Voltage drop upon acceleration?

BOB TEELE

MEMBER
2012 GMC ACADIA
105,000 MILES

When I first start my engine, the voltage is under 13v and in a few seconds there's a tiny rattle sound and then a noticeable click (like something locking into gear) and it jumps up to above 14 plus volts as I accelerate it drops down below 13 again then when I stop at a sign it jumps right back up. There is a new battery, alternator, tensioner pully and idler pully and all the connections are tight.

1 Reply

AL514

MECHANIC
4,795 POSTS

Hello, how are you monitoring the charging system voltage, by the display on the instrument cluster? The clicking noise you are hearing is most likely a relay in the passenger compartment fuse panel. More modern vehicles can run at lower charging system voltages now because they employ a battery current sensor, which also includes a battery temperature sensor. Are any codes setting in the engine computer, any check engine light coming on?
They are multiple manufacturers that will drop charging system voltage down as low as 12.8volts. Service information for your vehicle states that in Economy Mode the target charging system voltage is 12.5 to 13.1volts. I'll post this for you. See what happens when you put some electrical load on the vehicle, like headlights, AC system, etc. It should rise to 13.9 to 15.5volts.
Hondas will drop down to 12.8 at idle, and it can appear that the battery or Alternator is at fault when its normal operation. But I will post some of this service information for you.
A code will set a (DTC) if there is an issue with the charging voltage level and the Check engine light will come on plus a message on the display (3rd diagram below).

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