Hi everyone, I've recently fixed this problem and wanted to share my story. Firstly, it's not a complete fix. Everything works as before except the 'boot open' button on the fob is now redundant. The 'fix' kinda grew out of a workaround I came up with instead of replacing the BCM!

I have a 2018 plate MK3 Focus Estate 1.5 deisel. I've only had the car a month. When I got it, everything worked. I fitted heated seats and a dash cam. The car also got a software to update to the new maps, called 'F11', but that's another story. So I was being busy about the car when the boot popping problem appeared. It could all be my own doing, though I don't know what I did wrong!

I bought the Haynes Manual and got a few diagrams off the net. I started hitting the forums and couldn't find anyone with a ford focus with that particular problem. Reading the forums pointed to bad wiring or componentss caused by water ingress. I removed all the trim in the boot lid and inspected the loom for broken wires. I ordered a new boot switch/number plate light. I wondered about the loom. There was one on ebay and the VIN was almost idetical to mine with only the last digit being different. Only £30 so I ordered that too. I put a terminal block in at the boot latch to measure stuff and the yellow/violet wire sits low ( 0v ) until you press unlock, press the boot latch button above the number plate or press the boot button on the fob. It then gets a momentary high ( 12v) which opens the boot. I disconnected the loom at the connector above the rear fuse box behind the trim (big black plug) I thought "When you unlock the car the lights flash. A short from the lights flashing could maybe open the boot". Testing the loom showed no breaks or shorts with other wires between the yel/vlt wire at the latch and at the big black connector. Moving over to the 'live' side of the connector, the yel/vlt wire is still sitting low and going high momentarily when any unlock is operated. THE WIRING IN THE BOOT IS APPARENTLY OK. I then pulled fuse F65 out! According to the diagrams, pulling this fuse should stop power getting to the relay suppling the solenoid in the boot latch. Nothing should open it! What this did do was stop all the designed behavior i.e. the switch above number plate and boot button on fob stopped working BUT every way of unlocking the car still pops the boot open! This wire is supposed to go from the relay in the BCM to the latch solenoid and nowhere  else. This left me thinking of a couple of possibilities. A short between the big black connector in the LHS of the boot and the relay output at the BCM, (apparently on pin 12 of the BCM connector C2280E) or a fault with the BCM. Unofix kindly gave me all the info I needed.

I cut the yel/vlt wire coming out of C2280E to put in terminal block so I could measure it. Sure enough the wire sat low but pulsed high anytime the car was unlocked even with fuse 65 out! The relays for all the locks are built in to the BCM and appear on schematics as 'non-servicable' A fault in the BCM. The new boot switch arrived and was fitted.

Two options. Down the rabbit hole, V-Linker, laptop, new BCM reprogrammed, Argh! Or workaround. It's only a solenoid needs switching after all. I chose the workaround. I used two 12V 40A relays and a length of lawnmower cable stripped for wire.

Firstly separate the BCM from the boot. Cut YE/VT wire 12 going into connector C2280E This supplies power to the boot latch solenoid. Remove fuse 65 and put in a tap fuse without the bottom fuse in and a 10A in the top holder. This supplies a fused 12V always on. Cut brown wire 49 going into connector C2280B. This is the wire that is pulled to earth when the switch above the number plate is pressed. Tap into but leave connected GN/VT wire 55 going into connector C2280B. This is from the drivers door latch and is pulled to earth when the car is unlocked. There are two GN/VT wires going into that connector. The other is pulled to earth if the door is ajar. Use a meter to get the right one!

Secondly connect the first relay as follows. Pin 85 to fused 12V. Pin 86 to tapped GN/VT wire 55. This will switch the relay on or off depending on if the car is locked or not. Connect pin 30 to brown wire 49 from boot. Connect pin 87 to pin 86 of the second relay. So the first relay is to connect or disconnect the brown wire from the second relay depending on whether or not the car is locked. If the car is locked, GN/VT wire 55 is 'floating high' and the brown wire is not connected to the second relay. Connect pins 85 and 30 on the second relay to the fused 12V from the tap fuse ad lastly connect YE/VT wire 12 from the boot to pin 87 of the second relay.

That's it. Everything except the boot button on the fob is restored. (Pity it didn't use the brown wire too, to connect to the BCM. It might have worked). Now I need to tidy it all up. Replace the terminal blocks with soldered shrink wrapped joints connecting wires the right length. Replace glove box and trim. Hoover up. Put stuff away. All that stuff I hate!

The first relay is 'on' when the car is unlocked and does heat up slightly. I might leave everything hanging out till I see how warm it gets. If it gets too hot, I'll replace it with a latching relay.

This is a fix for this particular fault with the BCM only and depends on the boot latch, boot switch and loom being OK, but I hope it's useful to someone. The loom I ordered still hasn't arrived!

 

That wiring is a mess.

Carry a fire extinguisher

It can't be a fault with the BCM if you've changed the BCM and it's still doing it.

I would investigate why you are having the issue first

Did you fit a "NEW", "GENUINE" Ford release button?

Hi Dave,

20 hours ago, birdman2015 said:

Replace the terminal blocks with soldered shrink wrapped joints connecting wires the right length.

This dogs dinner does work. 😎 It replaces what the BCM should do with the boot lock. I led the relevent wires out from the back of the glove box up to the passenger seat so that I could sit in the drivers seat and make sure it all worked. Once I was sure it did, I tidied it all up.

I originally traced the fault back to the BCM sending an 'open' pulse to the boot latch, down YE/VT wire 12 from connector C2280E for all 'unlock' methods. That was the issue. Only the boot switch and the boot button on the fob should pop the boot open. It was happening even when fuse 65 was removed! This solution was instead of replacing the BCM. The brown wire 49 from connector C2280B that goes to the boot switch is a 'floating high' The boot switch earths it 'telling' the BCM to open the latch using YE/VT wire 12.

Yes, I'll need to get a fire extinguisher.