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Old 07-20-2010, 01:45 PM
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Voltage drops under load when engine is warm

I have a 1996 Astro Van. When the engine reaches operating temp the voltage will slowly drop till the light comes on when ever I put a load on (fan, lights, etc). If I shut off the load the voltage will rise back to 14V. Only happens when the engine is warm. The alternator is less than a year old and checks out OK. Did not notice this till a few weeks ago. Battery was 5 yrs old so I replaced it. I am thinking this is a loose/corroded connection. I have checked the obvious places and now looking for suggestions.

Thanks

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Old 07-20-2010, 02:07 PM
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Quote:
Originally Posted by heyjude076
I have a 1996 Astro Van. When the engine reaches operating temp the voltage will slowly drop till the light comes on when ever I put a load on (fan, lights, etc). If I shut off the load the voltage will rise back to 14V. Only happens when the engine is warm. The alternator is less than a year old and checks out OK. Did not notice this till a few weeks ago. Battery was 5 yrs old so I replaced it. I am thinking this is a loose/corroded connection. I have checked the obvious places and now looking for suggestions.

Thanks
you still got the rubber coating on the battery cables ?
they corrode between the innards ,cut off and clean
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Old 07-20-2010, 05:05 PM
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Quote:
Originally Posted by heyjude076
I have a 1996 Astro Van. When the engine reaches operating temp the voltage will slowly drop till the light comes on when ever I put a load on (fan, lights, etc). If I shut off the load the voltage will rise back to 14V. Only happens when the engine is warm. The alternator is less than a year old and checks out OK. Did not notice this till a few weeks ago. Battery was 5 yrs old so I replaced it. I am thinking this is a loose/corroded connection. I have checked the obvious places and now looking for suggestions.

Thanks
The battery is a good suspect as it should have ample reserve capacity and hold a charge well for a good length of time. You should be able to hit it with a load test ( after charing the battery with a charger) and get 200 amps without falling below 10 or 9.6 volts and sustain that for about 15 seconds. If you have a midtronics tester they are pretty accurate and take all the guess work out of it. If the battery tests good then I would suspect the alternator has an internal problem like diodes open circuit or a regulator that is washing out with heat.
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Old 07-21-2010, 04:59 AM
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Originally Posted by latech
If the battery tests good then I would suspect the alternator has an internal problem like diodes open circuit or a regulator that is washing out with heat.
Since it was doing this before the battery was replaced I am doubting it is the battery. I did notice yesterday that it does this with out a load only not as radical. The voltage slowly drops to <12V and then climbs back to 14V. It is almost like a hot/cold cycle. This is pointing me to the alternator even though it is almost new. Will swap it out this weekend.

Thanks for the suggestions.
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Old 07-21-2010, 08:52 AM
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You most likely have a bad connection. I would check all grounds and main feeds. I would be willing to bet you find a bad ground connection somewhere. As the engine warms the connection will get worse if it is loose.
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