WP Fiesta 2005 The Car Cannot be Locked (Fixed)

[Webel117]

I recently bought a three-door Fiesta in great condition in every way except that it couldn't be locked, by the Remote or by the Key. When I pressed the Remote the Drivers Door Lock 'Clicked' and Locked, then 'Clicked' again immediately and unlocked itself. The same thing happened when I used the Key. However, if I turned the key it would Lock, then unlock, but if I held the key in Locked state, it double-clicked and the Drivers door was locked, but not the Passenger Door. When I started the car the 'Door Ajar' light went On, and no amount of Door slamming and Hatch closing would make it go out, and that was the answer: the 'Ajar' switches.
After a lot of work which included changing the Drivers Lock mechanism with one from the wreckers and discovering the joys of the GEM, I found the answer and offer it to you in case you have the same problem. Each Door Lock has a Microswitch buried in it which switches between two positions depending on the state of the Door- either fully closed, or 'not closed'-open. I spent twenty years Teaching Industrial Electronics and have plenty of experience with Microswitches which often have Gold Plated contacts and are usually reliable. But in this case the contacts on one Door were dirty (as measured by my Multimeter), so even when the Door was closed the dirty contacts made the GEM regard the door as being 'Not Closed'. The other switches were not brilliant including the newly replaced one: after all it was just as old and dirtier inside than the original.
The GEM expects to 'see' an 'Earth' or Chassis connection when the Doors are all Closed, so I wired the three inputs to the Module (Pins 18, 19 & 20) directly to Chassis and disconnected the Door switch wires. Now the GEM is happy, the Doors all Lock, and I don't have a false 'Door Ajar' light. But since I never had one of those on my 64 GT Cortina in '64, or my later XK, XL and XP Falcons, or many others, so what? Hope this helps.