I'm going to use this thread to just dump information as I gather it, and perhaps think out loud about what I'm busy with. Chime in if you have anything to add, please!
Also, all information here is possibly correct, possibly not. Use it at your own risk! (especially because what I do here might interfere with the vehicles ability to stop)

Vehicle:
2001 4.6 HSE Thor.

Aim:
1) Develop a controller that will allow me to intercept and correct the speed signal from the ABS controller to the BECM to correct for different diameter tires.
2) Have a way to disable ABS at the push of a button without interfering with the working of the traction control.

Gathering data.

Rough Road Signal:
ABS Controller C0504-Pin4 --> ECM C0637-Pin34
Amplitude: Battery Voltage
Frequency: 2.38Hz
Duty Cycle: 25% = Rough Road OFF
75% = Rough Road ON

ABS Warning Light Signal:
ABS Controller C0504-Pin18 --> BECM C1288-Pin10
Amplitude: Battery Voltage
Frequency: 5Hz
Duty Cycle: 50% = Warning Light ON
100% = Warning Light OFF

Traction Control Warning Light Signal:
ABS Controller C0504-Pin17 --> BECM C1289-Pin12
Amplitude: Battery Voltage
Frequency: 5Hz
Duty Cycle: 50% = Warning Light ON
100% = Warning Light OFF

Speed Signal:
ABS Controller C0504-Pin3 --> BECM C1289-Pin11
Amplitude: Battery Voltage
Duty Cycle: 50%
Frequency: Varies with speed (detail to be added)
221.7Hz = 100mph = 160.3km/h

ABS Schematic Diagram (2001 Petrol)

Why do you want to alter the speed signal? If purely so the speedo reads correctly with different sized tyres, unless you are going really silly with tyre sizes, you can almost certainly do it using the speedo calibration pots on the instrument panel pcb.

Yes, I've already adjusted the pot on the back of the cluster, and the speedo now over reads by 1km/h at 100km/h. Perfect, but the trip meter still under reads by around 9.8% (if I remember correctly).

While I'm busy with the ABS disable I might aswell get the speed signal to the BECM sorted too 😀
(and do it in such a way that removing a bit of plug and play hardware reverts things back to standard).

ABS Controller & Connector Info

C0504
Junior Power Timer, 17P
Partnumber: 1355204-1 (black housing, not available)
Alternative Partnumber: 1355204-2 (red housing, unconfirmed fit)

C0505
Junior Power Timer, 9P
Partnumber: 1-967621-4 (Grey)

C0506
Junior Power Timer, 15P
Partnumber: 1-967623-1 (Purple)

ABS & Traction Control Solenoid Drivers:
Infineon BTS612N1 (Smart Two Channel High-Side Power Switch)
Various output protections as well as open load & short circuit diagnostic capability.

My first attempt at disabling ABS without influencing the working of the traction control was to prevent the ABS ECU from "seeing" the brake pedal input. The thinking here was that the ECU used the brake pedal as in input to decide when to activate ABS.
I passed the C0504 Pin7 and Pin14 connections through a DPDT pushbutton switch in the cubby that fed the ECU with a fixed state (NC for Pin7 and NO for Pin14) when the pushbutton was pressed. This meant that the ABS ECU always thought that the brakes were released even when pressing the brake pedal. Releasing the pushbutton switch meant everything worked as normal.

This didn't work! The ABS still activated even though the ECU didn't see the brake pedal state changing.
My assumption was wrong and the ABS ECU doesn't use the brake pedal as an input to activate the ABS. This also means that ABS will activate even when you are not pressing the brakes but only slowing down using engine braking. I've have since experienced this on a steep muddy descent.

I think this means that the ABS is only monitoring the wheel speeds and if the average is decelerating then ABS is "armed" and ready to intervene if one wheel is decelerating quicker than the others.
The inverse would also then be true for the Traction Control (TC). If the average of the wheel speeds is accelerating then the TC is "armed" and intervenes if a single wheel on an axle is accelerating faster than it's counterpart (or the average, I'm not sure, but RAVE seems to imply the average).

My current plan is to intercept the drive signals to the ABS Booster unit and prevent the solenoid valves booster unit from being energized when the ABS ECU attempts to do so.
This should allow me to either prevent ABS activation or to prevent TC activation (loose sand).

Any thoughts?

This also means that ABS will activate even when you are not pressing the brakes but only slowing down using engine braking. I've have since experienced this on a steep muddy descent.
Seems our cars have hill descent control avant la lettre?
Brake or TC? How did You realise?
Maybe the late four channel ABS has more abilities, than I knew ... very interesting.

Not quite hill descent control, but rather ABS that prevents wheel lock even when the brakes are not being applied.
I was heading down a wet muddy hill with large, flatting rocks that made the surface very slippery. I had the initial version of my ABS disable switch installed and had the switch pressed for this descent, so the ABS ECU had no way of knowing I was pressing the brakes. Even a light application of the brakes would lock one of the front wheels, which was then immediately released again, even though I kept applying the same amount of brake pressure. The vehicle just kept increasing in speed down the hill and I just had to hold on and try to keep things under control using steering, applying the brakes or not made little to no difference down the steep sections.

