Alternator testing?

[nutt7]

For those of you that have read many of my recent posts, you would know that I have been upgrading my electrical system, and struggling through some parts. Here I go again.

So I put on a junkyard (bench tested ok) 140a CS144. When cold and unloaded, it keeps the voltage at 14-14.4v. Warmed up and unloaded it's about 13.7v at the battery. Load that sucker with lights, and front and rear AC, and I get like 12.7v at the battery at idle (13.0-13.3v loaded and driving). Seems kinda low to me, for a 140a alternator (am I right?). Also when its loaded, the dome lights lightly flicker.

I took it in to O'Reilly to test in on the vehicle. The guy hooked it up, I started it and he said it tested ok. Aren't they supposed to check it under a load too? If I remember, the last time I took it in, I turned everything on and even held the rpm at 1500 for a few seconds.

Thoughts?

Also, my old alt couldn't keep up with the load (but more drastically) which makes me think there is something shorting in my wiring, but I have been through it and I can't find anything (no worn/hot wires, no shorts to ground)...

 

[1BEATK5]

Those little hand held midtronics testers can't load the charging system like an old carbon pile. Find a real alt/start/batt shop. They will have a real tester and know how to use it.
The o'Reilly's or autozone worker just knows how to push yes or no buttons on a little screen.

 

[Fordum]

Some alts are temperature compensated so they put out different voltages at different temps.
Mainly because batteries are happier at different volts at different temps.

But, regardless, wire resistance goes up with temperature, so the hotter it gets the less its gonna put out.

Having said all that, the only way to know if it meets specs is to find out what its supposed to put out at what temp.
I'm sure that info is out there somewhere, but I'm not sure where to find it.

However, even that info is useless unless you find out what its actually doing. If you were close by, it would take me about 10 seconds to snap my Fluke clamp-on AC/DC amp probe onto the output wire and read what amount of current it is actually putting out.

These things are rated at amps, but actually they should be rated at watts. ( volts X amps).
It may say it will put out 140 amps, but at what voltage? Watts would give you a clue.
If it was rated at 2000 watts, that would equal 140 amps @ 14.29 volts. If you saw that, you would know the alt was doing its job.

On the other hand, if you saw 13.5 volts, but it was putting out 148.15 amps, then it would still be meeting specs. ( 15.3 X 148.148 = 2000 watts)

The low voltage would be because it was overloaded.

You need to get your hands on a ammeter that will read to at least 150 amps or so, hook it to the output of the alt, and see what it is actually putting out when the voltage starts dropping.
If you are drawing more than 140 amps, or close by, then its not the alt.

Also, when the voltage is getting low, measure the voltage at the output of the alternator.
It may be putting out 14.5, but the wire between it and the rest of the truck may be too small and dropping too much voltage, especially when it gets hot.
If you upgraded the alt and not the wire, its a good possibility.

 

[nutt7]

Thanks. I know the 140a rating is at 14v at 27 degrees C, or about 81 degrees F. I also have a 4 awg (stock was 8 awg) charge wire that drops like a tenth of a volt when warm and loaded. All grounds are good, heavy, and clean. I need to borrow an ammeter for sure, or get it to a specialty shop. Or try a new one. Any other ideas that I should check out?

 

[Fordum]

If you can borrow a meter, then that is the cheapest way to go.
Just measure the amp and the volts at the same time with the engine revved up. I would not expect a used alt to meet the specs exactly, but if you get somewhere close, then I'd call it good.

Most alts that are in heavy use setups do not put out full output, or often even enough output, at idle.
The idea is that they do not normally see all that much idle time, and the battery takes up the slack.

Vehicles that do see a lot of idle time with heavy loads, often have special alts or smaller pulleys to compensate.
Limos, ambulances, things like that.

If your alt is doing ok at speed, but you need more power at idle, you can experiment with a smaller pulley.
But, there is a maximum RPM that the unit is rated for, and you need to be darn careful not to exceed it.
The least dangerous is for the wiring to fly out of the rotor. Or the rotor can just explode.
Either way, its charging days are over.

