I just picked up my '01 XJR with 135000km on the clock after getting a raft of preventive maintenance done (timing chains and tensioners, waterpump, transmission fluid). 5km down the road there was a minor thump from under the bonnet, and a few km later the battery light came on and the water temp shot up. Turns out the ancillaries belt had come off. Got the mechanic who did the work to come out and get it reassembled enough to drive back to the shop, and he now reckons (at first glance, it hasn't come back off yet) that the belt tensioner has failed.

This is a heck of a coincidence of timing - it has to be related to the work that was done. I can see where de-tensioning the belt for removal will put more load on the tensioner and therefore maybe cause an already weak one to fail soon thereafter, so maybe it's just an unfortunate "one of those things" job. On the other hand, it's also possible the guy screwed up. We haven't had the conversation about money yet, but I'd like to be fore-armed.

Can any of the experts advise whether it's possible/easy to damage a belt tensioner by mishandling? Over-twist it de-tensioning the belt, refit it wrong, something else...