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Gm Tech (Cam)
Gm Tech (Cam), GMC Mechanic
Category: GMC
Satisfied Customers: 1672
Experience:  GM Grand Master Technician 2007. 14 year experience.
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I bought a used 2009 gmc seirra sle 4x4 and shortly after

Customer Question

I bought a used 2009 gmc seirra sle 4x4 and shortly after the volt meter would slowly drop to 12 volts and comes back up after a while. the trans starts slam shifting and the cruise stop working, then the other night the lights started flashing and door locks started cycling and speedometer started bouncing. pulled over and stopped and restarted and worked ok the rest of the night. whats up?
Submitted: 1 year ago.
Category: GMC
Expert:  Tim Mohr replied 1 year ago.

Hi, my name is ***** ***** you installed the new alternator, you said it dropped down to 13 volts, but did the light flash and the transmission slam still, or just the voltage drops?

Customer: replied 1 year ago.
I did not drive, just put old alt back on. when it does this you can rrestart and volts come back. the lights and door locks was new. the trans only does the hard shift occassionally and get check engine light sometimes when volts drop. the code was like 700 something which is trans problem. I also am a master mechanic and have been building cars since the 60's and have a very good expertise with electronic repairs
Customer: replied 1 year ago.
a gm service manager told me that there is a problem with dash units on some of these trucks
Expert:  Tim Mohr replied 1 year ago.

This vehicle uses an Electrical Power Management (EPM) for the charging system. This system monitors electrical items such the state of the battery, vehicle electrical draw and driving conditions. . If the EPM system selects a "Fuel Economy Mode", the battery voltage will drop to about 12.5 volts until another mode is needed. So it is likey the old alternator was bad, so I would reinstall the new alternator and see if the driving problems are gone npw, it is normal operation for this system to drop the voltage, as long as it does not go below 12 volts,Tim

Customer: replied 1 year ago.
dont buy that. my son has exact same truck only 08 and his has never done this. the trans is intermitent and the door lock and flashing light and dash module has only done this once. thanks anyway i'm going to unload this pos
Expert:  Tim Mohr replied 1 year ago.

this comes straight from the GM manual , you dont have to buy it, but it is the truth,Tim

1. This vehicle uses an Electrical Power Management (EPM) for the charging system.

2. This system monitors electrical items such the state of the battery, vehicle electrical draw and driving conditions.

3. If the EPM system selects a "Fuel Economy Mode", the battery voltage will drop to about 12.5 volts until another mode is needed.

4. Check for charging system related codes in the Body Control Module (BCM), and Engine Control Module (ECM). Verify the battery tests good and all wiring is in good condition.

5. If all OK, this is a normal condition.

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