if i rev to 3500 rpm the battery light turns on, alternator and battery are 3 months old,
d16z6,

 

how old is the drive belt??? sounds like you have to much play in it

 

i think that was changed with the alternator i have to check

 

Belt tension, it's slipping, too loose.
If it was a new belt it has stretched a little and needs re-tensioning.
Or you have a flakey alternator or connector.
I believe the battery light can come on on over voltage too (not sure) which means you may have a flakey battery or alternator (either or both).
Connect a dc volt meter to the battery and check voltage while reving 3500, if it goes way above 14.2 with lights etc on it's your regulator circuit, which may be in your ecu (expensive) Idle should be arround 12.8 to 13.8 vdc loaded. Then try it again with no load. Check for high or low voltages out of these specs.

 

is there a way to tighten the tensioner

 

i just checked the belt, theres just a little bit of play