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Old February 3rd, 2011, 13:31   #1
skicat
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Default alternator wiring

i have a 2001 jetta 155000 miles manual trans
i have charging issues
my battery would die while driving so i figured it was my alternator. i pulled and had the alternator tested and it was good.
so i pulled the battery and tested it. it was great. i disconnected my stereo and sub thinking maybe they are drawing to much. still the battery dies.
the battery reads 12.3V while off, 11.8 while on.
hooked my meter to the alternator and it read 14.2v while running. so some how power is not getting to the battery. i followed the wires they all look good. sprayed some with contact cleaner just cause but cant figure it out.

does anyone know what i can do next where can i look for a short

Last edited by skicat; February 3rd, 2011 at 13:35.
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Old February 3rd, 2011, 13:45   #2
oilhammer
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Bad ground? Electricity travels in a circle.

Bad alternator clutch pulley causing poor charge under load? It is a sprag.

Check the fuse holder atop the battery for melting or corrosion.

Correct 80Ah battery in the car?
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Old February 3rd, 2011, 14:16   #3
skicat
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pulley is good
checked the fuse holder cleaned it up appears good no melting or corrosion
yes its the correct battery
where can i check to see if there is a bad ground
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Old February 3rd, 2011, 14:21   #4
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Start at the negative battery cable and follow... The cable goes under the battery tray, attaches to the body, then continues on over to the bellhouse bolt nearest the #4 cylinder glow plug.

FWIW, I never trust the off-the-car alternator tests. I never can understand how you can test them that way, no more than you could test someone's pulse with their heart taken out of their chest. Just see too many bad ones that Autostone said were good.

You can also be doing voltage drop tests on every circuit from the alternator to the battery to the body back to the alternator case.
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Old February 3rd, 2011, 15:10   #5
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You can follow the big grounds... and the "big hots"... they are pretty short too.

Multimeter on the alternator output reads 14.2 V.... what doe it read by the time it travels over one big red wire gets to the alternator fuse on top of the battery, extreme left hand side? On the other side of that fuse?
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Old February 4th, 2011, 10:05   #6
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skicat- I agree with Vince Waldron. Believe you're (both) on the right track- if you've got 14V at the alternator and a significantly different reading at the battery, that strongly suggests the alternator is working, but it isn't electrically connected to the battery. Definitely check the ground cable as recommended above. Also check (trace) the fat wire off the alternator post (where I presume you're measuring 14V) its full distance up to the terminal block on top of the battery. There's a fusible link there. Search for my recent posts. Out of coincidence yesterday I made a reply and included a couple links to info on this forum which shows pictures of the battery and terminal on top, focusing on the alternator feed wire. Two possibilities I can think of are: [1] the alternator feed wire has gotten salt corrosion in it over the years and may have rotted through inside, you can't see it from outside. [2] the alternator fusible link on the terminal block is open.

Good luck. Let us know what you find.
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Old February 11th, 2011, 15:54   #7
skicat
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after too many hours of wondering i found my problem to be a short on fuse box on top. i follwed the main power wire from the alternator testing the voltage at any point i could. it was 13.7 where it goes in and 12.1 where it comes out. i took the whole thing apart and found a small burn mark on the bottom of the pseudo fuse thing. it was minor but enough i guess.i sanded it down and now it works. thanks for the help.
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Old February 11th, 2011, 15:58   #8
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That's good news... and thanks for taking the time to come back to this thread and post the solution. So many threads end up unresolved and that's where all the learning is!!

Vince
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Old February 12th, 2011, 08:34   #9
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Quote:
Originally Posted by oilhammer View Post
Bad ground? Electricity travels in a circle.

Bad alternator clutch pulley causing poor charge under load? It is a sprag.

Check the fuse holder atop the battery for melting or corrosion.

Correct 80Ah battery in the car?
Hey, sounds like that guy hit the nail on the head, eh?

FWIW, that is somewhat common. I would REPLACE that fuse holder and any and all fuses that look the least bit suspect, as well as carefully inspecting the end of the lead from the alternator, and if it too looks suspect replace as well.
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