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[x1xv45]

I've checked and it's charging OK, but the warning light stays on. I could almost put up with it (not really) except it seems to have triggered a shutdown in the auto gearbox (Vauxhall dealer admitted this is a possibility). If I turn off ignition, restart, gearbox is fine, then after a few minutes an autobox warning appears and I am forced to use gears manually. I'm thinking it's an electronics issue, but where to look? I picked up a second-hand alternator and will try a swap, but it seems off target as the alternator I have is working fine. I'd appreciate some thoughts on this puzzle.

[jonathanh]

are you sure it is charging ok?  could be overcharging throwing the ECU on the gearbox

[MutantCav]

Have you checked the fluid levels in the auto box? Perhaps the warning message is genuine?? Low fluid levels will cause the gearbox warning message and needs to be dealt with...

[x1xv45]

Quite sure it's charging ok - checked myself and have driven 75 miles with it, and checked with proper testing box at local garage under various loads and shows OK.
As there were no signs of any problem with gearbox - smooth going - until immediately the alternator light started showing, it would be a big coincidence if there were no connection between the two, also given the comment from the Vaux dealer. My plan is to try to fix the warning light and then if there's still a gearbox warning to move on to that.

[Marks DTM Calib]

So, what is the voltage across the battery terminals with

a) Engine off
b) Engine on idle
c) Engine on idle with heated rear window and full beam headlights on
d) Engine at 2000rpm with heated rear window and full beam lights on

[x1xv45]

a.13.4
b.12.1
c.11.6
d.11.6

Different from yesterday - in that I just took off the sensor wire from the alternator - warning light goes off of course, but gearbox warning still comes on - would have said something if the gearbox stayed ok, but now it's looking like alternator and gearbox needing attention at same time.
I was going to change the alternator but couldn't find a way to prise the pulley to release the belt - nothing solid nearby to lever against - is there a secret to this?  

[Welung666]

a.13.4
b.12.1
c.11.6
d.11.6

Different from yesterday - in that I just took off the sensor wire from the alternator - warning light goes off of course, but gearbox warning still comes on - would have said something if the gearbox stayed ok, but now it's looking like alternator and gearbox needing attention at same time.
I was going to change the alternator but couldn't find a way to prise the pulley to release the belt - nothing solid nearby to lever against - is there a secret to this?  

Yes there is! Undo the tensioner above the alternator

[Loch View]

engine on idle less voltage than engine off

[Marks DTM Calib]

Alternator is NOT charging!

There is a tensioner that simply requires a spanner on the front nut, rotate and it will release the tension on the belt allowing it to be removed.

The autobox will throwing a wobble and a low voltage error code.

[Marks DTM Calib]

engine on idle less voltage than engine off

Yep....alternator not charging and the load of running the engine electrics is bringing the battery voltage down.

[Loch View]

but I find 13.4 volts accross the battery terminals quite high with engine off

[Marks DTM Calib]

but I find 13.4 volts accross the battery terminals quite high with engine off

Nah, just the sign of a healthy battery

[Loch View]

but I find 13.4 volts accross the battery terminals quite high with engine off

Nah, just the sign of a healthy battery

I always thought a fully charged battery is around 12.6-12.8 V  :-/

[Baron Von Spongebob]

Alternator not charging..You might even notice the dash lights pulsing and it will throw up a warning for the auto box..

Change the alt and it will be fine..

[philhoward]

a.13.4
b.12.1
c.11.6
d.11.6

Different from yesterday - in that I just took off the sensor wire from the alternator - warning light goes off of course, but gearbox warning still comes on - would have said something if the gearbox stayed ok, but now it's looking like alternator and gearbox needing attention at same time.   

That explains the low readings - somehow i think you might have readings for c and d about 15-17 volts with it connected....

With no sense wire connected, it won't charge.  Try it again with the wire connected.  If you get anything above 14-15v then the alternator is goosed (or rather the regulator is).  This can ause all sorts of problems - especially the autobox.