MGF Central locking & alarm broken

I ordered the pscan tool, thank you for the recommendation! Will do a full check-up once it arrives, and resume the project at that point (also ordered a replacement 5AS unit, YWC000980, as i found one for good price).

Hi, long time no see

I wanted to provide some closure to this, as i've finally (FINALLY) solved it. Yes, this has remained a problem for 2+ years since i started this thread, and as nothing seemed to solve it, i saw no reason to keep on piling here about my unsuccessful attempts.

But, now that it IS solved since this weekend, and i don't like threads that remain unresolved (i've read so many posts that have no real ending or answer when searching for solutions myself), i wanted to report it.

The reason turned out to be so bizarre, yet so trivial, that i went back and forth with the parts to prove it reproducible before reporting back, and i'd be really curious as to your knowledge & experience on it. But more on that in a bit

So i got the Pscan, and it's been amazingly useful for a lot of stuff, but for this specific problem it didn't provide me with answers: all doors/boot/bonnet sensors reported as ok, detecting correctly their state. Everything it reported, was fine. I got a replacement 5AS, which didn't change anything, so i knew that my original one was good (unless i was really cursed and both had the same identical fault).

And so it went for 2 more years: erratic immobiliser, opening driver's door with the key and the other by hand, sometimes opening the boot would set off the alarm, etc. And more recently, at 2 separate occasions, the immobiliser would not deactivate with the fob at all, refusing to start the motor and just beeping away. In the most unfortunate fashion, both times it was my wife that had taken the car and called me with unkind words about it.

And so i studied the electrical schematics of the car, and around the central locking and alarm specifically, and decided to just start at one door with a multimeter and progress from there.

I unstripped the passenger side door, and started. I knew that the orange wire at the plug should have a 12V signal during locking, and the pink wire should have the same during unlocking. When i measured them, i found 2.5V on the orange wire (during locking), and zero on the pink (during unlocking). Then i measured the continuity between the 5AS plug and the door plug. It reported fine.

So the communication between the 5AS and the lock loom was ok. And supposing the 5AS itself was fine, it was then something before 5AS that messed with the signal. It kind of started to make sense, given that there were several things that kind of worked, and some that didn't work at all, so maybe what 5AS itself got was erratic?

I went to the under-bonnet fuses again, and inspected them visually to find them all fine, and then decided to actually measure each side of them in-situ for voltage with the multimeter. And all of them, except for fuse #3, gave an identical measure at both sides of the fuse. #3 fuse (15A, classic little blade-type fuse) is described in the Rave docs as "Fuse 3 (C0571) in the under bonnet fuse box supplies a permanent feed to the alarm ECU (C0061) on a P wire". Possibly interesting. So i took it out, and measured it for resistance: it had a resistance of 26 ohms. Weird. Any other fuses of the same kind have zero. I swapped it for a known good fuse, and... everything worked fine: central locking, alarm, hazard lights flashing when locking... everything.

Here's where i swapped it back and forth just to check if i wasn't crazy: with that fuse the central locking didn't work but immobiliser kind of did, and with the replacement fuse everything worked. Without a fuse, of course nothing worked, it takes down the whole 5AS. Checking the fuse again, the continuity test was successful but it had 26 ohms of resistance. So clearly it was not fully blown, but it was not good either. All that for that?

I then took macro photos of the fuse (i needed to try and understand), and saw that there was indeed a gap in the wire. It was small enough for me to not see it with my bare eyes (well, with glasses), and presumably small enough to let current pass (and the continuation test to succeed!). I'll attach a photo of it below.

And so, underwhelmingly, this is the closure: it was a fuse that looked visually fine to my eyes and worked enough to fool the 5AS into functioning partially, but in reality was never fine. 3 years of problems (i'm crying inside, this is so silly). I guess the 5AS got current, but not the current it expected, and for that reason just threw it's arms up and did whatever.

I do know that, personally, going forth i'll be always measuring my fuses with a multimeter in case of a problem before moving onto other diagnosis.

Thanks again for everyone who participated

The fuse that trolled me for 3 years:

Ahh the dodgey fuse........I had much the same situation, only it worked to my advantage.
The bloke I bought my Ford Falcon ute off, hadn't been able to start it for two years, when I went to inspect it I figuresd the it was something to do with the Smartlock security system, noticed the interior light didn't work, went home and studied the wiring diagram, and I twigged that the Smartlock system uses the interior lights to know if the doors are open.
I returned, made the bloke an offer, he accepted. I then replaced the interior light fuse and started the engine......you should've seen the bloke's face! :omg: :bust:

That is an unusual and interesting fuse!

If you look at your photo you will notice a "grain of wheat" bulb at the top, connected to each terminal.

Connect 12V across the fuse ansd see if it illuminates.

The idea is, when the fuse is good, the bulb is shorted out, so will not illuminate.

When the fuse blows, the shorting (or more correct shunt) is removed and the bulb will illuminate, which indicates a blown fuse.

This method is often used in industry to quickly identify blown fuses in production machinery.

The bulb must be rated for the voltage passing through the fuse though (12V).

However there are two problems.

The bulb will conduct very low currents when fuse is blown, hence why the meter showed it as high resistance but not blown.

Also for it to actual illuminate you need both power to it and a load, to enable it to draw enough current to illuminate the bulb (The bulb will act as a current limiter, so it will not blow its self)

As the load was only there when locking or unlocking, then you would only see the bulb illuminate during locking or unlocking, for a very short time, indicating it has blown!

If you have any other similar fuses, get rid of them and replace for conventional non illuminated fuses.

I was thinking the same as G0RSQ when I saw the photo.

These "glow" fuses are ok in circuits where you get continuous current but not in circuits which have pulses.

Have you noticed that the pscan now has its own section on here.

James

P.S. thanks for the update.