yes,that's right...for a better picture,this gap must get tight ,and the alternator will pop out

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Ah-hah! Thanks. I'll give it a go.

Success! Thanks again, trundlenut, and also IulianE. It turns out I have a 90A alternator.

All that remains now is to work out whether the fault lies with the voltage regulator or deeper within the alternator itself. Not whether this is just a case of "try a new rectifier and if that doesn't work, it's the alternator" or whether there's a better way.

I think I've now tested everything I can. The alternator looks in good shape, inside and out, the brushes have plenty of life in them and the diodes are behaving in a diode-like manor (conducting - at least somewhat - in one direction but not in the other; I'm not sure whether this is too simplistic a test, though). I can't think of a way to test the regulator but it looks like that's probably the culprit.

However, I need the car at the crack of dawn on Monday and no parts suppliers are open on Sundays. On e-Bay, I can get a new regulator for £15 but nowhere nearby will let me collect one on a Sunday. However, I've found one seller who will let me collect a complete (used) alternator for £35 from nearby tomorrow morning. Fingers crossed it works!

I've taken some more photos, too - I'll upload them next. EDIT: Can't do it as this forum only seems to allow me upload 1MB in total, and I already have. You can see most of what I found in somebody else's post about a similar alternator, though.

Edited 6 January, 201313 yr by will_

Not sure if you got to the bottom of this but this thread might help you

http://www.briskoda.net/forums/topic/261286-getting-really-annoyed-with-the-car-charging-troubles/page__hl__+charging%20+problem#entry3085455

I have seen a few instances of the alternator to battery wires and plug connector break down and cause charging issues.

hope this helps

Thanks, Bowders. I ended up putting a second hand alternator on. It'll do for now, but the bearings are pretty noisy so I don't imagine it will last long. Also, having spoken to my local garage, it seems the freewheel / overrun pulley has seized on both the old and "new" ones.

I could have put the regulator from the new one onto the old one, but I think I may have mangled one of the cooling blades inside the old one in my attempts to get the pulley off, so now it will probably be off-balance. Totting up costs of a new pulley, getting the old one removed a new one fitted, plus all the time it'll take and the uncertainty of how long such a repair would last, I think I'll probably leave the noisy one in until it dies or become more horribly noisy, and then replace it with a new or one. It looks as if GSF will give you a partial refund if you return the old one (and I now have two).

iirc the freewheel pulley was only fitted to alternators on the diesels? anyway, if yours is seized then they can have a habit of breaking the aux belt tensioner too.....

For anyone having difficulty in doing an alternator on a mk1 octavia vrs.... I did this yesterday. All in, not too bad a job. 
removed battery to charge it. Took of big jubilee clip with 7mm socket holding rubber pipe to throttle body. 5mm Allen x4 to remove throttle body and took electric connection off this plus a 7mm jubilee clip. Removed plug off alternator that’s easily accessible. 16mm spanner on auxiliary belt tensioner and then put a 4mm drill bit in the tensioner holes to hold tension off belt. Removed belt. Two 13mm bolts hold alternator on. Bit of wiggling and alternator comes away. You then have earth cable strap still on alternator. Remove an 8mm nut holding strap in place then the bigger 13mm is easy to take off. Alternator then comes straight up and out. New one was a napa make but has valeo written all over it like the original. Mine was a 90amp. 136 quid for new and kept old. I believe the old one just needs a regulator. Bearings in old one sound and feel fine and quiet. TPS wanted 303+vat for theirs. Battery back in, but before I did. I checked earth strap under battery tray and it was starting to rust a little bit in good nik. Cleaned it all and added anti corrosion to it and put all back. Having started car battery shows 14.27v across it. All good 👍

You can also change the regulator/brush pack in situ on the MK1 without removing the alternator, its much cheaper, easier if like me you are away from your workshop & only have limited tools and its nearly always what has failed.

 

I did mine at about 200k miles & it was still fine at 325K when I scrapped it.

 

The warning light came on on the M25 heading for my ferry back to France, it left me in a quandary, could I continue on the journey or should i lose a few days and the price of the crossing?

 

I carried on and learned just how little electricity the MK1 with its mechanical injection uses when running, basically just the ECU and a few sensors plus use of lights & indicators etc. I drove about 170 miles including one engine start at Calais including one hour with the headlights on, there was still enough battery capacity left to restart the next day.

 

A MK2 or later vehicle with electric power steering, injectors etc would be fault code city when the battery voltage dropped even slightly. I dont think that my MK1 even had an electric fuel pump.