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300 alternator troubleshooting

Z

Zachary Hoyt

Well-known Member
I was using my 300 a lot last Thursday-Saturday and on Saturday
morning when I started it the ammeter did not show charge after
startup like it has been doing. Didn't have time to fix it at the
time, but I checked the voltage at the battery with the engine
running and it seemed to be all over the place on my cheap
digital meter. I just converted it from 6 volt to 12 volt three wire
with a Delco 10 SI over the summer and rewired and I am
wondering if I should be checking the little diode in the line or if
I should assume alternator trouble and rebuild the alternator.
Any advice will be much appreciated.
Zach
 
In most cases the alternator will tend the battery so well that (if it starts quiclkly) the needle will stay in the middle most of the time. Use an analog meter to assess the real voltage. Turn on the lights if they are bright (as much as they can be) and the needle stays in the center all is well. Jim
 
Thank you, that is how it did work up until Saturday. I tried the lights at the time and showed about 15 amps discharge, vs about 3 with just the distributor drawing current. Before Saturday it would jump over to about 5-10 amps charge right after the engine started and then after a minute or less it would go right back to 0 and stay there for the rest of the time the tractor was running.
Zach
 
Assuming the wiring is OK, could be the diode has failed open. Try shorting across the diode and see if it now charges. If so, replace the diode.

If it still does not charge, bypass the internal regulator by shorting the test tab inside the D-shaped opening in the rear of the alternator to ground (photo). If it charges hard with the tab grounded, the VR has failed - a new VR or replacing the alternator is the fix.

However if it doesn't charge with the tab grounded there's something else wrong with the alternator. Best solution is replace with a rebuilt alternator.
IMG_635612SIalternatorbackside-crop.jpg
 
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