Courtesy light issue.

[NafemanNathan]

But... That 10v is constant at the moment, so if it was the feed from the door switch, when the ceiling light switch is in the central postion the light should be on all the time (Whether the door's open or not).

But it isn't, and looking at the picture of the light itself, it looks like I need connectivity from the door switch to the blue wire, but there is no connection from the light harness to that blue wire.

Ideally I need a 5th genner to just pop out their light an check their harness is the same as mine. And if they have a multimeter and are able to check the readings from all colour wires with and without the doors open, that would be smashing

[wurlycorner]

Ok, now had a chance to look in the 5th gen manual. Pleased to say I wasn't talking cobblers

There should only be 3 wires on the (loom side) of the 4 pin plug.
WHT/BLU = permanent 12v
BLK = permanent ground
WHT/BLK = switched ground.

WHIT/BLK runs to the ICU (called a multiplex control unit on a 5th gen apparently - and lucky 5th gen owners have 2 instead of just 1, one on the passenger side and one on the drivers side )

In theory, you should have no voltage on the WHT/BLK wire, but looking at the diagram the WHT/BLK from the courtesy light splices into the ground side of the "ignition key light", which has a permanent live to it, so when the switched ground isn't connected, it's possible this wire might well float around 10v. (would need to verify against another 'working' 5th gen.)

Anyway, regardless of that, when the door is opened, WHT/BLK wire should drop to ground.

Check the door switch is operating correctly (makes/breaks continuity ok) and also check the resistance across it (going across the terminal on back of the switch, to the car body itself or ideally to the battery negative). It's possible that although you have continuity, it's at very high resistance and that's cocking it up (fingers crossed).
If that checks out ok, then just to be sure, bell out the WHT/BLK wire from the light connector to the fuse box (ICU/Multiplex unit).
If that's ok then you're definitely looking at the ICU/Multiplex unit being the fault.

Page 23-96 is the circuit

[wurlycorner]

But... That 10v is constant at the moment, so if it was the feed from the door switch, when the ceiling light switch is in the central postion the light should be on all the time (Whether the door's open or not).

Nope, because you're connecting +12v to +10v, so nothing will happen. It needs the +12v and a ground for the light to come on.

But it isn't, and looking at the picture of the light itself, it looks like I need connectivity from the door switch to the blue wire, but there is no connection from the light harness to that blue wire.

Ideally I need a 5th genner to just pop out their light an check their harness is the same as mine. And if they have a multimeter and are able to check the readings from all colour wires with and without the doors open, that would be smashing

Page 23-94 is the plug wiring. It shows that on the plug (i.e. the loom side) you should only have 3 wires; on pins 1, 3 and 4. There should be no wire on pin 2 on the plug.

Ignore whether there's an extra wire on the other side (the socket) that might just have been a convenient location for them to run a spare wire to, or for future planned mods/accessories etc.

[NafemanNathan]

but looking at the diagram the WHT/BLK from the courtesy light splices into the ground side of the "ignition key light", which has a permanent live to it,

Ahh yes, my ignition key light doesn't come on either

There's another clue then

Ok, I'll have a looksee at the manual when I've got time. Cheers Iain

[wurlycorner]

but looking at the diagram the WHT/BLK from the courtesy light splices into the ground side of the "ignition key light", which has a permanent live to it,

Ahh yes, my ignition key light doesn't come on either

There's another clue then

Ok, I'll have a looksee at the manual when I've got time. Cheers Iain

Yep, definitely the ICU, high resistance on the switch or break in continuity where the WHT/BLK wire connects into the ICU then.

Unless the previous owner has been a muppet and cut through the WHT/BLK wire in amongst all the other messing on the car, my money is on the ICU...

Hopefully stripping the fuse box/ICU unit and giving it a good spray over with electroclean will sort that though

[NafemanNathan]

Ok...Where are these ICU's supposedly hidden then?

[wurlycorner]

On a 4th gen it's on the back of the drivers footwell fuse box, hence why I'm saying stripping the fuse box out and giving it a good clean up is a decent starting point.



Don't know on a 5th gen, but would guess the same?
If it's not on the fuse box on a 5th, don't strip the fuse box out

Incidentally, the circuit shows the passenger door switch/ICU drives the light as a slave off the drivers ICU, so if your light doesn't come on with either drivers/passenger door open, if it is an ICU problem my guess is it will be the drivers side ICU not the passenger side ICU at fault (common part of circuit).

[NafemanNathan]

Cheers Iain, you've saved me a whole bunch of searching

[wurlycorner]

If you like 'em, you could always 'like' one of the posts?

[NafemanNathan]

I rep'd you. You should be happy with that