WHEN MY 03 COBRA UNDER BOOST BATTERY LIGHT FLICKERS ON AND OFF? ITS BOOSTIN 15PSI

 

Check your alternator. Could be when you enter a good amount of boost the alternator isn't doing its job. Could be a pulley making it do that, I'm not sure of your setup tho.

 

It has a Brand new Alt. and I wonder If It is belt slip causing it to come on only a thought but I have a 100mm upr Idler but i'm running a stock belt should I go with a smaller belt??

 

Yea check the belt, slippage could be the problem. It seems like when you enter boost since you're running that much boost, it sounds like your belt is slipping on the alternator pulley because whenever the alternator drops below a certain amount of output, it will trigger the battery light.

Check the battery, if the battery is holding a charge at the normal level, then its most likely the alternator pulley slipping on the belt.

 

+1 on belt slipage