I was driving my 56 the other day and noticed the battery light was barely on at idle. As I would give it some gas the light got brighter. I'm running a 292 Y-block with a gm 10si alternator, It's been working fine for 3 years. I checked the voltage at the battery. With the engine at idle and electric fan off it shows 14 volts. With the fan on it drops to 13.3. At high rpm volts go up to 14.8 but that is when the batt. light is the brightest. That doesn't make sense to me. I took the alternator to the parts store to be tested, they said it tested good. Why is the light on when the volts never go below 12 ?

 

Would this indicate that the internal regulator is not working right?

 

Almost certainly, but check the battery too (load test).

 

Thanks for the reply. Battery seems to be in good shape, it holds a good charge and it's not giving me any trouble. The alternator should be putting out close to 14.2 volts almost all the time right? I'm still not sure why the batt. light is acting like it is.

 

I believe the light is hooked up with alternator output on one side, and battery voltage on the other. When they are the same, it is out. When either one is more than the other, it is on. Since your alternator is putting out more than battery voltage, it's coming on.