Easy Alternator test? Options

[ChrisReale]

Is there an easy multi-meter test you can do to see if the alternator has crapped out? I guess the fact that my battery doesn't charge is proof enough, but I just wanna make sure.

[type47]

I'd like to listen to a discussion on how the VR works. Here is a link to the PP elec diagram containing VR and Battery:

http://www.pelicanparts.com/914/parts/Elec...lectric_74E.jpg

[Dave_Darling]

I'd like to listen to a discussion on how the VR works.

It compares the battery voltage to the alternator voltage. When the alt voltage is higher than the battery voltage, it disconnects the field windings of the alternator, making it no longer charge. When the alt voltage is lower than the battery voltage, it connects the field windings of the alt, making it charge. It effectively turns the alternator on and off, over and over again, pretty quickly.

That is a simplified version, at least. There are more details, and I think those start taking into account how quickly the electric fields in the windings build up and decay, and how the compared voltages are actually biased so you charge at ~14V rather than ~12V, but the description is a half-decent overview.

I believe the alt light runs off of the alternator voltage wire that goes to the VR. I'm not sure exactly when the power for the light has to go through the VR, but it probably has something to do with the way the light is used to "bootstrap" the alternator to get it to start charging.

--DD

[mikesmith]

I believe the alt light runs off of the alternator voltage wire that goes to the VR. I'm not sure exactly when the power for the light has to go through the VR, but it probably has something to do with the way the light is used to "bootstrap" the alternator to get it to start charging.

There are two rectifiers inside the alternator. One supplies what we think of as the "output" current, the other one is used to supply current for the field coil.

The alternator "works" by moving the field coil and its magnetic field around relative to the stator coils. You have to have current flowing in the field coil for it to create a field, and that current has to come from somewhere.

When the alternator is running, the second rectifier supplies the current. But to begin with, there is no current, and that's where the charge lamp comes in. One end of the lamp is connected to ignition power, the other end is connected to the output of the field coil supply rectifier.

To the earlier points about the charge lamp burning out; in some designs there is a resistor in parallel with the lamp so that current can flow even if the lamp fails.

This connection is also why some vehicles need a diode in series with the charge lamp; if your ignition current draw is low you can actually back-power the ignition circuit from the field coil rectifier.

Much more detail here:

http://brianesser.com/technical-informatio...nd-information/