What's the normal lifespan of an alternator? (Gen 3)

Right yeah, that makes sense now that I've done the job. That is a particularly bad design. I think my crank shaft pulley is ok though, gave it a wiggle and felt nothing.

The good news is that now my banshee squeal on start-up appears to be gone. I start-stopped it 3 times in a row with lights and heated mirrors - no unusual noise.

The bad news is that I am now getting what I'd call chirping when revving above 2,000 RPM (even when stationary). That's new. I think it's because I slightly damaged my power steering belt when installing it. I couldn't get it over the pulley and (stupidly) used an allen key braced against the engine to try to pop it on.

I only realised after that in doing so I nicked off a piece of the rubber off at the end of the belt... Sigh. I might just pop my old one back on and see if that fixes it. Now I understand how the idle pulley fits together it shouldn't take too long.

Ah nice, realised I just hadn't tensioned the idle pulley tightly enough and so it had a slight wobble if I rocked it back and forth by hand. Now it's all tightened up and properly tensioned - no noises at all!

Not a very fun job to do with the radiator still in place but did it and that's my first time swapping a belt

FWIW I replaced my alternator at around 150k as it was dying.

I believe it was the original one, that I replaced with a new old-stock Denso one I got for £90! 

Nice, it seems like if you've got an older or higher mileage Jimny then the alternator / bearings croaking it is something to look out for.

With respect to my earlier belt replacement... I thought I had mine tensioned enough to not squeal on start-up but 2 nights ago I heard it after testing with heated mirrors / lights. A very quiet squeak. That was after I used a bar as leverage to tighten it up as well. I'm getting just over 90 degree turn in the belt which is pretty tight but maybe it simply needs to be tightened more!

If that doesn't fix it and I still get slippage then I'll give it one last tighten before I accept that the alternator is probably goosed.