alternator type problems but not alternator mk1

[midge]

right the car doesnt start on the starter motor, battery dead ish, it will bump start then run fine for a few mintues, but the rev limit slowly drops and it shows its idleing at 500 but it isnt so must be reading wrong. the lights on the dash go dimmer when you turn anything on, lights e.t.c, i presumed this would be battery/alternator related, i changed the battery and it was the same, bumped it and it done the same, i then changed the aternator and its still exactly the same.

Anyone know whats wrong? gearbox earth's still attached and correct

prob worth noting the batteyr light doesnt come on when thsi happens but it does work.

 

[Dink]

my mum had a similar prob with her corsa

it was that the earth terminal at the batt was not making good contact with the earth clamp (was showing 12.6v at the batt and 14v at the clamp)

gave it a good clean (didn't look like it needed it tbh) and has been perfect ever since

i'd check all the earths tbh