Alternator squeak and intermittent battery charging

Technical Area Engine, Transmission, Exhaust etc
1
  1. My model of MX-5 is: 2004 1.8 Sport NBFL
  2. I’m based near: Glasgow
  3. I’m looking for technical help or recommendations on: Alternator

This is probably an easy one for most of you, but I’m needing better knowledge than my guesswork.

After a good, fast run out at the weekend, the alternator has started making a squeaking noise which wasn’t there before. On the drive to and from work on Monday, this turned into a battery warning light every time I stopped at traffic lights, along with the squeaking stopping.

Keeping the engine above 3,000 rpm the alternator charges ok, if it drops below ~2,500 rpm even when moving the battery light comes on again.

Aftermarket stereo has a useful function which shows the battery voltage, so I can see I’m getting ~14.8V when it’s charging ok and it drops to 12V and below when the battery light comes on. Alternator belt seems to be spinning ok whether the alternator is squeaking/charging or quiet/not charging.

Video of the squeak: https://youtu.be/6WLikcUkyfE

Is tightening/replacing the alternator belt likely to help here, or am I looking at replacing the alternator? Recommendations of any other checks I can carry out?

Thanks in advance

2

I’ve not had this but it sounds like a weak tensioner. Idea is it doesn’t work at low revs but once higher revs centrifugal forces cause it to do its job. Squeaking is usually slipping of some sort.
Just a guess, be interested what others come up with

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First check is spray a bit of WD-40 on the inside of the belt while the engine is running an observe if the noise stops.

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