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Hello:

I have been trying to troubleshoot an electrical problem on my tractor
(John Deere 300 with 16 hp Kohler single cylinder) and so far I am
totally stumped. The problem is this: The battery seemed to be out of
juice so I had to jump the tractor from my car. The tractor started
right up and ran.... but as soon as I disconnected the jumper cables
from the car, the tractor stalled immediately.


That says bad alternator/alternator path or bad terminal/connection... but
since this is the third time I have heard this exact same problem in
vehicles in two months, and each was fixed by replacing the battery...
Shorted battery/bad post comes to mind. But onward into the post

I am assuming the tractor should continue to run on its own power, so I
started testing the charging system on it. The stator voltage was
fine, around 30 volts AC. However when I went to check the voltage
coming out of the rectifier, my multimeter went crazy and I could not
get any consistent reading when connecting the negative of the
multimeter to the engine block.

That can
1) be a multimeter impedance problem. It can't read accurately if it's
too low an impedance relative to the circuit being measured, or
2) be a directional problem in reading the rectifier (normal is it reads
ok one way, doesn't read when the leads are reversed), or
3) the probe is inable to penetrate the corrosion, etc., on the contact
point. (Scrape block before touching tip firmly to metal)
or a combination of the three

or you were measuring the reference (negative) side of the alternator- which
means no voltage.
or you were trying to read AC on DC.

It was as if something was interfering
with the multimeter. But when I used the frame of the tractor for the
negative, I finally got a consistent reading of around 0.3-0.7 volts
DC.

That level of voltage sounds like the voltage drop across poor cables/from
high output from an alternator to a battery. It is also close to the voltage
drop across an alternator diode.

The negative battery terminal runs directly to the engine block and
that connection was good. So immediately I assumed the rectifier was
bad. After connecting a new rectifier still the same problem.

So next I hooked up the jumper cables again and shut off the car,
keeping the cables attached.. and the tractor continued to run just
fine. I checked the voltage across the tractor's battery and it was
around 14.5 volts.

You were not reading the battery, you were reading the alternator voltage
output into the tractor circuit.

Alternators output around 14 volts, fully charged auto-type batteries around
11.5, give or take a few tenths

I shut off the tractor with the key and the voltage
dropped to around 13.5.volts.

Should have dropped to 11.7 if the battery had been accepting charge and no
current was flowing in or out.


When I saw this I assumed the charging
system was working and that my earlier assumption of a bad rectifier
was incorrect.

First, remember that it's the rectifier path, not just the rectifier, and

second, voltage does not mean an appreciable amount of charge is going
into/being accepted by the battery. It could be 14.5 volts and milliamps
going through the battery

Then I removed the jumper cables and the voltage then
showed 10.5 volts.

So before disconnecting, charge was still flowing from the car battery to
the tractor battery, giving you an extra 3 volts until you disconnected the
jumper cables.

Is it possible that the tractor's battery is bad or
shorted, causing these issues???


Yes. It should read around 11.5/11.7 (memory here) when charged if the
battery is ok.

But it also has to deliver amps as well - so just reading 11.5 is not
enough - if it's outputting the amps of a watch battery it's not ok, but
reading 11.5 and delivering rated amps does.

Thanks in advance for any feedback...




--
Chris