Hello Jerry.The belt tensioner is powered both ways: Keeps the belts tight in one position, and pulls the alternator up to allow you to more easily change/replace the belts. That's the lines to both ends of the cylinder, just like a hydraulic ram.The system is also tied in with a pressure switch to a dash warning light, maybe the no gen light? or maybe low air? Can't remember. No pressure for belt tension, you get warning light up front, I can't remember what is actually going on with the controls in that Penn switch attached to the rear junction box as to the functioning, just remember what happens, sort of!happy coaching!buswarrior