Odd Altenator Problem

[kona]

Im having a Intermittant altenator failure (more off than on), when its working its throwing out just under 15V and when its not working its around the 12V mark.
What makes it interesting, I have a voltmeter in the car and can see this happening and sometimes the battery light is off when its making only 12V, other times its on. I have swapped it with the oil warning light and its the same issue. The lights all come on when the ignition is set to on, then go off when the engine is running, then the battery light comes on.

The alternator will always give 15V when I have the battery fully charged via a trickle charger, then after maybe 10mins driving the voltmeter drops and the light comes on.

Its doing my nut in, I took the alternator off the other day and took it apart, the commutator was corroded so i took the rust off, I couldnt see anything else obviousl wrong.

The little wire of the 3 in the multiplug, I understand gives the altenator a voltage signal from the battery, this reads around 12V with the car not running, drops to 7 with the engine on. I presume the other two energise the field windings and carry the power out. Im totally lost with this problem. I suspect the alternator but the fact its a intermittant fault, is making me wonder if its a voltage reg or a wiring/earth issue.

[Merlin]

The reg is bust Kona you can get them for about 15 bob and just replace it
Rgd
Paul

[kona]

I cant find the reg tho! Theres so much changed on the car Ive no Idea what the story is, I read its actually part of the circuit on the back of the clocks?

Its a 1991 cooper but the clocks defo aint what came with the car. I presusme I could buy a random one and just install it?

[Merlin]

Nope is the reg in the Alt that's gone noth the voltage reg for the clocks.
You would need to dissamble the all to find it

[kona]

Think I know the part, is it a little 2x2" box beside where the multiplug goes. You see it when you take off the plastic lucas cover? If thats it, mine is a " lil" corroded.

Thanks Paul I'll Get the part for next week