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Tool Talk Discussion Forum

how to test alternator on lawn tractor

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Bill Rolland

03-14-2007 18:55:38




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Have a lawn tractor (Craftsman), 12-volt system, battery won't charge. Methinks it's the alternator. How do I test if an alternator is putting out enough juice to recharge the battery? I know this is pretty elementary stuff, but I barely made it out of preschool...




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dave guest

03-15-2007 17:30:18




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 Re: how to test alternator on lawn tractor in reply to Bill Rolland, 03-14-2007 18:55:38  
You got good advice. Also alt on mowers is very small sometimes and does not have brushes. Voltage increase over batt static voltage might only be half amp. Example batt reads 11.5, you could see only 12 when running. Good luck



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Ralph - Ohio

03-15-2007 05:21:47




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 Re: how to test alternator on lawn tractor in reply to Bill Rolland, 03-14-2007 18:55:38  
I'm not familiar with your particular engine but a very common no-charge problem on lawn tractors is a poor ground connection at the rectifier/regulator. The mounting screws can get loose or rusty or there is sometimes paint or corrosion between the rectifier and the metal on which it's mounted. For the rectifier to work it has to be well grounded. I'd suggest checking this first before replacing anything.

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MarkB_MI

03-15-2007 03:33:56




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 Re: how to test alternator on lawn tractor in reply to Bill Rolland, 03-14-2007 18:55:38  
More likely a bad battery than a bad alternator. Just to add to the previous posts, if you have at least 13.5 volts at the battery and the battery doesn't seem to charge, replace the battery.



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cat236

03-14-2007 21:33:09




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 Re: how to test alternator on lawn tractor in reply to Bill Rolland, 03-14-2007 18:55:38  
Lets start with basics. Take a voltmeter and place it on the 20VDC scale.Connect leads to battery terminal and test battery voltage. Lets say your static voltage is 12.5VDC. Now start engine and run up to WOT, wide open throttle. You should get a reading anywhere from 13.2 to 14.5VDC. On many Sears units the save you money by installing low amp charging systems usually 2-3 amps. Therefore typically you will only have one lead from the stator being changed to DC voltage, your other line out will be AC voltage dedicated to the light circuit. There are numerous arrangements of these wires and the easiest way to determine what charging circuit you have is by the color of the wires coming out of the stator.If you find your voltage does not change it would be helpful to identify your system but not entirely necessary. If voltage does not change test for voltage at battery side of solenoid with KEY ON/ ENGINE OFF.If you have voltage here and there still is no change most likely a problem with a failed diode or external regulator.If no voltage here trace wire and find defect in wire.

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ebbsspeed

03-14-2007 20:09:51




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 Re: how to test alternator on lawn tractor in reply to Bill Rolland, 03-14-2007 18:55:38  
More likely to be the rectifier/regulator module than the alternator itself. To test the alternator, first, with the engine off, unplug the two leads from the alternatot (coming out from under the flywheel somewhere) and using an ohmmeter, measure between each lead and ground. You should see infinite resistance. Now measure the resistance between the two leads, and you will have somewhere between 0.5 to 3 ohms, depending on the current rating of the alternator. Now set your meter on the AC Volts setting, and start the motor. You should see somewhere around 25-45 volts AC measured across the two leads. If that's what you have, then your alternator is probably OK.

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old art

03-16-2007 11:25:30




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 Re: how to test alternator on lawn tractor in reply to ebbsspeed, 03-14-2007 20:09:51  
on my sears mower the altinator has a diode in the lead for the charging system it is in the plug in the wire that comes out of the engine to test use a ohm meter take meter test to ground fromeach wire in plug then reverse the leads of meter and check again one wire should read one way and not the outher way if the diode is shorted it will read both ways you can replace it your self get a new one at radio shack . you can put it in backwards so checkit out first.

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