Bit more of an update.

Every fuse in the front fuse box has been metered out and is giving continuity when testing with the meter.

The white cable on the car loom that becomes white with a brown stripe on the step loom does have a constant 12v. There's obviously a splice in the step loom somewhere though as that colour doesn't come out of the loom at the other end.

David. 2002 4.6 Vogue SE - Alveston Red with Lightstone Leather
2007 Range Rover Supercharged in Java Black with Ivory Leather
2012 Range Rover 5.0 SC Autobiography in Indus Silver with Jet/Ivory Interior
2012 Range Rover 4.4 TDV8 Autobiography in Baltic Blue with Sand Interior

What about the red/green? That should have a battery positive supply (from fuse 49, CJB) The white although showing live all the time, will switch off once the infotainment shuts down.

Yep, getting battery voltage on the red/green with the ignition off too.

David. 2002 4.6 Vogue SE - Alveston Red with Lightstone Leather
2007 Range Rover Supercharged in Java Black with Ivory Leather
2012 Range Rover 5.0 SC Autobiography in Indus Silver with Jet/Ivory Interior
2012 Range Rover 4.4 TDV8 Autobiography in Baltic Blue with Sand Interior

So the big question, are these getting anywhere near the sidestep control module?

I’m not even sure where they should go.

I’ve gone nuclear and have removed the entire step loom from the car so I can try and pin the whole thing out this evening. 2002 4.6 Vogue SE - Alveston Red with Lightstone Leather
2007 Range Rover Supercharged in Java Black with Ivory Leather
2012 Range Rover 5.0 SC Autobiography in Indus Silver with Jet/Ivory Interior
2012 Range Rover 4.4 TDV8 Autobiography in Baltic Blue with Sand Interior

Right, I've done some more metering out of this loom tonight and now I'm really confused!

Black ECU Plug


Grey ECU Plug


Roof Switch


The loom at the other end is what's confused me...

Side Step Loom


Here you can see what goes between the plug and the socket and what comes back to go to the step loom.

That is very strange and completely against the principles of control/switching segregation!
CAN does indeed carry the communication to the control module - but no further.
I'd make a guess that the orange and blue are LIN to the motors, the only conclusion I can draw is that the switch is 'smarter' than I've given it credit for and is signalling to the sidestep control module on LIN or similar using the green or blue wires.
The grey/red is clearly the illumination supply and does no more than that.

All of which leads me to think that there are two options - a failed switch or poor pingrip at the switch or sidestep control module, if you can remove the cover of the sidestep control module, you should be able to check continuity from the connectors to PCB header - it's unlikely there is anything wrong with any soldered joints on the header as you've substituted for another module.

I notice in your table there is no continuity check between the yellow that becomes 2 x red and the connection to the sidesteps - is that correct or an omission? I'm fairly certain that will be the control power for the motor position electronics.

A lot of questions and guesses I'm afraid - I do commend you on the conciseness of your data, and persistence, a lot of people, tech's included, would have lit the fuse on the parts cannon by now...

Thanks Phoenix.

Yes, that’s an omission on my spreadsheet. They were tested without fault.

I’ve come to the conclusions:

1. The ECU is likely OK, 2 failed ones is highly unlikely with the new one being brand new.
2. The wiring loom is likely OK after all of the testing but I’ll check continuity with it connected to the ECU.

A new switch is £13.57 from Land Rover.

I might refit the wiring loom tomorrow and as everything else is testing OK I might order a switch. Nothing much else to loose to be honest.

If nothing else, at least there’s a greater understanding on the forum of how the steps work

David. 2002 4.6 Vogue SE - Alveston Red with Lightstone Leather
2007 Range Rover Supercharged in Java Black with Ivory Leather
2012 Range Rover 5.0 SC Autobiography in Indus Silver with Jet/Ivory Interior
2012 Range Rover 4.4 TDV8 Autobiography in Baltic Blue with Sand Interior

I still think you should check pingrip at the control module, if, as it seems, the module is powered by the 'heavy' wiring, then (with the harness disconnected from the AHU) you should see CAN voltage at the AHU connectors - that will prove, in the absence of 'scoping the signal, that the module is powered up and capable of communicating as well as good pingrip on the power & CAN lines.

You could also meter to ground the (assumed) LIN connections to the motors on the Orange & Blue lines, LIN should be around 8-10VDC - also check the yellow>red x 2 for what I think should be 12V to the motors - could be 5V though. The switch seems too cheap to have any integrated control electronics, I could be wrong though given some of the silly prices LR sell stuff for!

ETA
This whole process would have been so much easier with an annotated wiring diagram like the generally excellent (compared to other marques) ones for the standard equipment, it's really frustrating having to make best guesses with no frame of reference.

Here’s the ECU.

Click image to enlarge

Click image to enlarge

Click image to enlarge

Click image to enlarge

Bit more testing today.

I’ve taped the loom back up with fabric tape and refitted it to the car this afternoon.

Surprise, surprise, the steps are still dead.

I tried pressing and holding the switch to enable them to no avail.

I then removed the switch and had a look at that.

Have also just done the following:

Isolated both step motors from the ECU and tried to enable using switch - nothing.

Cleared all faults on IID Tool and tried to re-enable the steps - nothing.

Pressing and holding the switch is meant to make the LED blink to confirm the steps are on or off, this one is totally dead.

I’ve ordered a new switch as it’s the only bit I’ve not changed and it was cheap enough but I’m not holding my breath.

I did wonder if something under the car on the steps had got full of salt and shorted which was why I isolated the steps from the ECU but it’s all totally dead.

Loosing the will to live with it now.

David. 2002 4.6 Vogue SE - Alveston Red with Lightstone Leather
2007 Range Rover Supercharged in Java Black with Ivory Leather
2012 Range Rover 5.0 SC Autobiography in Indus Silver with Jet/Ivory Interior
2012 Range Rover 4.4 TDV8 Autobiography in Baltic Blue with Sand Interior

Got to admire your persistence David Somewhat ironic if it ends up being the £11 switch but here’s hoping  Jonathan

2007 Defender 90 XS Hardtop

The CAN to the AHU essentially comes from the instrument pack, which is a bridge to the PATS module (most likely source of door open/closed data) and also a gateway between the MS-CAN & HS-CAN.
Did you get chance to check for CAN voltage at the AHU jumper plug/socket with the harness disconnected? - obviously you'd need the heavy power connected.

I know you've ordered a replacement switch, did you try metering it through to see if there was any state change when the switch was pressed? In the absence of any CAN connection, the switch should operate the sidesteps autonomously. It's a pain having to work through the whole harness to work out what's connected and how it all functions, unfortunately without a schematic or even a 'description & operation' document, it was always going to be a drawn-out process.

Coincidentally, I've spend most of the day tracing an intermittent chassis CAN fault that was taking down a gateway and an instrument pack, this vehicle has five broadcast CAN and three private CAN networks, even with the schemas available, it took several hours with logging scopes and two voltmeters... got there in the end, the steering rack was intermittently broadcasting rubbish on a private CAN which the gateway was faithfully reproducing onto the HS powertrain and two HS chassis CAN's - the instrument pack is a second gateway to the MS-CAN which also got a dose by proxy.