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Old 03-25-2013, 07:33 PM
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Alternator not Charging Under Load?

Hey fellas, got a charging question. Swapped out my alternator a few months back under warranty. Tested as having an open diode. It would charge fine most of the time but under load (headlights on), it wouldn't charge at all and the voltage would drop. Now it's doing the exact same thing again.

Just running during the day it does fine, but as soon as I cut on the headlights, the voltage drops. I'm wondering if my replacement alternator could be bad or if there is something else that could cause it. I'd hate to swap the thing out again and it not be the actual problem. Any suggestions on things other than the alternator itself that could cause it to behave like this? I've checked my wiring and it looks good after a quick lookover. The main plug does have a little corosion in it, but I don't know if that would cause it.
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Old 03-25-2013, 08:14 PM
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is it only when you use headlights or do other big loads do this too? could be a short in the headlight harness
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Old 03-25-2013, 08:18 PM
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Didn't think of that. I don't have anything else that really puts a load on it so haven't tested it any other way.
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Old 03-25-2013, 08:35 PM
Can you hear the derail??

 

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Know anyone with a trailer you can hook up and hit the brakes?
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Old 03-25-2013, 08:43 PM
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Yeah I have access to a trailer. Think that'll put enough load on it to notice a difference?
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Old 03-25-2013, 08:47 PM
Can you hear the derail??

 

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Does you problem only happen with head lights or park lamps? Kause a trailer's park lamps combined with yours should draw about as much as the headlights, that and electric brakes as well
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Old 03-26-2013, 05:18 AM
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Ok, I'll hook it up today during lunch break or after work and see what happens.
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Old 03-26-2013, 07:35 AM
Can you hear the derail??

 

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Don't need to be hitched, just plug it in and try it
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Old 03-26-2013, 09:42 AM
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What? Really? I thought the ball had to be hooked and secured for the lights to work.

Tested it out a few minutes ago. Here are the results:

Idling, no load: 14.0-14.2
Idling, headlights: 12.5-13.0
Idling, running lights w/ trailer plugged in: 13.2-13.5
Idling, running lights & brakes w/ trailer: 13.0-13.3
Driving, no matter what the load is: 14.0-14.2

Basically I would just let the alternator be at full voltage, then cut running lights/headlights/brakes/whatever else on and it would drop down to somewhere in the above ranges depending on what I had. Charges fine under load while driving. I'm thinking it's just a bad alternator unless you can think of something else that would cause the same conditions. I suspected bad alternator in the first place, just would hate to swap it out again and it not be bad.
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