Checking an alternator

Charlie M · Feb 24, 2011

About 20 years ago I converted my M to an alternator and 12 volts and has worked great the whole time. Now I don't see any movement on the amp gauge. I'm either not charging or have a bad amp gauge. What's the test for the alternator - is it just measure the charging voltage at the battery? If so what would indicate the alternator is charging OK. I'm thinking most likely the amp gauge is bad.

Wisconsin Cowman · Feb 24, 2011

Get you volt meter and take the red led on the + battery wire on alternator and the black led to ground on alternator somewhere and see what you get should around 14.0+/- volts if everything is working right. Also like to check it at the battery too.

old · Feb 24, 2011

Best way and the only safe way is to use a volt meter. Check battery voltage with out the engine running first. You should get 12 volts maybe a tad bit more. Then start the engine and and recheck and you should see 13-14 volts give or take a little bit. If you do not get those 2 readings then you have a problem. If battery volts do not read 12 plus then you have a low battery of a bad battery. If when running you do not get 13 plus then the charging system has a problem. DO NOT pull a battery cable off with it running to check it can/will kill the alternator or could also kill you

IBorange in TX · Feb 24, 2011

just a quick way to see if the alternator is charging, place a 16penny nail close to the aluminum color center of the alternator. If the alternator is charging, the center will be magnetized and will hold the nail.
HTH
mb

Bob · Feb 25, 2011

Yeah, RIGHT!

Brownie 450 · Feb 25, 2011

With this much time on the alt. the brushes are probably worn or hanging up in the brush holder. Especially in dusty conditions. If your voltage checks show the alt. isn't charging, I'd take it apart & look at the brushes. They are cheap.

gurneejoe · Feb 25, 2011

If the unit you have is the internal regulator Delco type, there is a 'D' shaped hole in the back of the case near the plug in which you can take a small screwdriver and touch the tab of metal at the bottom of the hole to the case of the alternator while it is running and it will bypass the regulator giving you full output at the battery connection if it is working. If you get full voltage, the regulator has failed. If partial voltage, the regulator and possibly one of the diodes is bad. If you can get the separate components anymore, they are very simple to replace. Rebuilt units are pretty reasonable now too. Most likely NAPA if they are to be had.

Janicholson · Feb 25, 2011

Go with Old's process below. Max voltage should be 14.6 Maybe 14.8, If it was that high you would see needle movement. If the lights work, the amp gauge will respond to their draw if the tractor is off. This tells you that the gauge is working. Which alternator, and how it is connected is very important for determining what specific things beyond these, to suggest. The nail placed near the rear frame center bearing will show magnetic attraction if the field is getting electricity. It does not indicate that it is actually charging. Jim

teddy52food · Feb 25, 2011

That only tells you that the field rotor is energized. If the diodes are bad or stator windings are open or shorted it still won't charge.

Checking an alternator

Charlie M

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Wisconsin Cowman

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old

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IBorange in TX

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Bob

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Brownie 450

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gurneejoe

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Janicholson

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teddy52food

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