need help with multi meter testing an alternator
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Powerstroke Doctor
Joined: Aug 2002
Posts: 474
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From: Loxahatchee, FL
need help with multi meter testing an alternator
well for the past couple of days when my truck sits more then 8 hours it doesn't want to start. so i went a bought a multimeter today and i checked the voltage across the batteries when its not running its 12.86v when runing it is 13.65. I then disconnected the batt wire from the alternator to check to see if there is drainage but i wasn't sure how to do it. i put one probe i=on the alternator and the other one on the cable. i wasn't sure what to check so i checked volts and it was 12.86 then i checked mA and it was beeping 1.000 with 2mA selected so i put it on 200mA and it was 2.5. could somebody please help me
Of the next Riffraff East
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From: Concord NC
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Senior User
Joined: Sep 2007
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From: VA
..038 mA in fine I have to agree with zx250 the cranking voltage is the next step keep us updated
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Post Fiend
Joined: Feb 2006
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From: Middle Tennessee
Originally Posted by 88f1504x4
well for the past couple of days when my truck sits more then 8 hours it doesn't want to start. so i went a bought a multimeter today and i checked the voltage across the batteries when its not running its 12.86v when runing it is 13.65.
Post Fiend
Joined: Feb 2007
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From: Blairsville,Ga
Originally Posted by Tenn01PSD350
I am not so sure I like the alt output # either. I like to see about 14 or so when running.
Post Fiend
Joined: Feb 2006
Posts: 11,892
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From: Middle Tennessee
Originally Posted by rbaker6336
think that reading was across batteries,if they ain't up to par niether will the alt reading be
Post Fiend
Joined: Feb 2007
Posts: 7,191
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From: Blairsville,Ga
my batteres were just over a year old when both bit the big one,while trying to get a charge in them thinking one had to be good charger was readng 14.8 bateries were reading 12.7 with charger attached
bad thing was 700+ miles from battery tester
bad thing was 700+ miles from battery tester
Last edited by rbaker6336; Sep 2, 2007 at 07:13 PM.
Elder User
Joined: Dec 2006
Posts: 718
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From: Chattanooga, TN
Suggest you drive it to a local auto parts store and have them do a battery load test -could be one battery is bad. If batteries are determined to be good, then remove the alternator and have them do an output test on it.
Sometimes an alternator going bad can cause the primary battery to break down.
A mechanic friend (works on large trucks) told me how to check the output of the alternator without a VOM meter - take a compass and hold it at the battery terminal on the alternator while engine is running; if the arrow points to the alternator, there is no output; if the arrow points away from the alternator, it has output to the battery. Interesting.
Good luck,
Sometimes an alternator going bad can cause the primary battery to break down.
A mechanic friend (works on large trucks) told me how to check the output of the alternator without a VOM meter - take a compass and hold it at the battery terminal on the alternator while engine is running; if the arrow points to the alternator, there is no output; if the arrow points away from the alternator, it has output to the battery. Interesting.
Good luck,
Elder User
Joined: Mar 2003
Posts: 699
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From: northern Michigan
If your battery post and connectors are clean and your alternator shows 13.65 volts, I would suspect either a load in the truck drawing juice that shouldn't be or a battery with an internal short. two test to try are:
1). measure the battery voltage while cranking with charged up batteries. It should remain above 9 vdc or you need new batteries.
2). With the batteries charged up, remove the ground connections and leave sit 24 hours. Read battery voltage then, if you have one or both showing low, you have an internal short and need to replace both batteries.
In twin battery applications like this, if one is bad, you replace both.
1). measure the battery voltage while cranking with charged up batteries. It should remain above 9 vdc or you need new batteries.
2). With the batteries charged up, remove the ground connections and leave sit 24 hours. Read battery voltage then, if you have one or both showing low, you have an internal short and need to replace both batteries.
In twin battery applications like this, if one is bad, you replace both.