These days, it seems like everyone has to have the newest and hottest thing. The newest phone, the latest television, the latest and greatest sheetmetal to roll into showrooms. This particular 300M might not have been the latest and greatest 300, but it was pretty obvious the owner didn't care. In fact, the AC belt on this car wasn't even broken; the owner just wanted to replace it as a matter of preventative maintenance to keep her Chrysler pristine. Those 300C buyers could learn a thing or two about the virtues of caring for a great used car, even if it's not the newest, hottest thing out there.
Popping the hood, I saw the 300M had two belts sandwiched between the engine and the passenger-side wheel well. The "front" belt ran the alternator and power steering pump on the top of the engine; it had to come off before getting to the AC compressor belt, which ran along the bottom-rear of the engine.
This engine didn't have spring-loaded, self-adjusting pulleys. On both belts, it used an adjustable idler pulley that had to be loosened and moved to relieve tension on the belt.
I loosened the bolt in the center of the alternator belt's idler pulley, between the crankshaft pulley and the alternator, and slightly above both. With the pulley loosened, I turned to the jack bolt -- aka "push bolt," and "adjuster bolt" -- behind the idler pulley. This bolt pointed sideways to the idler pulley, just above the crankshaft. I turned the jack bolt counterclockwise to "push" the pulley away from the center of the engine, releasing tension on the belt. When the pulley was far enough in, I reached down into the engine bay to pull the belt out.
The AC belt came off the same way. After loosening the bolt in the center of the idler pulley on top, I turned the jack bolt to move the pulley and pulled the belt off. This being a 2.7-liter, the belt came off pretty easily; but if it had been the 3.5-liter, I'd have had to go under the car and loosen the idler pulley from the bottom.
The moment I pulled the belt off, I saw that the 300M owner had indeed been wise to replace the old belt; not only was it dry-rotted and cracked, one edge was highly frayed and there was a cut running right through the center of the belt. This belt was about three inches away from splitting in half entirely.
With the new AC belt on, I turned the jack bolt to tension it. With the belt properly tensioned, I snugged up the center bolt by about 1/4 turn. The jack bolt is actually what holds the pulley in place; the center bolt just holds the pulley steady in the bracket. The top belt went on in exactly the same way. With the belts on and the customer properly horrified after seeing her original A/C belt, the pristine 300M drove off to live another day.