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Strange Alternator Problem

My car goes into my OPC in January for it's MoT and a small service. I'll tell them to change the belt - definitely the cheapest option for starters!
 
Well I borrowed an alternator from a local breakers and fitted that today,an easier job than I expected but voltage drops straight away again. I put a meter across the battery and neither alternator shows a charge increase so I'm either unlucky and have two duff alternators or could it be the cable at the back of the alternator? Which cable is this that may be a problem?
If anyone is looking to remove their alternator it's probably one of the easiest jobs I've had to do. I found when you remove the long bolt with the pulley wheel on it,drive that side of the alternator down and lever the other side up,after removing the short bolt obviously.This puts the alternator in an easy position to pull out.When replacing,the boss with the short bolt, the engine mount,has a split bush in it,tap this out slightly towards the rear of the car.I then filed a few thou off the inner faces of the alternator mount bosses,when putting the alternator back in it slid back into position a lot easier,no hammering required.Anyway I haven't solved the issue but I've now attained Formula One pit standards of alternator removal,so not a complete waste of time.
 
To rule out the cable, measure the output of the alternator when the engine is running by trying to get the positive from the multimeter round the back of the alternator to touch the nut where the power cable is attached to. Obviously be careful for moving parts.
 
To update re my original strange alternator problem, after changing the power steering pump and then refurbishing the alternator, the dry bearing noise was still present. I then changed the 11 month old original Porsche drive belt and the noise disappeared along with the low voltage when the car had been in the rain or washed. Now after 1 month, the low voltage after the rain or washing the car has returned. It turns out the belt is slipping when wet and after a minute or so of the engine running it grips again. The belt is in as new a condition with no evidence of glazing. There are no oil leaks near or around the belt. Anyone has any clues why this has happened again? Rare as it is, could it be the drive belt tensioner that is not applying as much a tension as it should? When I changed the belt, I had also cleaned all the pulleys with brake cleaner.
 
Does anyone have instructions/guidance regarding replacement of the drive belt tensioner? I am hoping the housing it is attached to doesn't have to come out.
 
Keep us posted as my problems stopped after changing the belt but then started again after about 4 weeks. The belt is slipping again when wet. I have a new Gates belt and power steering pump pulley in the garage to fit sometime this week. Was going to do it tonight but have you seen the gales outside?
 

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