The ABS light on my Jaguar XJ6 X300 4 litre sport started illuminating once the car reached about 15 mph.  I have connected my oscilloscope to the ECU and checked the signal from each of the wheel sensors, all gave similar signals when rotating the wheels by hand, so I presume they are all OK.  I had a spare ABS ECU, so I thought I would give it a try.  I had read that a fault motor connection on the ECU can cause these symptoms.  First of all I unplugged the motor from the ABS ECU and connected the motor to a 12v battery.  I could hear the motor operating, so the motor was clearly OK.

The ABS unit prior to swapping the ECU.

The abs unit is fastened to a bracket by 3 rubber mounts, two at the front (shown here) and 1 at the back.

The ABC ECU is fastened to the ABS module by two long thin bolts screwed in to the module from underneath. The bolts have female Torx heads.  After unclipping the brake pipes from the firewall, it was possible to gently lift the ABS module just enough to get my hand under to unscrew the bolts.

The original and spare ABS ECUs.

The two long bolts and the tools I used to remove them.

Unfortunately the fault was still there after swapping the ECUs.  Since I had nothing to loose, I thought I would check to see if there was a bad motor connection on the original ECU so I cut a small section out of the back of the ECU, directly opposite the motor connector.  I used a small angle grinder with a 1mm thick cutting disc to carefully cut an opening.  The motor connector did have a bad solder joint, so I re-soldered it and refitted it to the car.  However once again the ABS warning light illuminated at about 15mph.  I will have to do some more investigating.