I've been wondering why the V12 doesn't have all the pulley accessories on one belt? Or if it's possible to shift components forward or backward so they run on a different line.

There is a belt which is the exact length for the Jaguar ac compressor to the main pulley wheel (bypassing the idle arm or air pump) but the AC sits behind the other belts, its 3+ hrs to re&re the arrangement. Pain in the ***.

I'd like the aftermarket belt to be the most accessible in case of adjustment or replacement which would involve moving the AC forward? Alternator back?

 

I think, based on other people’s comments, that this might be a viable strategy. Replace the ac compressor with a newer smaller more efficient one and put it where the alternator is. Then do the same thing with the alternator and put it where the air pump is. Replace the fan with an electric one. After that it remains to be seen if a serpentine belt can be arranged to run power steering, water pump, ac, and alternator. It’s probably possible, but I haven’t gotten there yet, although I have been thinking about it.

 

It doesn't have a serpentine belt, as more modern cars do, as it wasn't a common thing back in the design days. I think the first Jags with a serpentine belt were the XJ40 models 0and those based upon that platform.

A simple fanbelt, as fitted to the XJS auxiliaries isn't really able to transfer all the grip and co required to power everything - especially later cars with the 120A Bosch alternator (got a ribbed belt).

So just rrplacing it all wirh say a Chevy Corvair belt wont really work. Youll need to align all the powered parts, fit new, matching pulleys and most important, route the belt properly to get all parts powered...

 

What year is your car?
If l moved my alternator "back" any more it would be driven off the flywheel. It is already driven by most rearward damper groove.
running electric fan setup helps by reducing belts to 3.
l guess Murphy's law will always dictate the belt needing attention is always the hardest to reach.

 

The first Jaguar imported to the US that has the single serpentine is the XK8 (MY1997).

Earlier Jags use multiple drive belts for accessories.