Tractor Alternator

plenty - if you don't mind opening it up a bit and putting  the alternator on the bench a full diagnosis is possibke. You will need a voltmeter, an ohm meter, an ammeter a source of voltage to act as a 'battery'  and a series indicator lamp for initial excitation, and eventually  the ability to turn the thing for testing. A power drill can sometimes be pressed into service.
Is it the sort with the regulator built in  or in an external box. ?

The external reg tyoe you can energize the rotor externally and see if it then generates or if the rotor draws no current, look at the brushes and slip rings. The built in reg sort usuallly needs the end cover to come off to separate them

If you have a photo of the thing, it will speed up the explanation.
M

Thanks Mike, at least you didn't suggest an oscilloscope! 

I'll take it off tomorrow and have a poke about. Yeah it's built in.

It's a compact tractor with a loader so it is unbelieveably fiddly. I nearly had a nervous breakdown changing the oil filter.

Thanks again.

Some explanation so you know what you are looking at.

The electricity is generated in stator coils that 'stay' still and are wound into the frame. Usually 3 phases, and rectified with 6 diodes, On the ohm meter all 3 coils should look very low resistance, and the diodes will conduct 1 way, not the other. You can only sensibly test the diodes with the coils not connected to them, as all you read is another diode via the coils.
Spinning in the middle is an electro magnet, that is exited by a DC, through the reguator  and this connects by 2 brushes. Again, the resistance should buzz out very low on the ohm meter.
The regulator alters the rotor current, winding it down at higher revs so that the product of velocity and magnetic field strength, and so generated voltage is more or less constant.

In a car type system, the 'ignition light' provides the initial excitatiokn to the rotor, until it generates enough to sustain itslef,  for testing the reg can be removed, and the rotor just excited from 12V via a lamp - the lamp should glow and on spinning some volts should appear at the rectifier.

For any given model of alternator all this sort of thing, with numbers is usually available from the makers, but in order of easy fix and then liklihood, , igniion lamp fault, brushes fault, blown rectifier diode, dead reguator,

And look for mechanical things, like wet/corroded contact, or moving parts full of sand or even cobwebs.

So the rotor in this example the bent pole pieces create  alternating North and south poles in the stator windings,  even though the rotor winding is a simple cylindrical solenoid wrapped around the shaft. The brushes are shown emerging from the regulator blok like 2 fangs ready to meet the slip rings.

Mike.

well if you do have a scope it can be instructive, but it's probably overkill.

Mike.