For cars that need low RPM power and high speed running, they make special alts that are rated at a much higher RPM.
The wiring is clamped in and the rotor is stronger.

I once saw a custom setup where one alternator was hooked up normally, and a second was hooked up with an air conditioner compressor clutch and a smaller pulley.
It dropped out when the engine got over a certain RPM, but was turning at about the equivalent of 2500 engine RPMs at idle when the clutch was engaged.

 

[dyeager535]

I wonder what the 9C1 (Police) Caprices used? Not like there are tons of them around, but a smaller pulley (to boost idle output) would be a no brainer.

Still think there is something else up if the alternator can't maintain voltage while the vehicle is moving.

 

[nutt7]

I actually have one, but I want to make sure everything else is squared away before installing it. If it held 13.8v loaded while driving, I would be happy to install it.

 

[dyeager535]

I know on my CS130 that if the idle starts to fall below 750 or so (and even at 750 with heavy load) the voltage gets VERY erratic...I've seen it dip below 12V if I really lug the engine or am tuning it and the idle starts to decrease. Even at idle it will dip if I put the heater on high, and hit the tailgate window switch.

So my assmption is that at idle speeds, these alternators, while having better output at idle than the 10/12SI models, still need some RPM's to keep up with heavy loads.

A rehash of your previous post, but does the alternator voltage mirror battery voltage? I know you upgraded the wiring, but need to ensure it's the output of the alternator that is the problem.

With a bit of thought and calculation, if you watched voltage at idle under full load, then increased engine RPM until voltage was "adequate", you could probably calculate if whatever pulley you have, would put alternator RPM at idle it needs to be. That again, assumes the wiring is not the issue.

 

[Fordum]

Its been years since I was behind the wheel of a diesel, and I was not watching the RPM then, but overall diesels rev lower than gas engines, plus the big trucks do a lot of idling.
I wonder if they have different setups. Anybody have an idea?

 

[dyeager535]

Just some quick searching, the 9C1 and 9C6 (apparently taxi) in 1991 w/5.7L apparently got the CS144 rated at 124A, while the "standard" cars got a CS130 rated for 100A. Using Rockauto as my source there.

They nor acdelco.com no help determining what diameter pulley(s) they used. Wonder if you could go to a parts store and ask to see both applications. Don't really trust parts stores to sell something that has the exact right part in things like this, but would be a free check.

Looks like a 6.2L truck the same year could have had anything between an 85 and 120A CS130.

A 1992 7.4L 3/4 Suburban could have had anything from 85-124A, and that appears to be either a CS130 or CS144 for the higher output.

 

[Irish1941]

The pulley ratio is all you need. Im 99% sure it'll be a 55 mm on your 144. Same as CS130.

To figure pulley ratio, you to take crank pulley O.D divided by alt pulley.
Then multiply that number by engine rpm to get alternator rpm.
You should get a number around 3:1 ratio, what most GMs leave the factory with. At idle, CS144 is spinning around 1900-2400 rpm depending where your idle is.
With these trucks, over spinning the alt is very unlikely unless you see the other side of 6k rpm. Anything over 18k and the alt will die but live just fine up to 16-17k rpm runs of short duration. Keep that 18k rpms in mind when swapping pulleys.
Smallest pulley you'll find is 49mm. Brings ratio to 3.65:1, making alt spin at 2700 at 750 engine rpm. At 3000 rpm, alternator would be spinning at 11k.

 

[nutt7]

I have a tenth a volt drop across my charge wire. All other points are within a few tenths of alt output.

My current pulley is 56mm, and I read that the stock crank pulley is 7.75 inches, or 197mm (might be wrong). Giving a ratio of 3.52, so at 600rpm my alt is turning 2100rpm which should give me about 100a in a perfect world. With my smaller 48mm pulley, I should get about 2460 alt rpm and 110a at 600rpm in a perfect world.

 

[euzoa]

Find a local shop with a VATS machine, and have them run it with a load.

Just remember, if your running over 80% of the rated output for any length of time you really shorten the life of the Alt

 

[Blue85]

Where is the sense wire connected? Can you get it closer to the battery?

 

[nutt7]

No sense wire. It just has a one wire plug, the exciter.