A/C Pulley

[micmak]

It’s a nice above zero day here in Ontario, so I decided to start up my toys. As I stood there looking at the D’s engine running, I wondered what was the idea behind two adjustable pulleys on the A/C belt. Why did they not just run a belt directly to the compressor and leave it at that? Why the two pulleys? OK, so I can understand one pulley to adjust the belt tension, but couldn’t the compressor be adjusted to take up any slack? Just wondering, that’s all. The two pulleys I refer to are shown below.
pulleys.jpg

[DMC5180]

It’s a fairly long distance between the crank pulley and compressor. My guess is they where trying to limit belt whip and the potential for throwing the belt off.

The upper pulley is fixed while the lower is adjustable.


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[mr_maxime]

What's the purpose of the upper pulley though? Seems redundant, but I never really questioned it until a coworker asked why it was there.

[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.


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[Jonathan]

Beautiful day here indeed! Didn't think to fire up the car, but at least got myself outside for a while.

Got thinking about being able to adjust via the compressor itself. If it was set-up similar to the alternator, and physically moved a little up or down/side to side, it might put undue stress on the refrigerant lines and create a leak one day from that movement. Moving a few wires around at the back of the alternator when doing a belt adjustment is no biggie, but pivoting the Freon lines in a similar way might be asking for trouble.

[Ron]

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.

I'm thinking it's CW not CCW and the tension is on the upper (idler) pulley side when engaged. Which really doesn't matter anyway, since there's a pulley on the lower (adjuster) pulley side too -- It's all about the very long run(s) the belt makes only. ...You rarely see such a skinny V-Belt run that far pulling a compressor, on a single V-Belt AC system anyway. A loaded tensioner instead would be nice (esp. in stainless;-).

[DMC5180]

I stand corrected, it is CW. I just watched a video in another thread of the engine cranking over.

Thanks Ron



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[DMCMW Dave]

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. k

I've seen a few cars running without the upper pulley. The only drawback I've seen is that if the belt is even slightly loose, it can start cutting grooves in the large coolant hose. Not a good thing.

[DMC5180]

I've seen a few cars running without the upper pulley. The only drawback I've seen is that if the belt is even slightly loose, it can start cutting grooves in the large coolant hose. Not a good thing.

You just refreshed my memory of this very issue. When I got my car way back when, it was missing the upper pulley and bracket. The belt created a notch 70% into the wall of the hose. I was lucky I discovered it before the hose failed. When the belt was static, there was about 3/8 clearance to the surface diameter. The belt penetrated the hose wall an additional 1/8-3/16. Meaning it was whipping at least a 1/2”. Some of this clearance issue was also due to the P-shaped hose support not in the correct location.


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[micmak]

Hmmm.... I never knew the belt could slap that much. Nor did I ever think it could damage the hose. Interesting. Thanks for the responses.