generator third brush problem

Could be a generator fault or perhaps a battery problem. Not knowing what type of cut out it has, it could be a fault with either type. The ammeter may need testing as well.
 
There are all kinds of reasons why this is so. More information will be helpful. As ndnchf asked, what is the charging rate. I would add at idle and at a fast engine speed, about 1,800 rpm.

There could be a voltage regulator in the system somewhere and the 3rd brush is not used. There could be a bad brush or all three brushes bad. As rotorwrench said it could be a bad cutout or a bad ammeter or both in combination. Or bad wiring somewhere. Use a voltmeter to get some more data.

Where are you located?
 
all good suggestions, the ammeter is showing just slightly negative, When I try horn it shows nstrong negative, when i gun engine it does not move to positive. Maybe that will help with diagnosis, thanks Pete
 
It sounds like either the generator is not putting out, or the cutout is not closing. With the engine running at around 1000 RPM, put a voltmeter to the generator output stud and see if you get 6.5 - 7.5 volts. If so, then the cutout may not be closing. If you read nothing, the generator is the problem.
 
If the cutout is not closing the gen can experience runaway voltage and burn up. Otherwise worn brushes most likely.
 
As with every repair job, you should start with the simplest fix first, and then move on to the more complicated ones.
 
Its been 6 weeks since I made comment #7. It still applies. Did you do the suggested test?
 
With belt loose or off, apply power to generator and see if it motors. It if motors slowly the generator is probably good. If not, its probably the cut-out.
 
The first thing I would do is test the battery for voltage and specific gravity unless it is an AGM battery which would require a high rate discharge test. If the starter turns over then turn the headlamps on and look for a discharge on the amp meter. Hit the starter with the lights on and see if the headlamps dim out. If they do with a fully charged battery then the battery is likely getting weak or connections are needing attention.

If the battery and amp meter give normal function then the cut out relay or rectifier need to be checked to insure function. A rectifier or power diode, like many folks are using for a cutout replacement, will pass current one way but not allow it to flow back the other way. If that part works then there is only one thing left to check and that is the generator. A number of things could be wrong with it so it is the most difficult part of the troubleshooting procedure. Motoring it is not the best way to test it so I'd at least do a tear down inspection and see if there are any obvious faults in there. Using a growler and a multi meter to test the armature will eliminate it as suspect. The shunt Field coils should have some resistance when tested for continuity with the ohms test of a multi meter. A clean & smooth commutator with some undercut left between segments is a good thing. New bushings/bearings and brushes don't hurt either. The field pole shoes can be remagnetized with correct polarity before reassembly by using a 6-volt battery and observing positive ground polarity between the case and the connection to the 3rd brush.

If you don't have the facility to overhaul a generator then exchange it with a reputable rebuilder of model A generators.
 
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