Full field test (alternator) ???

[tsteiner61]

I want to do a full field test on my alternator. I know how to set the test up (jump the appropriate field term. to a good ground), but I don't know how to read the results of the test.

What RPM? What should my voltmeter read if alt. is good? bad?

Thanks in advance.

[jelsr]

This test is for current output (amps), a 70 amp rated alt should put out 70 amps, a 100 amp rated should put out 100, etc. This test should not be run any longer than to get a reading as the voltage is uncontrolled and high RPM is not necessary, 1500-2000 will do it. You will also need a guage that reads amps not volts.

[6 packin]

You should be able to tell with a volt meter. Should read between 15-18 volts output.

[John Kunkel]

Yes, the test is for voltage output. I'd expect to see 15-16 volts with a full field.

[dave571]

The guys are right but, it does depend on the state of charge of the battery.

It may take a while for the volts to get up that high, if the battery is low when you start the test(ie: the alternator hasen't been putting out prior to the test) No matter what the state of charge, you should see an immediate increase in system voltage when you full field the alternator(if the alternator is any good)

Make sure you've got 12v on the terminal you don't ground. Otherwise you can ground the second one all day, and you won't get anything out of it.

[6 packin]

Originally posted by dave571
The guys are right but, it does depend on the state of charge of the battery.

It may take a while for the volts to get up that high, if the battery is low when you start the test(ie: the alternator hasen't been putting out prior to the test) No matter what the state of charge, you should see an immediate increase in system voltage when you full field the alternator(if the alternator is any good)

Make sure you've got 12v on the terminal you don't ground. Otherwise you can ground the second one all day, and you won't get anything out of it.

Huh??? If is gage works in the dash it should peg out, no matter what the battery is doing. He's not reading the battery, just the out put on the altenator. Because he is bypassing the voltage regulator so to speak. It will not matter if his battery if completly charged or laying on the ground beside the truck It will be under full load.

[dodgeman360]

I just went through charging problems, bought alt , voltage ref, still didn't charge, Guess what it was just the gauge, on old mopars the gauge is part of the circut, if the gauge faills, it quits charging.

[dave571]

I'm not talking about pinning the amp gauge, I'm talking about the voltage level of the charging system.

If the battery is in the vehicle, and it is low on charge, it will take a while for the voltage to come up to the high voltages mentioned earlier.

Charging systyem voltage is a result of the resistance of the charging system. If the battery is fully charged and the alternator is full fielded, it will go up much quiker than if not.

That said, it definately makes a difference if the battery is on the floor, or in the vehicle, as to the voltage output. It won't be under full load at all, it will be under full output. There is a difference.

I'm talking about voltage, not amperage on the dash gauge. A good voltmeter is considerd by most, the most accurate tool in diagnosing charging problems.

Usually when doing a full field test, a volt meter is placed on the battery(most do it your selfers, don't have an inductive ammeter, and the accuracy of dash gauges can always be questioned) Full field the alternator, and view the changes in system voltage, to see what effect you have had.

No effect, with good supply voltage to the field wire, and the second wire grounded, and the alternator is no good. Voltage goes up. ..it's ok.

I worked on a car last year that had steady alternator output. The regulator circuit was not wired properly, and the alternator charged about 15 amps all the time. This was ok for short drives, but would drive the voltage up to scary levels(17 volts plus) after about 20 minutes of driving. It weould take that long, because the slow output would take that long to recharge the battery from the starting load, then to puish the voltage so scary high.

Hope this makes sense to you.

[jelsr]

The purpose of a "full field test" is for maximum output, it's not necessary to do so for voltage readings. Voltage is simply a difference in potential and does not indicate current flow. During this test the regulator is removed from the circuit and voltage is uncontrolled. If not for the battery the voltage would go off scale.

[dave571]

obviously the voltage would go extremely high without a battery.

What I'm saying, is that voltage is a product of current output and system resistance. Increasing the output amps increases the voltage.

That said, a charging system can be diagnosed extremely accurately without need for an ameter, or trusting the 10 cent ammeter the car came with.

I have expensive charging system tools at the shop. The one tool I use most for charging problems, is a voltmeter, and it's cheap.

[jelsr]

Not questioning your expertise or equipment, just pointing out that the "full field" test (actually current output test) which is performed with volt/amp/carbon pile equipment should not be approached lightly. Components can be ruined. My info shows 1250RPM and within 8-10 % of rated output.

[6 packin]

I agree with the last post, you can definatly fry some things if your not carefull. It just a test to see if you altenator is working, and to check the integrity of the Voltage regulator. Dont run it more than a few seconds if possiable.

[tsteiner61]

you can definatly fry some things if your not carefull

Yeah, I just found that out.
But at least I know my alternator is good.

Thanks.

[Buddy Branon]

Don't full field to long you might let the smoke out

[jelsr]

Buddy,
I've heard that term "Magic Smoke" used on AC electric motors also, when it escapes you're done. When I was still with KDOT the radio man had a term "VTSC". When questioned he explained it stood for "voltage to smoke converter". Seen em both!

[Buddy Branon]

jelsr,
Thats a good one, never heard it.LOL. I know this is off the subject but I just can't resist one more.
I usually have a part or two left over after a big project...So, my theory is "If you take something apart and put it back together enough times, you will eventually have two of them"