This meant only one thing, there was a short further up in the loom.
Even worse the boot wouldn't open. So ended up having to go in through the back seats
Then forcing the mechanism by hand
Turns out I fitted the lock catch upside down. For reference the microswitch should be at the bottom of the door and the pushrod fitted like this
In my day job there is the popular phrase of "Divide and Conquer". Basically if you have a problem, divide it up into smaller bits to find the root of the issue. No different from car electrical problems.
A bit of searching found a pinout diagram for the UCH (body computer) and that noted pin 39 is the magic wire that goes from the boot lock to the computer. I removed this wire and turned on the ignition to confirm that it thought the boot was shut. This is good as it means this UCH is fine.
Checking continuity on that wire found it was shorted to the body from this point.
I had contemplated splitting the wire at the boot connector as it joins there for both the boot light and wire to the body computer. I could then solder it back up after. However when doing anything like this, you open up the risk it could fail in the future. So I'd rather not chop up the OEM loom if I didn't have to. Thus I went rooting for another areas I could probe and found a new connector for this loom at the front.
Now I knew the circuit is made up of four bits. First is the wire from the boot lock to the boot light. There is also a second wire from the boot to the front. When this gets to the front of the car it hits another connector in the passenger footwell.
Disconnecting this connector and checking continuity from the body computer wire to this connector showed there still was a short. Also removing the wire from the connector and replugging in showed that interior light worked as intended.
Great! I now know that the short is only in this front part. Makes it massively easier to diagnose than having to pull the trim off inside all the way along to find it.
Now at this point I didn't really take anymore photos. Mostly because it was gone midnight, I wanted to sort it and I forgot.
Anyway the alarms on the 172 aren't factory but they're not aftermarket either. They were installed at the port, presumably to meet UK insurance specs. The newer 182 had it wired in and talked over the CAN network. So these only needed two wires and power into the UCH. However these 172 didn't have that and so were installed by tapping into the loom directly.
Thus the loom was mucked about by a third party and how well that was done depends on the person doing it. Thankfully the person that did mine didn't do too bad a job. Especially as they probably had a lot of cars to do and so in a rush. Unfortunately as I mentioned earlier, anytime you cut into a loom wire, you risk it failing later on. After 18 years and 150k miles it seems like that time was when its sat in the garage...
I say it was installed well, some of the installation is a bit shonky. Like this earth connection.
I only have one fob (which is corroded inside anyway) and the alarm has been disconnected shortly after I bought it as I don't trust aged alarms on any car. The car is insured without an alarm and it didn't work properly anymore anyway. If I ever wanted an alarm, I'd get a more modern unit installed. Probably another 5 to 10 years at least though before these are worth enough to do that...
So I ripped out the last of the alarm loom and chopped out the connections into the main loom. Also removed the LED on the dash and the ultrasonic sensors on the A-Pillar. If anything it will mean I don't get passengers questioning what they are everytime they get in!
I'm not entirely sure where in the alarm loom it was broken, but I removed the entire wire from the UCH to that connector and inspected it. There was no damage on it nor anywhere it could rub through into the car body. So it had to be somewhere on that spliced section.
I also took the time to clean up and resolder the loom wire for the boot switch with some heatshrink over the top.
Powered the car up and re-tested everything. Thankfully not only the problem is fixed, I didn't add any new electrical problems in! 😁