Alternator question
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Senior User
Joined: Apr 2002
Posts: 201
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From: Schaumburg, IL
Alternator question
I've had ongoing alternator problems. I've replaced it several times, and had the charging system checked several times. Each time it's gone out suddenly and totally. Recently, it's started giving real low output, and I have to put my battery on a charger every few days. The battery seems fine, and charges right up. If I rev the engine up to about 2,000 rpm the charge goes into the normal range, so it seems like the alternator is capable of charging ok, but it's just not doing it at normal idle speed. Any ideas/suggestions?
Also, with the alternator problems I've had in the past, a friend suggested adding a second ground off the battery. Any thoughts about that idea?
Thanks for your help.
Also, with the alternator problems I've had in the past, a friend suggested adding a second ground off the battery. Any thoughts about that idea?
Thanks for your help.
Cargo Master
Joined: Dec 2004
Posts: 2,232
Likes: 3
From: NE Texas
You need to do a voltage drop test.
Using a digital voltmeter on DC setting, measure between the alternator case and the ground post of the battery. You should not drop more than .1V at the most. Then measure between the output post on the alternator and the positive post of the battery. Again there should be VERY little voltage between these two points. If there is a voltage drop with either of these tests, find and fix the connection, wiring, ground or whatever problem you find.
Good luck,
Doc
ASE Master
Using a digital voltmeter on DC setting, measure between the alternator case and the ground post of the battery. You should not drop more than .1V at the most. Then measure between the output post on the alternator and the positive post of the battery. Again there should be VERY little voltage between these two points. If there is a voltage drop with either of these tests, find and fix the connection, wiring, ground or whatever problem you find.
Good luck,
Doc
ASE Master
Posting Guru
Joined: Oct 2005
Posts: 2,209
Likes: 1
From: Louisville, Kentucky
Originally Posted by MBDiagMan
You need to do a voltage drop test.
Using a digital voltmeter on DC setting, measure between the alternator case and the ground post of the battery. You should not drop more than .1V at the most. Then measure between the output post on the alternator and the positive post of the battery. Again there should be VERY little voltage between these two points. If there is a voltage drop with either of these tests, find and fix the connection, wiring, ground or whatever problem you find.
Good luck,
Doc
ASE Master
Using a digital voltmeter on DC setting, measure between the alternator case and the ground post of the battery. You should not drop more than .1V at the most. Then measure between the output post on the alternator and the positive post of the battery. Again there should be VERY little voltage between these two points. If there is a voltage drop with either of these tests, find and fix the connection, wiring, ground or whatever problem you find.
Good luck,
Doc
ASE Master
You may also want to check the serpentine belt and belt tensioner. 
Senior User
Joined: May 2004
Posts: 170
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From: DFW, TX
I had this exact problem at about 125k miles on my 99 5.4L. The problem turned out to be the alternator field fuse. The fuse wasn't blown and would check out good on a multimeter. The fuse was cracked internally and would make and break contact causing what looked like a bad alternator. You could see the crack if you looked carefully at the fuse body.
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Senior User
Joined: Apr 2002
Posts: 201
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From: Schaumburg, IL
I tested the alternator as Doc suggested. The readings were .055 and .015, so apparently within specs. But I'm still getting a very low charge unless I rev the engine way up, then the charge is in the normal range. The belt seems fine, no slipping that I can tell. The fact that it is charging at all, and normal at high revs, makes me think that the alternator (and fuse) are ok. I even hooked up a second ground, in case the cable was bad, but it made no difference. The battery charges normally on a charger, and takes a full charge, so the battery seems ok. Any other suggestions, guys?
Thread Starter
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Senior User
Joined: Apr 2002
Posts: 201
Likes: 0
From: Schaumburg, IL
are you talking about the fuse in the fuse box, or is there an internal fuse in the alternator?
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Senior User
Joined: May 2004
Posts: 170
Likes: 0
From: DFW, TX
There are 2 fuses:
Fuse #11 Alternator Field 20amp located in the Battery Junction Box (Under the hood, drivers side, by the fender) Check this one first! Very carefully examine the metal part inside the plastic case for a crack. Wiggle the legs while checking it.
Fuse #30 30amp located in the central junction box (under the removeable dash panel, drivers side, left knee level)
Fuse #11 Alternator Field 20amp located in the Battery Junction Box (Under the hood, drivers side, by the fender) Check this one first! Very carefully examine the metal part inside the plastic case for a crack. Wiggle the legs while checking it.
Fuse #30 30amp located in the central junction box (under the removeable dash panel, drivers side, left knee level)
Last edited by jcstratt; Feb 25, 2006 at 09:05 PM.
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