blue wire / battery light, can anyone clarify?

[casinoblue]

hey guys,

been searching for a bit more info on the above, but can't seem to find a definite answer.

had my first AA call out last week  . the battery warning light came on on the way to work, then when i went to leave, nothing, totally flat. 2 hours later AA guy's replaced battery, and tells me alternator is also goosed. made it home (18+ miles) in the dark with no problem.

read up a bit on here, and decided to replace the brush pack on the alt, as much cheaper. next day the battery warning light stays on until revved to 2.5K, but then goes out and everything back to normal.

a week later, randomly warning light stays on even when revved up. thought i'd better get home quick, don't want 2 AA call out's in a week :embarassed:. half way home the light goes out, and all seems well again. this seems to happen randomly.

have checked / cleaned all the connectors in the engine bay, and all seem ok. recently replaced the earth cables (battery-head-coil-box).

should the blue wire bring up the battery light when disconnected? (it didn't when i disconnected it engine running.)
the battery light comes on with ingition, then mostly goes out when revved to 2.5K, but sometimes doesn't

battery voltages seem healthy when running and charge from alt is ok when warning light is out, no charge when light is on?

does it sound like my alternator is gradually failing, throwing up random not charging? or does it sound more like dodgy wiring to the clocks maybe?

the randomness of it has me baffled..............


thanks guys

[Jay]

Get a multimeter on there when the light is on and again when it is off.

It could be a failing alternator and you've changed the brushes... I think GSF take the old alternator in part ex for a refurbed/new one if you want, best check with them first though!

[casinoblue]

thanks Jay,

did quick voltage checks at the weekend, when everything seemed to be ok (warning light wasn't on).

12.2V on battery, 13.9V running with no warning light, 13.8V running with no warning light and with headlights on full beam.

did the same test again with the warning light on (didn't rev past 2.5K), and the voltage stayed at 12V. so there is def no charge when the warning light is on.

i'm a bit stuck because randomly the warning light will stay on, even when i rev past 2.5K. can't risk going anywhere in this state, it just fooks the battery.

it's weird bacuse the alternator seems ok when the light is off, but sometimes the light just will not go off

cheers,
james.

[rubjonny]

when you cleaned up the brush pack, did you also clean the metal ring round one of the bolt holes, as well as the mounting face on the alt?  If so you need a new brush pack, and/or alternator.  If the blue wire was the problem the light would never come on at all, and the battery wouldnt charge.  The fact the light comes on when bat not charging prooves the blue wire is working correctly.

[danny_p]

the blue wire supplys power to the alternator to excite the windings,   that battery light comes on when the alternator is drawing power to excite itself. once it's excited ( powerd up ) and turning  at pritty much anythign over 500 rpm the alternator will start to produce power and no longer draw any power from the blue wire  so the light goes out.

to me it sounds like your alternator is dead on it's feet.

[casinoblue]

excellent replies, thank you guys, that's exactly what i needed to know.

didn't fully understand the blue wire circuit, thought the light would come on if i disconnected the blue wire, so when it didn't i was a bit confused. it doesn't take much though really.

thanks again all,
james.