Test Alternator Output

John T

• Hobby farm

As you know, removing the battery cable with the engine running on an alternator-equipped machine can cause all of the smoke to leak out of the alternator!

Why even suggest it to a "newbie"?

90% of the time, doing so won't hurt anything, but sometimes either the electronic voltage regulator or the diode trio, or the bridge rectifier will be shorted. Why take the risk, when a resonably accurate voltmeter is only $10.00 or $20.00 at WallyWorld.

For use around the electrically-noisy ignition and charging systems of these old beasts, an analog meter is a better choice, unless you have access to a high-dollar, high quality digital unit that will not behave erratically from the electrical noise.

I have an ancient Simpson VOM (analog meter) I use for tractor electric work. The harmonics and "hash" in the old generator systems drives cheap DVM's crazy and renders 'em useless.

Re smoking the alternator, been there done that! Years ago I was messing with my folk's car after dark. Thought I'd "test" the alternator by pulling pulling the battery ground cable off with the engine running and the lights on. I was rewarded by a brilliant flash of light as every lamp that was lit burned out and the motor quit. It also smoked the car radio, burned out all the dash gages and ruined the alternator. About the only thing that survived was the battery.

I learned a VERY expensive lesson that day...

I was working on a AC unit at a 50KW radio transmitter one day when my meters went bananas. At the time I had a couple good anolog and three digital VOM's and when taking a voltage reading would spike to 800vac or so.

So off 60miles to town to the local utility company to complain of a bad transformer that needed replaced. The FM radio transmitter had it's own power supply so I thought that's why the voltage spikes were not bothering the TX.

Well the utility engineer was not impressed with what I told him was happening with his transformer, so off to the job site to see for himself.

After there for a hour we finally observed one voltage spike at 900v. Well he said it's not there problem as a transformer can't do that. Well then please tell me what's causing the high voltage spikes? I can't and he goes back to the office.

Well I finish changing the compressor and have my meters on for start up and I'm rock solid at 231vac. I'm setting my digital ampmeter and notice my voltmeter going banana's again. Then it dawned on me as I heard the DJ talking.

My meters couldn't take the scattered magnetic field of the 50kw transmitter while the DJ was broadcasting and caused my meters to read false. As soon as the DJ quit transmitting, my meters would read normal.

T_Bone

• h

I didn't miss a word of your post.

You said to do it "quickly"... quickly is a moot point when working with electrical things, unless your as fast as some superhero like Superman! A voltage spike that may damage the alternator will occur far faster than you can react!

And, as far as it being a "poor man's test", if someone tries it, and does have the bad luck of "frying" their alternator, they WILL be "poorer"!

I stand by my other post... removing the battery cable intentionally while the engine is running on an alternator-equipped machine is a bad thing to do, and telling someone to do it was bad advice.

Please don't get torked off about this, and leave in a huff, as you have done before, and sometime when I goof on a suggestion, I will accept your constructive criticism between friends in the same fashion!

• Hobby farm

I just try to tell folks not to do the dumb things I've done!

• Hobby farm

• Hobby farm

If you have a voltmeter, IH2444's test below is a good one.

However if you don't have a voltmeter there's a "full field output" test you can run requiring only the tractor's ammeter and a screwdriver:

First look on the back of the alternator for a 3/8" dia round (or sometimes D-shaped) hole. About 1/4" inside that hole find a small metal tab. The tab is the regulator bypass terminal - grounding it causes the alternator to go to maximum current output.

To test, with the engine running at about 2/3 throttle take a small screwdriver and touch the blade to the tab while grounding the side of the screwdriver against the edge of the hole. If the alternator is good it should peg the ammeter on the charge side (+20 amps) when the metal tab is grounded.

Tim

If the battery is low, and the alternator is charging heavily, there will be a strong magnetic pull.

The problem with this test is that if the battery is nearing full charge, the voltage regulator will cut back the current to the rotating field, and there will be MUCH less magnetism to detect because the alternator is not being required to charge as heavily.

In other words, high rotating field magnetism = high charging rate, lower charging rate, but still working properly = low magnetism.