Alt not working (2840)

I get the same voltage reading with tractor running as I do with it shut off. I am pretty sure it is not charging, my question is how can I tell if the problem is the Alt or the Regulator? On the 3 plug wire coming from the reg I get 12 volts from 2 plugs to ground but nothing from plug + to plug -. (service Manual shows a + and a - plug.I read battery voltage from the alt to bat. Not the normal 14 +- volts. Again the ? is is the problem the Alt or reg or Both. Sorry but I have not messed with a exciter type alt before.

Not sure at all what you mean by exciter type alternator, but I assume you have an external regulator system. Your readings may not mean anything without the key on, so missing - not a clear indicator that the regulator is at fault there. No telling from my couch exactly what's going on in your shop either, but we'll try. It may be that the - wire is grounded inside the alternator just to keep the VR and alt on the same page voltage wise, OR the other way around. Until we know for sure which way it is, just can't use that information for much.

It might be cheaper to pop for a standard rebuilt Delco at $65 total and just take off the regulator in order to use the replacement alternator with built in regulator.

Since solid state regulators rarely fail, it would be more likely that your brushes have finally worn out and need replacement. Not a standard item anymore, you'll need to hunt down a shop that even has a selection of similar sized ones in order to fit a replacement if you want to even go that far into it. Consider the price you are paying yourself and $70 for a 20 year fix starts to look pretty good three days into this mess.

Engine off and harness unplugged from alternator you should have 1.5 ohms to ground on one of those connectors sticking out of the alternator or from connector to connector. If you can't find it, this would indicate that the brushes are worn down beyond contact. If you had them or could get them easily, it's not hard at all to replace them. Parts stores will not carry such items since they sell rebuilt units and don't want you rebuilding your own for cheap. And I'm talking about five dollars here. Sorry, it's just not a fix it at home world anymore. Have had some luck buying alternator brushes from Rock Auto but no clue if the size is right for your application. Electrical motor rebuilding shop your best bet for a brush selection assortment box to choose from, be sure to bring an old one with you. Their price likely to be $10 range for two. Some numbers and dimensions that may or may not help, only wish I had more to post.

JX107 Brush Set .192 x .269 x .632
RX103 Brush Set .202 x .318 x .647 Delco 10SI

According to JDparts.com a 2840 has Robert Bosch alt. the regulator from JD is pricey.
AT31413 Voltage Regulator - REGULATOR,VOLTAGE ADD 130.03 USD
The reg from A&I is some what cheaper
A-AT31413 Voltage Regulator - VOLT. REGULATOR - 69.68 USD

I think I will just replace the alternator. May go with the single wire type.