[oil pan 4]

Originally Posted by Baltothewolf

OK seriously though, why not just use an A/C clutch, but upgrade your alternator to compensate for the bigger pulley size?

A typical A/C clutch is so much bigger than a regular alternator pulley that the alternator over drive ratio would go from a 1:3 over drive down to a 1:2 or 1:1.5 over drive.
Then at idle being over driven at 1:2 the alternator would only spin 1200 to 1400 RPMs and make almost no power. Getting a bigger alternator would not fix this.

This is on a 6.5L mechanically injected diesel. I cant hook a computer up to it and look at the difference in load. The only way to see how much the load changes with out the alternator hooked up is to watch for the RPM increase at no load with the injector pump metering lever set in a fixed positions. Such as idle and fast idle.

All the air compressors I have built and rebuilt use a idler tensioner. This is my preferred method but I don't think loosening an idler can prevent the belt from rubbing on the crank pulley.

I would use spring to tension the alternator so that if the cable broke, the actuator or its air supply failed or what ever mechanism I used failed, the alternator's belt would go to tension and start supplying power.
This is a fail safe mode.