Morning everyone. So I have a headache. So I ran a test over the weekend due to a dead battery Thursday and now buying a new one. I went ahead and checked the voltage with the new one, 12.4 resting and then 14.9 after starting. Now, once it’s warmed up and been driven, occasionally at idle my voltage needle lightly bounces back and forth. And also once the truck is warmed up the voltage eventually settles a notch below 14 volts, when at start it’s a hair above 14. Any suggestions on what could be causing this fluctuation? Thanks.

 

You may want to have a look at this:

https://www.underhoodservice.com/alt...rrect-voltage/

https://www.w8ji.com/battery_and_charging_system.htm

 

What year is your blazer? Btw, gauges in Gm aren’t very accurate...I wanna say it was somewhere around 00’ or 01’ the Ecm controlled the charging of the alternator....which would easily explain higher voltage at startup! 😉 There is info. Somewhere on this site regarding it. I wanna say Captain Hook was the the thread starter....? It’s been a ‘few’ years since I had my blazer though...🤔

 

You can check with an OBDII reader. I think that system voltage as seen by the ECU should show. That is quite accurate as it is needed to properly pulse the injectors. Else it's digital multi-meter time.

Mine is a 94, so the voltage is not controlled by the ECU.