On my 97 f150 the battery light has been coming on for a few months now but only when you get on the gas hard and turn the engine to 3000rpm or higher. Battery checks good and have 14.5v at idle. causing shifting problem, Is it the alternator?

 

my 1999 E250 did that for a year or so and then the alt went out

 

Is there any way to check it before it goes out?

 

Just replace the alternator before you get left on the side of the road. If the light is coming on, the alternator is bad.

 

Thanks, I just checked voltage at idle 14.2 and when you rev it up it drops to 12.1. will be replacing alt soon.

 

My alternator started dying by giving me the ALT light only at high RPM. It then got worse to the point where the light was on all the time.

I found a guy on E bay who sold new alternators removed from vans that were converted to ambulances. They install a much larger alternator (the reason for the removal). It was a much better price than a new or rebuilt unit from any local store around here.

Chris

 

Check the brushes in your alternator. You have the classic symptoms of worn brushes - battery light at high RPMs. Brushes are a LOT cheaper than a new or rebuilt alternator.