After a period of the ABS light illuminating at random, it decided it was going to stay on permanently so I finally got round to pulling the fault codes from the ABS modulator -
61-1 Battery Voltage Failure
53-1 ABS Pump Motor Relay Short Circuit
51-1 ABS Pump Motor Failure
This all points to motor failure, and with a bit of confidence from
this excellent post by Brendan976, I removed the motor from the pump to investigate further.
It is a bit fiddly to remove the motor, but fairly straightforward. First of all, isolate the ABS modulator by disconnecting the multi-plug. After marking the top of the motor casing to show orientation on reassembly, remove the two 4mm capheads and withdraw the motor carefully, it pulls out about an inch or so before the contacts are free and you can manoeuvre the motor around the brake pipes and out. Make sure you don't lose the gasket.
The motor was not seized or tight, so I split the motor apart. After cleaning the carbon build up, it was obvious what the problem was - the motor brushes were worn out, and at the extent of their travel. I have cleaned the motor up and reassembled for the time being, I managed to tweak the braids of the brushes a little to allow them to move in a fraction, but it won't last long. Now I know what the problem is, I can source or improvise some suitable replacement brushes.
Refitting the motor to the ABS modulator is fiddly too, roughly align the offset on the shaft with the corresponding hole in the pump before inserting, and gently ease the two contacts back into their holes. If it's lined up correctly, it will go in very easily - do not force it. Replace the two capheads, making sure the gasket is in place.
Splitting the motor is a bit delicate, but if anyone is interested, I'll post a procedure and maybe some pics when I get some new brushes.
