Gen. not charging

[Bruce Parker (RIP)]

There is a section in the Chev MB-C2 manual that talks about reverse polarity. It says that "if the polarity of the generator is reversed the circuit breaker contact points will vibrate and burn". Ii goes on to say how to polarize the regulator by momentarily connecting the GEN to the BAT terminals.

What's not clear is how the generator finds itself with a revered polarity. I expect 'new in the box' will most likely be correct...I say this because for the first two 6 volt negative ground systems I installed years ago (MB jeep and C-15A), I was completely unaware polarity even existed. It only came up when I was figuring out the differences between positive and negative ground systems when I bought my first Ford.

Originally Posted by Lynn Eades

Bruce, If your generator is not "polarised' the cut out points stay closed when you turn the ignition off. (the two brush geny has a cut out in the regulator)
The result of this is that the generator is "fed" by the battery. The generator in this situation tries to (be a) motor. The "motor" cannot turn because the fan belt stops it. The end result is that the "motor" (generator) burns out.
Not much noise, but a bit of smoke!

If David's geny was charging, at all, the ignition light would go out.(or at least would appear so) I have no experience with adjustment of the brush in a 3 brush geny, but I would expect it to still put out a some sort of charge, not none at all
A geny that "motors" will usually be o.k.
I would expect the fault to be the cut out, a wiring issue, or an earthing issue.
If the geny is removed to an auto electrical shop it can be tested.
The big wire on the generator is the one hooked to the battery. The little one goes to the cut out.(the cut out only has 1 wire????? The other side goes to earth on the case???
Have I got that right?

In a three brush generator the third brush controls the voltage to the field windings and so the strength of the magnetic fields) The strength of the magnetism from the fields is what dictates the output voltage of the generator. I hope these snippets help, David.

[Dave Schindel]

Hi guys, yes,I did polarize the gen. And it is a 3 brush design. My issue seems to be more with the F wire from the gen. and what it should have as far as voltage. This is on a carrier so the switch may be different than on other vehicles. With out the engine running, and with the switch on,the field wire on the gen. should be fed by power from the battery, through a light, and to the F wire,right? As it is now,with the switch on,there is no power going to the F wire.Thanks for all the ideas and discussion.

[Lynn Eades]

Bruce, the soft iron pole shoes are the magnets which are energised by current flowing in the field windings. These pole shoes hold residual magnetism. The polarity is easily set up and just as easily reversed.
To polarise the generator, all it requires is to run a wire from the live battery terminal (in the vehicle the geny is to be fitted into) to the field terminal (wire)of the geny. Striking the wire onto the "F" terminal a couple of times will do the job.(expect little sparks)
The process of testing and rebuilding a generator sometimes results in the polarity being reversed. The unit you get back may not have been your one and also may have come from a vehicle that had a different polarity to your veh. This is why it needs to be polarised for your particular vehicle.

BTW, this "residual magnetism" is what voltage "builds" from, when you tow start a vehicle with a dead battery. If you can get the generator turning over fast enough, it will produce enough voltage to fire the ignition. If you keep the revs up the geny will not only produce enough to "fire" the ignition as well as put some charge into the battery.

You cannot do this with a vehicle fitted with an alternator because the design is different and an alternator requires battery voltage to "exite" it. (get the charging started)

Dave, Does you ign. warning light go? If it goes, there must be a complete circuit. I don't have enough knowledge to help you from here.

ASK an auto electrician if you can do this;
What happens if you momentarily touch a wire from the "F" terminal to to the battery? (engine running above idle)
This would be an un regulated generator which will deliver 30 to 50 volts (the geny wont last long at this out put) The field strength regulates the out put.

Does the generator start working? You should be able to detect the engine loading up.
If it then works, I'd say it is the cutout at fault.

Be careful, those are good bits until the smoke gets out of them. Once the smoke gets out, you'll need new bits.
I don't want to cause that.

[Dave Schindel]

Yes,the gen. light comes on when the switch is turned on.Dimms a wee bit when cranking,and stays on when the engine is running.I will check with a jumper wire or maybe rig a 6v bulb with a jumper wire to test further. Thanks!