The automatic belt tensioner keeps the serpentine belt and timing chain tight and in position. It’s fitted with a coil spring that applies the amount of force required, against the belt to keep it in position. It also provides some free movement to absorb shock loads generated by the A/C compressor during the immediate starting up of auxiliary components. The tensioner automatically compensates for these effects and keeps the belt under constant tension. The pulley guides the belt and the chain.