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Duramax Second Generation: 2004.5-2005 (LLY) Discuss the second generation (2004.5-2005 LLY) of the 6.6L Duramax diesel engine & associated components. 2006 LLY engine discussion belongs in Third Generation forum due to the differences of the 2006 LLY and the 2004.5-2005 LLY. Engine related discussion ONLY.

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Old 02-28-2017, 02:45 PM   #1 (permalink)
mnnmaz
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LLY dual alternators HELP

How to troubleshoot dual alternators? Which one is it? Batteries are dead every week (dont drive it but once a week, its mainly used to pull 5th wheel)

HELP, this all just started the last 3 weeks.

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Old 02-28-2017, 03:39 PM   #2 (permalink)
davester
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Is it the alternators not charging the batteries (gauge indicates it isn't charging), or is something in the truck draining the batteries while it sits?

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Old 02-28-2017, 04:15 PM   #3 (permalink)
CHEVYJEFF
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disconnect one and put a volt meter on positive terminal of the other and look for about 14 volts at 1500 rpm then repeat process for other one?
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Old 02-28-2017, 07:09 PM   #4 (permalink)
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Gauge states its charging at 14V....not sure if its draining from something
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Old 02-28-2017, 07:32 PM   #5 (permalink)
mnnmaz
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Quote:
Originally Posted by CHEVYJEFF View Post
disconnect one and put a volt meter on positive terminal of the other and look for about 14 volts at 1500 rpm then repeat process for other one?
Are you saying disconnect the wires (plug) from the alternator?
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Old 02-28-2017, 07:54 PM   #6 (permalink)
Ramblinrodney
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Quote:
Originally Posted by mnnmaz View Post
Are you saying disconnect the wires (plug) from the alternator?
Yes that's what he is saying.

There's a lot of YouTube vidios showing how to test your alternator output with a volt meter.

Found this
Try this procedure to verify alternator outputs:

With the engine running at a high idle, 1200-1500 rpm: Check the system voltage with a load on (headlights, heater blower). Should maintain 13.0-15.0 volts.

Turn the engine OFF: Disconnect the regulator/control plug on one of the alternators (only do one of them at a time). Run the engine again (load on, same as above) and check system voltage again. Should still be 13.0 to 15.0 volts.

Turn the engine off. Reconnect the plug on the one alt. and disconnect the other. Repeat the test again.

If the system voltage is substantially lower with either alternator unhooked, then the one still connected is bad.

Also: disconnecting the alt. reg/control plugs may throw a DTC (and maybe give an "SES" light). It should clear itself after a couple of drive cycles.
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Last edited by Ramblinrodney; 02-28-2017 at 08:11 PM.
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