Originally Posted by
DMC5180
The upper pulley is on the slack side based on CCW rotation when viewed from the rear. Ever time the A/C engages along with power pulses from the pistons there is a momentary loosening on the upper belt run too the Compressor. This creates a slacking or whipping effect. If you didn’t have the upper idler to shorten the span, you would see allot of belt slacking vibration under load cycles. I’ve observed high speed video (slo-motion) of this effect on other applications. It’s surprising how much movement the slack side can have.