@KenONeill - I've just gone and taken new measurements. My earlier ones were off a little, here's new measurements:

 

Off: 12.7 V

Idle: 13.1 V

2,000: 13.6 V

5,000: 13.4 V 

 

Is it possible to service the alternator brushes or slip rings if this is the problem, or easier to buy a reconditioned alternator?

 

 

Off values are good, running values are rather low, 14+V is much better.

 

Try measuring the voltage directly at the alternator output stud, with the negative meter probe on a bit of scratched clean alternator body casting.  If the charging voltage is significantly higher when you do so, it would point to a poor connection somewhere in the high current leads, either the positive output cable, or the main earth cable between engine and chassis/battery negative.  This sort of thing can often be resolved by simply disconnecting and cleaning the contact areas of the wire crimps on the ends of the heavy-duty leads.

 

 

 

 

8 hours ago, Lewys said:

Is it possible to service the alternator brushes or slip rings if this is the problem, or easier to buy a reconditioned alternator? 

Buy a reconditioned alternator.

@Wino Okay, I’ll check the alternator and see if there’s any difference in voltage. 

 

@RicardoM I’m looking at alternators but am struggling to find many of my older low ampage type for a reasonable price.

 

There are many of the VAG Bosch versions with the regulator hidden behind a rear plate. Would this be okay for me to use, or must it be a direct replacement?

 

Thanks 

The dropping voltage at higher rpms is a very common felicia problem and it's caused by one of the bridge rectifiers starting to fail it an pretty hard task to replace them but it is doable I did that on my felly and it's now perfectly fine

1 hour ago, Lewys said:

I’m looking at alternators but am struggling to find many of my older low ampage type for a reasonable price.

Why low amperage? Go for the 70A type no worries.

@Lewys - Seconded what Ricardo says about fitting a higher peak amperage alternator when you need to (ex)change anyway.

On 29/12/2018 at 01:34, KenONeill said:

@Lewys - Seconded what Ricardo says about fitting a higher peak amperage alternator when you need to (ex)change anyway.

 

Thanks for the info - I’ve ordered a reconditioned higher amperage Bosch alternator.

 

@Wino - I checked the voltage directly at the alternator and there was only a 0.1 increase in charging voltage, which should rule out a connection issue in the leads then.