update on 2-135 alternators

got a different alternator yesterday. put it on the tractor. check voltages before starting. had 12.58 at alt. starter, etc. don't see any bad grounds. the #1 terminal on the alt is hooked to a switched wire that goes to the gauge. this runs through a diode. #2 terminal on alt is hooked to the "bat" terminal on the alt.
started tractor. voltage went to 14.4 but had 14 on one side of diode (coming from the gauge) and 14.4 on other side of diode closet to #1 terminal. alt got hotter and hotter and started to smoke. unplugged and waited a minute. plugged in again and same thing happened in about 30 seconds. The alt on there is from "carquest" a 7127-3a 63 amp. 5th alt I have put on.
so, could I have a pinched switched hot wire causing this thing to over amp itself?
Gary
 
Firstly, alternators are very reliable, so it is your tractor which is at fault.

All diodes have a voltage drop across them. Germanium has about 0.4V and silicon devices about 0.6V.

Does this diode defeat the battery voltage sensing for the alternator? Is the alternator attempting to charge at 14.4V , but is actually seeing rather less?

If that is the case, both the alernator and battery would be adversely affected. Just seems the alternator fails before 'boiling' the battery dry!

Most modern alternators are 'machine sensed', which generally means any high resistance connections in the charging circuit would result in low battery voltage (so under-charged), but normal load voltage while running.
 
from what I understand the diode is only there to keep the system from backfeeding on a gas engine so it stops run on. I found my book and see there are two different types, a 2 wire and a 3 wire. question is, why wont a 3 wire work? not sure what would be in the tractor that would cause the alt to want to overcharge so much. was showing 14.4 volts and it started to smoke so I unplugged it.
 
Simple, as I see it. The alternator tries to attain a battery voltage of approx 14.4V, but as it senses less than that it increases the rotor excitation in an attempt to keep up with the apparent 'battery drain' (it expects the circuits are using more current than is being supplied, as detected voltage is low) in an attempt to balance the drain and generation. The cycle repeats itself until the alternator output fries the diode bridge or the stator windings. As I say - simple - a lack of truthful feedback to the alternator regulator.

There are millions of alternators out there, controlling the battery voltage at 14.4V and reducing or increasing output to maintain that 14.4V and not burning themselves out in umpteen years, let alone a few minutes. Yours is clearly not doing that. Tho only other alternative is that you have some high voltages which are breaking down the diodes in the bridge rectifier.

Turning off the ignition key should prevent any current going to the coil and distributor, thus stopping the engine.
 
last night I started checking for hot wires. once I turned all the lights on the fuse holder got really hot. jumped the wires on the back of the fuse holder and alternator got pretty warm but never started smoking. going to put a circuit breaker in. could still be bad switch downline but maybe not.
thanks for the help
Gary
 
I would take the alt in and have it tested first. Second is disconnect everything not needed to run the tractor and one by one reconnect them. Smoking usually means a short causing an over load.

Think of it this way, remember the old screw in fuses? Older homes electrical systems were fine for the day but as power consumption increased people plugged to many things in causing the drain to exceed the fuses capacity and they blew. That is where the "smoke" should be. The "clever" realized that if they put a penny in before the fuse it would never blow, the smoke however didn't get the message and the next weakest link burst into flames.

Either your alt has a problem or someone has stuck a "penny" in where the smoke is supposed to be and I'd check the fuse holder. Lighting wiring on these tractors seems poor to me, too many places for a short that would blow that fuse and the "clever" previous owner may have "fixed" it to never blow again.
 

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