Ok this is for my young cousin, he has a 2006 6.0 .....it is reading 12v at idle......when you Rev the engine the reading goes down to like 10v........just replaced both batteries and alt. Any help would be greatly appreciated.

 

How (where on the truck and with what) are you reading the voltage?

 

He is reading it thru the sct tuner...

 

That's quite a drop, is the battery light coming on? I'd check at the batteries and alternator output lug to verify there is a problem. Maybe someone here who is familiar with the SCT can help with info on what/where that number comes from and if there may be a set up option that could affect its reading. Checking and cleaning up the battery posts and other connections may help also, don't forget the grounds.

 

He must have an x4 tuner,it also doubles as a monitor.

 

I had not one but TWO bad high-output alternators right out of the box from O'reilys.

I would have preferred to order a better one but it went out 100 miles from home..

 

Thanks for the input......I think it is a bad alt.......just doesn't make sense