On a Mopar style dual field alternator, what happens when you swap the field wires from the stock setup?

The reason that I ask is because when I took my Mopar square back alternator (left below) into NAPA to have it tested, they tested it as the chart shown and it failed due to lack of voltage. Then I handed them my Powermaster (right below) unit and they hooked it up the same way as the square back and the machine told them to swap the field wires. Once they swapped the wires, then it tested PERFECT.
I just received an email from Powermaster discussing my current ammeter/voltage guage fluctuating at idle and they are suggesting that I swap the field wires.
Why would they suggest this and why did the PM alternator test fine at NAPA after they swapped the field wires? I have been running this alternator for a year now (wired like my stock unit) with no issues until a few weeks ago. I am just trying to understand the suggestion.

EDIT:
This is the response from Powermaster as to why to reverse the field wires:

""All the field wires do is complete the circuit, so it shouldn't matter if you reverse them. It really shouldn't do anything different, but I've seen it work in cases like these. It could be a sign that the diodes are starting to go out, but I've also seen this work, and work for years.""

I have had more discussions with them and since the machine at NAPA said to reverse the field wires, he thinks that the diodes may be the issue even though the machine showed the alternator to be in perfect condition after the swap. I am going to swap the field wires and see if it makes a difference at idle. If it does, Powermaster will take it in for repair and return it to me as new condition. Even if swapping the wires does not work, I will probably send it to them anyway due to the NAPA machine test and I am not out any driving time due to our Winter weather.



Thanks

Maybe Moparts member 68HemiB will chime in, he's an xpert on "dual field" alternators and their inner workings

Powermaster's observation is correct; if swapping the field wires changes the output, the alternator is faulty...even if it works that way.

I had no idea that swapping the field wires affected anything

I always thought that they had to be a certain way too. I was really surprised when the NAPA guy had to turn them around to make the machine test it. I should have brought up the question then.
If the output is the same on the car as it was on the machine, I am still going to get it repaired since it is just sitting in the garage for a few more months.

thanks allot

one more to chime in to say

ITS THE SAME no matter what. If there was a diff, you'll have some label at housing, or some ANTI IDIOT connector at brushes

Guys . . .remember that the field is setting up a magnetic field that the rotor (connected to the shaft which is turned by the belt) moves around. It needs a ground connection and positive connection. The voltage regulator senses the output voltage and varies the amount of voltage and current going to the field.

The stator is the stationary coil of wire around the outside of the rotor. As the rotor turns, the magnetic field induces current in the stator wires - although it is producing alternating current (AC, which goes positive and then negative, just like in the house), in what is called 3 phase (house is single phase).

Since the current is alternating between negative and positive, you have to use diodes to rectify the current into DC (direct current) that is used in the car. With 3 phases, you need 3 positive and 3 negative diodes. When you combine the output of each 3 diode sets, you then have 12volt DC current output from the alternator. This is a more efficient producer of electrical energy than a generator - i.e. it puts out current at low rpm (like at idle), that is why it was a standard years ago.

So with the single wire field alternator, one of the field connections was directly connected to ground and the other to positive from the voltage regulator.

For the dual wire field alternator, one wire (positive) is connected to the ignition circuit, and then the other wire is connected to the voltage regulator - which then varies the amount of current via resistance to ground.

You can test your alternator yourself (briefly), by putting battery positive to the field wire from the alternator (usually a blue with white tracer) and then ground the other field terminal. You should see FULL output and charging on your ammeter (if you still have one) . . . . if this is the case, then you have a bad regulator. If not, then get another alternator. Needless to say, this is with the engine running.

I have also heard of taking a screwdriver (metal one) and holding it to the back of the alternator, where the rear bearing is. If it is held in place, then the alternator is working, as a magnetic field is present from the rotor.

So the back to the question at hand - why would reversing the field wires on the test stand make a difference ??? You are basically causing the rotation of the magnetic field to be set up 180 degrees out - doesn't matter much as you are rectifying the AC signal back to DC through the diodes, however, you are getting one set of diodes working before the others. But it really doesn't matter, so long as you have output from the alternator

Hope that makes it all clear as mud . . .

Cheers

Mark

I agree, swapping the field wires should make no difference if the alternator is working correctly. One set of diodes is slightly different than the other. FWIW A lot of rebuilt alternators, starters,etc get right back out there as the original problem was never found. Just had another "new rebuilt" alternator go bad on a car I just bought, looked brand new but was intermittent after warm-up. I have a local outfit here that rebuilds alternators and starters, been using their Bosch alternators on my Mopars for 20 years, never a problem.

"" I agree, swapping the field wires should make no difference if the alternator is working correctly.""

I think that "WORKING CORRECTLY" is the key.

I witnessed what the test machine had the operator change to make the alternator test correctly. Since the alternator tested good after the field wire swap on the machine, I am assuming that since I have already swapped the field wires (temporary jumpers) that the next time I start up the engine, the ammeter/battery volt gauge may not show the bouncing issue.

As far as that goes an alternator don't care which way it turns either. (Other than the cooling fins)

no matter the fields order, as far you get magnetic field inside. Then stator captures the sine power ( alternate power ) and diodes bank takes the sine section variation what it cares ( positive section or negative section of the sine ) to feed the + and - busses ( stud or chassis ) No matter with section of the sine signal it happens first

so no matter either the rotor direction as stated, the important thing here is create the magnetic field at rotor

Lets say that the alternator does work correctly after swapping the field wires (as shown on the test machine), would you send it in to be rebuilt or see how long it runs?

Thanks allot for the replies.

if is in guarantee I will give to manufacturer. No headaches about something recently paid.

since I'm overseas, send to manufacturer IS THE HEADACHE so I would take care by myself.

soooo, all depends on your position

I see one thing missed on this dicsusion, you have two different field feeds, maybe one of them has a problem on it in the connectors or corrosion or I have seen a bad connector on one of the the feild input wires (It had broken most of the wires at the connector, maybe it had one or two still flowing current, not the entire bunch of wires like it should have been) cause charging problems. The feed wires are 18 gauge if I'm remembering correctly I replaced that connector and it got ALL BETTER Lots of gremlins out there of all kinds for Murphy to use to test us against our troubling shooting skills

I am just outside of the warranty period but the manufacturer said that they would cover labor and if parts were minimal, they would take care of that too and retest it and send it back to me as new. This is why I am leaning towards sending it back and just waiting for the rebuild rather than taking my chances during my driving season.
PM said that they have seen the wires swapped and then ran for years, but I am not that confident on electrical parts these days. I have had toooo many new electrical parts failures in the last year. (Powermaster Alternator, FBO ignition box, Mallory E-coil, and a FAST wideband controller (twice) all in the last year. These are some big names in the industry and I alone have had these failures.)

I will be able to have the dash back in this weekend (headlight switch issue) and then I will retest the alternator.

Cab,
I removed both field wire connectors due to loose connections and replaced them with new connectors and squeezed the female ends until both were very snug.
thanks

Thanks again.

If the machine knows the wires are backwards, there almost has to be a short to ground somewhere.
The machine applies 12 volts to the field on one wire and grounds the other. If there was a short, the logic in the tester would see that short and assume the person hooking it up did it wrong. When he reverses it, the shorted field connection is now hooked to the machines ground. The logic now sees a complete circuit with a good ground and less current draw on the 12V feed. Machine is happy, the alternator tests good. Unfortunately, because of the internal short, it will not work right in the car going to full output at all times. Powerquest is right, the alternator is bad and as I suspected, the parts store testers do not fully test dual field alternators.