I have an intermittant battery light and meter shows discharge at hi-way speed, but as soon as i slow down, light goes off and meter indicates battery is charging. Took it to the local o'reilly's and system tested fine, could not get it to repeat.
So not sure if alternator is going out or jusst need to replace belt and possible the tensioner. Any suggestions?

 

What tested fine, and how was it tested ?

 

O-rielly's used a portable test stand and checked alternator out put. I ran it at idol and slowly brought it to 4000 rpm. Out put voltage 13.2 to 13.6 the whole time. Drove the 3 blocks home and light did not come one.

 

Is that a Load test, or just output voltage for the RPM range ?

At what RPM do you see the issue ? ( highway speed depending on gears could be 1.7K RPM in OD at 65 MPH ).

Did you have the battery load tested ?