Two volts at A and F posts, what's wrong?

[nothercrash]

So I've been dealing with some kind of gremlin that stops my tractor for two years, through two engine rebuilds.

I've checked through everything else time after time, but I don't know much about electronics, so I think that's where my problem is.

I just did a few tests on my my starter/gen after tearing it down and cleaning it up, finding nothing irregular visually. I did the test where you lay the generator on a bench, ground it to a battery charger, contact the positive charger clamp to the A post to spin up the starter, and then switch it over to the F post, and see if it puts the brakes on the spinning. It did, it slowed it down much faster than letting it spin down by itself. Supposedly, that tells you that it should charge your system.

Next, I put the starter back on the tractor, started the engine, and tested the voltage at the battery. It was about 11. Then I disconnected every wire from the starter/gen and tested voltage at the F and A posts. I got only two volts, from each post, with the engine running, and no wires connected to the starter/gen.

What does it all mean???

Thankyou, Alex

[R Bedell]

Perform this test and let us know........ S/G Test

[nothercrash]

I'm heading back out to the site where my tractor is tonight, so I just read through the S/G test thread that you linked me to.

In step #3 you say to jump the F terminal to a ground (negative), and see if the unit charges the battery. Wouldn't the F terminal need to be connected to the positive side of the battery to charge it??

Like I said, I'm very new to electronics, so if I'm wrong, I wouldn't be surprised.

[ol'George]

No, Ground it to make it charge.

[IHinIN]

Only ground the F terminal for a couple seconds or else you could damage the field coil in the generator. It should jump up above 16 volts. If it does then the issue is with the voltage regulator.

[nothercrash]

So the charging current doesn't come out of the F terminal then? Does it flow out of the A terminal, backwards through the starter solenoid??

I'm used to an alternator in a car, with a big fat wire going right to the positive side of the battery. How does a S/G get power to the positive side of the battery?

[R Bedell]

So the charging current doesn't come out of the F terminal then?

The S/G turns On/Off by grounding the F terminal. By you temporarily jumpering that terminal to ground eliminates the V/R. This will tell you the output status of the S/G.

Do all voltage measuring at the Battery and NOT at the S/G.

[ol'George]

And an alternator is not a generator, ----two different animals.
You power the field terminal on an ALTERNATOR to test it on a vehicle.
You ground the field terminal on a GENERATOR to test it.
Trust us.

[nothercrash]

Oh I trust you! I know they're different. That's what I was saying, I didn't understand how a S/G worked.

Now though, after lots of reading, tests, and help from you guys, I understand them better.

I finally got a chance to do the tests Bedell linked me to, and if they tell the whole story, then it sounds like I've got a bad S/G. The Bat terminal on the VR has power, the blue and yellow wires have almost no resistance (less than .5 ohms, which is the same as my meter read touching its own leads together), and finally, most damning, there was no RPM drop when I jumped the F terminal directly to the Neg battery post, and the voltage at the battery terminals didn't change, 11.5 volts with the F jump disconnected, and connected.

Might I just need to repolarize it? I have no idea if that's even a sensible question to ask in this case, but I see it suggested on that page of S/G tests.

[nothercrash]

And if repolarizing won't fix this, should I try to install a new field coil? The S/G was supposedly replaced right before I bought the tractor, the zinc coating on the components is still shiny gold/green. Both end bearings are still dead tight, no wiggle, and the part the brushes ride on is unworn. I've rebuilt engines and transmissions in cars, but the electronics are not my area.

Is it worth saving 50 bucks? Or will I probably not get it perfect and end up buying a new one anyway?