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Old 09-09-2021, 06:11 PM #1
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Voltage Drops When Engine Warms Up

I noticed my voltage dropping when engine has been warm for like 30 minutes or so. When starting rig, my voltmeter, connected directly to battery terminals, reads 14.1, and then hovers around 13.9 but like 30 minutes later it drops to 13.5-13.6. When I run A/C it even drops to 13.4, have yet to see anything lower. My battery and alternator are kinda new. I have no flickering lights and otherwise all seems normal. What could it be, or is this normal?
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Old 09-09-2021, 08:23 PM #2
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100% normal, and operating as expected.
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Old 09-09-2021, 10:45 PM #3
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I thought anything below 13.7V could be an indication alternator is on its way out. Could you explain your point that my scenario is normal. Thx.
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Old 09-10-2021, 09:48 AM #4
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it just the operating temp.

is your alternator a rebuild for high output or just the replacement you get at the auto store.

if you are using high output, are you running a shorter belt.

agm battery? they will require a higher charge voltage.
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Old 09-10-2021, 11:03 AM #5
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The normal output voltage range is 13.2 to 14.2

There is a shot of the relevant maintenance manual page in one of the threads around here.
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Old 09-10-2021, 12:26 PM #6
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Quote:
Originally Posted by riceandpho View Post
it just the operating temp.

is your alternator a rebuild for high output or just the replacement you get at the auto store.

if you are using high output, are you running a shorter belt.

agm battery? they will require a higher charge voltage.
Alternator is a lifetime warranty rebuilt Denso which I got at Napa Auto Parts 3 months ago. It's not a high output alt, and battery is not an AGM, also 3 months old.
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Old 09-10-2021, 01:58 PM #7
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Quote:
Originally Posted by Urbanite View Post
I thought anything below 13.7V could be an indication alternator is on its way out. Could you explain your point that my scenario is normal. Thx.
That is false.

State of charge and temperate compensation matter when charging a lead battery. The voltage will be variable depending on those two conditions.

Voltage alone is a poor indicator of alternator health. There are more appropriate industry standard tests for this including looking at AC ripple and load testing. If you can't get the rated amps out of the device, that would be a much larger concern.

Anything over 13v you are basically still charging the battery when you consider maintenance / float voltages are generally about 13v +/-

For a DIY approach, you are better off using an amp clamp meter, throwing a huge load on the truck, and measuring it's output in amps to make sure its functioning as expected.

Last edited by Bumbo; 09-10-2021 at 02:51 PM.
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