alternator questions 10SI JD 4020 diesel

55 50 Ron

Well-known Member
Would this rust on an alternator cause it to put out low voltage? It was putting out about 9 volts. Also how do you keep the brushes in place for re-assembly? This is my first alternator repair (maybe it will be my last!) The brushes have tiny holes through them that obviously must have a pin of some sort to hold the brushes. Are these pins (or whatever they are) tied to a string of some kind to be pulled out after partial re-assembly has been accomplished.

Please don't laugh since I'm a "duffer" at this. Thanks

cvphoto36360.jpg
 
The rust is not the problem.

Low output is commonly caused by a bad diode trio, the little black square beside the regulator. It can be checked with an ohm meter, open between the common leg and each of the 3 terminals one way, continuity the other.

When putting it back together, be very careful to install the plastic insulators back where they came from.

There is a small hole in the back to insert a wire to hold the brushes in.
Parts View
 
Yes the rust will affect the output. Take a small nail or wire, push the brushes in place and you will see a hole through the back to put the wire through
 
There is a hole on the back side of the alternator case that will line up with the brushes. I insert a small plastic straw like on a can of brake cleaner, or a o-ring pick, to hold the brushes up until assembled. As far as the 9 volts, are you sure that is not just battery voltage.

Also on the back of an internally regulated Delco, there is another 'D' shaped hole that will allow you to bypass the internal regulator by inserting a pocket screwdriver into it. Make sure the screwdriver touches the side of the case as well as the spot on the regulator. Do this while the engine is running with a way to measure voltage attached.

Beagle
 
Good info already, new brushes come with a pin in them you pull out when back together! If apart I would put a new kit and bearings in.
 
Thanks for the good info. The 9 volts was measured at the alternator output terminal with engine running but not at full throttle. The belt was very tight. I just have the unit apart but haven't tested any internal parts. It is very possible that there was no alternator output and I was just measuring a bad battery at 9 V.
 
There are several failure modes for these alternators. The diode trio is a common one, but the main diodes can also fail, as can the internal regulator.

There are plenty of resources online to help you troubleshoot and rebuild the 10SI. You will need a multimeter, but it's not difficult at all.
 
I do have a nice Fluke multimeter and 3 tool boxes full of common and not so common tools. I have looked at some You Tube videos and am amazed at how good they are. Thanks again to all you who responded.
 

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