This was the type of hill that I would have preferred crawling down as slow as possible, but the ABS didn't allow this. I would, for example, like to slowly step down a rock with the front right wheel locked so that it is dropped down the step and then gains traction on the bottom stopping the vehicle immediately. This isn't possible because the wheel locks as it goes down the step (no traction as it drops down the sheer face), the ABS releases the brakes as it locks, the vehicle moves forward letting the front left get to the step and also drop down. The ABS then releases the front left too which adds to the momentum of the vehicle which then forces the front right to start sliding which the ABS then releases again and so the viscous circle continues.

A similar thing happens while driving at higher speeds on a gravel road. The stopping distance is greatly increased because of the ABS. Yes, you gain more control because the vehicle can be steered, but applying brakes correctly without ABS would still allow steering control to be retained. Also, sometimes it is more important to just stop as quickly as possible without needing to steer. A sudden ditch on a gravel road cannot be steered through, it is better to come to a stop before the ditch even if vehicle steering is compromised!

Anyway, above my reasoning for wanting to disable ABS. Lots of typing and reading, but it forces me to think through the scenarios which helps me understand what I actually want. Thanks!

I had a look at the hydraulic circuit diagram yesterday and it seems I am going to be able to disable the ABS and TC together or disable the TC on it's own, but not the ABS on it's own. I was hoping that I would be able to disable only the ABS, but keep the TC active but it seems this won't be possible. I'll look through the diagrams again this morning and see if I come to the same conclusion as yesterday and then move forward with the PCB design.

just an FYI for you . i disconnect my TC by lifting one of the wheel sensors, usually the right one as this works with my left handedness, it comes up ABS fault on the dash . it only disables the TC as i often have the ratcheting through the brake peddle, but not all the time. i think it depends on which wheel is locking up at the time . i do a lot of sand work and it would be super convenient if i could just turn it off when needed or separate the ABS from the TC as sometimes you need to separate them. this is done on new vehicles with the sand, mud, hill descent knob or buttons, so very interested in what you are doing as i normally disable and not fix for a month or so and then a week later its disabled again (back to the shack) its a real PITA to be honest . mine is a gems system .

Thanks for the info mad-as, and I am glad I am not the only one with this requirement!
I know the Landcruisers and some other 4x4 SUVs disable ABS when the transfercase is locked.

I have completed the prototype design and ordered the PCB and components.

So the unit should be able to disable ABS and TC with the push of one button. A second button will disable TC only.
The most basic implementation won't be able to disable ABS but keep TC active because the ABS and TC both use the same solenoid valve to each wheel station.

I have an idea to monitor the brake pedal input and then a third "mode" can be programmed where the ABS and TC are disabled only when the brake pedal is being pressed. When the brake pedal is released both ABS and TC will then be active again. This should work in a similar fashion to only disabling the TC, but will need to be tested first.

I have also added CANbus functionality to the board. My "want" is to install a secondary CANbus in the vehicle which can then monitor and actuate different modules as I add them. A little display can then be places somewhere in the dash which could display this information. It won;t be used for anything at this stage, but adding the functionality costs nothing if the components aren't populated on the PCB.

I should have the first prototype ready for code in about a weeks time.

This probably won't be helpful, but I'm grappling with an ABS issue that I have come to suspect stems from a previous owner having castrated my Callaway P38, and it is currently sitting on hideous coil springs.
Sometimes I inevitably have to make a sharp turn into the near lane of traffic from the apron of a narrow parking exit. This causes my right rear tear tire to become suspended in air for a brief moment. (jumping the curb) Every time that happens, the ABS locks down hard, and I mean hard. Even at 2 to 3 MPH (under acceleration), I nearly have to pull my teeth out of the steering wheel. Very annoying.
So finding a way to desensitize or the energy and time to put it back on proper air springs would be very nice.

Hi Greyhound,
It sounds to me like the connection to one or more of the valves in your ABS modulator might be iffy, either not connected, or shorting to a wire on one of the other valves. I'd check all the wiring between the ABS controller (C0506) and the ABS modulator connector (C0501). Check for both continuity, a short to any of the other wires or a short to ground. Also have both the connector at the ABS controller and the ABS modulator connector unplugged while checking.

I suspect what is happening is that as you accelerate and the right rear loses traction the TC activates and applies the brakes and starts actuating valves to stop the right rear from spinning. If the valves aren't actuating correctly, not releasing when they should or the wrong valves are actuating then it might cause the symptoms you are describing.

Or someone has replaced the pipes to the rear brakes and got them crossed so instead of locking the spinning wheel, it is locking the other one.

Agreed, that is probably more likely!
I'm obviously just thinking of the potential electrical faults as that is where my mind is at at the moment.

Thanks for the alternative view!

I received the Revision A PCBs today.

Should be able to get it assembled tomorrow and then I can start with the coding and testing 😀

Thanks for the replies guys. I'm not ruling out the electrics by any means (found plenty wrong there) but crossed lines is going to be my first check.
So-called mechanics have worked on this that shouldn't be qualified to repair a lawn mower.
We now return you to your normally scheduled broadcasting.