Thread: 10si alternator

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Does anybody know how to test an alternator ? With lights on at red light volts drop to about 10. As soon as i get rpm to 800 it charges 14 volts. 2-5/8 alt pulley. Standard size crank pulley. I have an old tandy engine analyzer but i forgot how to hook it to alt to read amps. Any help is appreciated

That's pretty normal if there was an underdrive pulley installed onto the alternator. See if you can locate a larger od pulley for the alternator. A lot of guys ran underdrive pulleys on their 5 point oh Mustangs, it's okay during the day but night time cruising can be a pain at the stop lights.

Like Mike said, 10 Volts under 800 RPM with the lights and what ever other accessories you running (like AC or heater) but charging at 14 Volts above 800 actually sounds normal with a 10SI.

I think Mike meant to say go with a smaller alternator pulley to speed up the alternator....the other option would be a larger drive (crankshaft) pulley......or you could just speed the idle up a little if it bothers you.


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Originally Posted by 33plym

Does anybody know how to test an alternator ? With lights on at red light volts drop to about 10. As soon as i get rpm to 800 it charges 14 volts.

This is often caused by resistance, either by a bad connection or undersized wiring. A bad internal diode can also be to blame.

Originally Posted by 33plym

2-5/8 alt pulley. Standard size crank pulley.

Those are the standard sized pulleys for this alternator and should work fine.

I guess there could be some wiring/ connection issues. The sat for a long time. Theres not much load on the alt, basically lighting. If i run ac then it gets pretty ugly. Didn’t know if upgrading to a 100 amp would be of benefit or would it start melting wires. Best bet is to pull alternator off and have it tested.

I would also ensure that none of the grounds have been compromised.

Originally Posted by NTFDAY

I would also ensure that none of the grounds have been compromised.

Can not over state how important good clean grounds are. Negative battery cable needs to be bright and clean and preferable bonded directly to a bare spot on the block with jumpers to the body and frame. Make sure all connections are tight and use star washers where at all possible.

i run a 1 inch SS braided cable from block to frame and a 1/4 in one to the body .
one thing on a car that sets is cables corrode over time. i just replaced the cables on my f250 and cut the sheath off the old ones. lot of corrosion inside . seems common on diesels .
at low rpm you should not drop below 12v unless your running a lot of high amp stuff . might check your battery also .

I took ground wires off today and cleaned em up. I also took the alternator to an alternator shop and it checked out ok. Cleaning ground wires did help at idle. With only the lights on at 625 rpm idle volt gauge is slightly over 12 volt. At slightly less than 700 rpm volt gauge is at 13.5. Guess i either need a slightly smaller pulley on alt or just bump idle speed up.

Bumping the idle speed up a tad is real easy and costs nothing..

grounds are your friend . i think without doubt that grounding problems cause the lions share of problems . thats why i started doing overkill on grounds . a quality buzz bar inside for grounding all your dash components is a plus also .

Originally Posted by shine

grounds are your friend . i think without doubt that grounding problems cause the lions share of problems . thats why i started doing overkill on grounds . a quality buzz bar inside for grounding all your dash components is a plus also .

Ditto what shine says - I use a busbar with a direct #10 to negative post on the battery. Solves a lot of issues. When reassembling my '63 Nova I also added ground(s) at all four corners for the lighting/turn signal circuits. Make sure metal is clean and bright and use star washers. Certainly worth the extra effort and kills a lot of the gremlins associated with electrical issues.

After cleaning all the ground wires, snugging the belt up and bumping idle up, i guess its as good as it gets. If my memory is half right 10si alternators were a little lazy at idle. Thanks to all for making suggestions

you should never be below 12v . if your amp usage is higher than your alternator out put then is will drop . but that is hard to do . i would also check my battery .