UPDATE - Is it the battery or the alternator that's at fault?

[Robert Stirrat]

Thankfully the winds had calmed down enough last night to work on the car
without fear of the bonnet being ripped off or worse still landing on my
head.

Anyway here's what I found.  With the accessories switched on I could power
on all the accessories such as foglights, hazard and hazard light and the
high drain heated rear window and side mirrors and windscreen wipers.  It
was only when I switched on the heater blower to a higher speed did the
power fail.

I cranked the engine and switching on all the accessories again only to meet
with the same problem.

I measured the voltage across the battery with the engine off, It read 12.2
volts which indicates a good battery as you say.  I then cranked the engine,
the battery voltage now measured 13.8 volts which also indicates the
alternator is providing a charge to the cells as suggested.

Next, I tried switching on all the accessories with the engine on and off
with using a higher current delivery by jump starting the car.  Again the
heater blower dimmed the lights or extinguished them depending on the speed
setting.
I noticed a distinct squeal from the alternator which could effectively be
silenced by the blower speed control knob.  Am I right in thinking this is
normal when the engine is idling?

I had the alternator belt changed at the end of last year.

This morning I thought I'd take my chances and take the Coupe to work.  I
cranked the engine without trouble, switched on the dips, tried the
winscreen washer only for the lights to lose power, so I switched off the
wipers to resume power.  Now driving the car, I tried the heater settings 1
to 4 whilst looking at the brightness of the dash lights - No loss of
brightness.  I then switched on the
remainder of accessories I need, including the CD player.  Again no loss of
power. The car's voltmeter was reading about 13volts as expected.

I indicated right to enter my work's carpark only for the power to go again.
Now I'm wondering if this happened because I lowered the revs of the car?
I'm thinking I may have a problem with the alternators brushes? Or Am I
barking up the wrong tree?

Should I set about dismantling the alternator next? Any ideas/thoughts would
be most welcome.

Regards,
        Robert Stirrat.