Can connect to Pektron SCU but not Engine, ABS, Airbag ECU’s

So after working through a few bits and pieces on the wiring, the final answer was much easier than anticipated. It was quite simply a fuse !

It would appear, for reasons not 100% clear to me just yet (and actually not sure I can be bothered to work it all through) that the same fuse supplying the interior light and radio (I don’t have one fitted) also powers in some way the diagnostic port/circuits for the engine, abs and airbag ECU, but not the Pektron SCU.

The fuse was blown (probably because of the radio wires left dangling). So one new fuse and a tidy up of the radio wiring and it all worked.

Thanks again for all your inputs

When I bought my Ford Falcon ute, it had sat for 2 1/2 yrs as a non runner.
The owner had tried allsorts of things to get it to start, all to no avail!

He had thrown the towel in and it was offered to me very cheap.
When I went to inspect it, I noticed that the interior light didn't work.
So I made an offer, which was begrudgingly accepted, once the deal was done I replaced the dirty (a bit green and furry) interior light fuse and it started 1st turn...... ya should've seen the look on the poor bastard's face!

It has Ford's Smart Lock security system, and it uses the interior light circuit to tell it if the doors are open, the system detects something is wrong, which in turn trips the immobiliser.......no light = no start.

It comes as no surprise to me that shitty interior light fuse was the cause of your problem.
Modern cars integrate so much of what seems irrelevant shit into essential systems, that a simple failure of something pissy like the interior light can grind the whole shitfight to a halt

I don't understand why the ECU would run the engine but not talk to the diagnostic socket. The engine ECU doesn't have different fuses for running the engine and talking to a diagnostic tool.

There is a fuse for pin 16 on the diagnostic plug, which powers the pscan, but then if the pscan didn't have power it wouldn't talk to the SCU either.

So I don't really understand why what happened happened.

Or is it actually not a running car?

The car is a running car and has been in occasional use. I can not explain it any more than what I described.

One thing I think I noticed (but would have check to be100 % sure) was before the fuse was replaced, the flashing led on the pscan box only came on with the ignition on, I am pretty sure after the fuse replacement it came on when it was plugged in without the ignition on. Although I am not 100% sure as I said.

I am away a couple of weeks now, so could check that when I get back to confirm if it helps anybody

Does the car have factory reversing sensors?

I think that the interface was being powered through one of the data lines via the protection diodes.

The only other line I can see apart from pin 7 is pin 8 going to the reversing sensor ECU.

So it doesn’t have parking sensors, but it’s a 2005 model MGZR, I guess it could have the wiring for them maybe ?

Happy to do some extra checking when I am back if it helps others in the longer term.

Please can you use a voltmeter and, with the ignition on, see if there is 12V present on any of the pins of the diagnostic socket?
I expect to see 12V on pin 7 and 16 but not on any others.