The air-con unit has caused the car to vibrate (only way to describe it) quite badly.

It started in January (which is winter in the UK) so I switched it off and it stopped - hence my diagnosis that it was the ACU at fault.

As spring approached and the temperature started to rise, I put it back on and it appeared to work fine no vibration and plenty of cool air.

Occasionally (one or twice over many 1000's of miles) it would do it, but cycle it off/on and it went.

I figured it was probably the serpentine belt that needed replacing - which happened on the service last month, they advised it was worn so I was right to replace.

Yesterday the vibration came back with a vengeance, and my suspicion is that perhaps the belt-tensioner should also have been replaced as these have a limited life so was slipping due to the extra load of a hot day and me switching the ACU on.

A small edit after submitting - I'm assuming it HAS an automatic belt tensioner that is based on some spring mechanism - can't believe you'd have to adjust these manually ? (I'm no mechanic BTW)

From my limited description above - would those in the know on this forum generally agree this could be the solution ?? (Before I lose my money at a service-garage....)

 

Yes it has an autotensioner. I'd recommend popping the hood and having someone else turn the A/C on and off while you watch to see what starts shaking first. If you know someone who has some experience working on cars, have him/her watch while the unit is turned on and off. You may be able to figure it out on your own before going to a mechanic.