I just changed my t-stat and hoses and went for a test ride, when half way through the battery light went on. I checked it with a meter and the battery is showing about 12.5v (not running) and 14.4v running (even if I rev the engine).

Any idea on why the light would be on if the regulator/ alt./ and battery seem to be ok??

 

Check all the alt connections making sure they're fully plugged in and or tight. Double check any other connections you may have unplugged replacing the thermostat.

 

Yeah, I did that and everything looks good. I did charge the battery and the light is now off. I guess I'll drive it to see if it drains again, or just get the battery tested to see if it's that or the alt...probably the battery since it's an old Optima I stole out of my drag car.

 

When it's cold out, usually happens after left out overnight, the battery light will blink. The odd time it'll come on for a bit, but go away after I restart the truck. It used to do it more a couple years back.
Chances are you're battery might be going out. I haven't replaced the battery in mine and I'm not sure if the previous owner did. I've had it for 4 years now.

 

here is a little suggestion.......when i changed my t-stat i got the same light and also metered the battery just like you did.....come to find out, i didn't re-install the drive belt...check that...

 

Never pulled the belt....