into a 37 amp case, the internals from any amp rating alternator will interchange, as long as the case is the same.

 

[uglytruk]

I noticed that the numbers are no longer ground off... Interesting... Ug

Well, the easy way if they haven't been monkeyed with (unlikely at the age most are now) is to look on the case. GM stamps them with the amp rating.

However, in my experience, most rebuilders grind those numbers off, so you have no idea on most of them that I've seen. Obviously to them it doesn't matter, as an example, you can fit guts from a 78 amp unit into a 37 amp case, the internals from any amp rating alternator will interchange, as long as the case is the same.

 

[dyeager535]

Maybe thats worse...now you think you are getting a 78 amp unit but it's really a 37 lol.

 

[uglytruk]

Exactly!!!

Maybe thats worse...now you think you are getting a 78 amp unit but it's really a 37 lol.

 

[uglytruk]

I met an owner of a small but succesful starter/alt shop at the local hangout tonite. We talked about the older GM alternators.
Two things I was unaware about was the "air gap". This is the space between the rotor and field coil. When the gap is small, the "cut in" speed is lower. Best for snow plows and rock crawling. When alts are re-built, the rotor or coil could be cut down to remove rust. A larger air gap results.

2nd thing was the small Cadillac pulley used on the 27 SI monster 100 amp alt used in the mid 70's to mid 80's. Also found on Ambulances, military and the occasional FS GM.
He's all outta' this mini 2" pulley.

Got any?

 

[lectric80]

The rotor and coils plus the air gap control when the amp kicks in at lower RPM's. The tighter the gap, the faster the alternator starts producing power. Controlling the output is the role of the diode's. You can change the diodes in any alternator (don't recommend it) to increase the overall amperage of an alternator. My buddy's Sub is using a 200 amp custom alternator, which is just a stock 12SI with high amp diodes. The downside to this is 1)Diodes frequently get fried when installing them, even by professionals, and they are pricey. 2) The life of other components such as brushes and rectifier bridge, is usually lessened by doing this.

We used to have a test bench that consisted of an electric motor, belt, and a bunch of stuff behind the scenes that I never found out about. Basically we would start the motor, apply current to the alternator, then apply load to find maximum amperage being output. Most 12SI's are actually only rated to 80% of their maximum load, so they can actually output higher, but are rated lower.

Hope some of this helps.

 

[dyeager535]

I tried using a smaller pulley to get RPM's up so alternator output was increased at lower engine speeds, it didn't work.

It's been quite some time, there is a thread about it.

After my experiment, my opinion is that the solution is to use the correct size alternator that handles the output requirements at idle. If you can change the output via "hard mods" to the alternator then there is nothing wrong with that either, but a pulley didn't make a difference in my case.

As a matter of fact, thinking back on that, reduced output was probably because that alternator was on it's way out. Rebuild (myself) brought voltage up quite a bit at idle RPM's.

 

[uglytruk]

Kool!
I plow with one of mt TEERUKS, a tired 305 1/2 ton shorty. I plan on retiring it, and putting the plow on my 3500 SRW 6.2. Shorty has the Cadillac 100A.
I also may put a plow on one of my 6.2 Subs.
When plowing, all lights are on, heater on high with the window open, and your driving slowly. Sometimes the battery will go dead.
I just came from another starter shop. This guy says he has lotsa' small pulleys...We'll see... Ug
Oh yeah... I did notice that the double pulley's are the smaller 2 1/2 size. Low RPM trucks, I guess... Ug