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The only place for a coil spring is up Zebedee's arse
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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.

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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)

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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.

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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).