Central Locking advice - Issue with black/red wire

[redcar]

Good evening all,

Central locking suddenly stopped working completely (key in any door or boot is not triggering lock/unlock)
Did the black/red wire loop mod which made no difference

After some testing, the black/red wire is only getting 0.3V
As soon as I put 12v through the wire the central locking starts working perfectly!

Does anyone know why this might be?
Where does the black/red wire come from and why am I only getting 0.3V at it (I'd expect either 0 or the full 12?)

Many thanks in advance for the help!
Merry Christmas and Happy New Year to all

[redcar]

Here is a photo of the black/red wire mod just to confirm i've done it correctly

there is 0.3v measured here
upon application of 12v from a jumper pack the central locking starts working perfectly

[Brianmoooore]

Is one of those wires red/brown?

[redcar]

Is one of those wires red/brown?

Hi Brian, they're definitely the red/black wires

the red/brown wire to the central locking module remains untouched.

Many thanks

[Brianmoooore]

The red/black starts at fuse 27 in the fusebox, goes down through the duct underneath and into the cabin behind the glovebox, down in the front left corner, back along the LH sill, across the car, then all the way over to the front along the RH sill - a victim of the original design as a LH drive car.

[redcar]

Ok thank you, I'll trace this wire and see what's going on. I don't understand what could have happened to it for it to work and then suddenly not work.

Any idea why I'm getting 0.3v rather than the full 12v or 0v?

Thanks for the help Brian

[Brianmoooore]

I suspect that the 0.3 volts you are measuring is not a 'real' voltage - just leakage on an unloaded circuit that your meter interprets as 0.3 volts. Connect a 12 volt lamp between the red/black and earth for a true reading.

[redcar]

Got to the bottom of it:

At the black/red wire I was getting;
12.4v from the fuse box
12.4v all the way along the LHS sill
12.4v at the b-pillar
then after this it would drop 1v ever 50cm
until it got to 1.5V at its final destination at the RHS speaker hole

after much fiddling and investigating for any damage to the wire, the wire just snapped at a point near the n/s rear seating area and the copper was stiff/blue/corroded
Chopped the corroded part of wire and replaced
12.4v measured at all points along the wire
central locking works perfect

Bit worrying that this wire was corroded; i'm guessing at some point the insulation has been damaged very slightly and then moisture has slowly caused it to corrode
I couldn't see any obvious damage to the wire
although the car is a track-car and semi-stripped, i've kept all the factory insulation and tie down points

Thanks again for the help Brian!

[fixedwheelnut]

Check where the wiring goes in to the drivers door, the feed for the central locking relay usually goes into the drivers door and then back out again then to the relay, a common quick fix is to jump the two red/black wires at the inside of the drivers door plug to effectively bypass the loop in to the door until someone correctly replaces the corroded pins in the plug.

[biffer]

Hats off ,Redcar, for nailing the fault down to that section of loom.

[Brianmoooore]

Check where the wiring goes in to the drivers door, the feed for the central locking relay usually goes into the drivers door and then back out again then to the relay, a common quick fix is to jump the two red/black wires at the inside of the drivers door plug to effectively bypass the loop in to the door until someone correctly replaces the corroded pins in the plug.

The problem here was still no power AFTER this had been done.

[fixedwheelnut]

I have a pdf of 1989 325i Electrical manual I have put it on my Google drive hopefully you can download it via this link. https://drive.google.com/file/d/1jiMLQA ... sp=sharing

section 5126 covers central locking and includes a diagnosis check sheet after the diagrams that should be helpful C104 and C105 are the front door wiring plug connecters.