Thanks Leo, I'll check that but as both are switching I suspect that isn't the problem. Can you check the inlet and outlet voltages when the brakes are applied for me please? I would have thought the voltage should change when the brakes are applied but they aren't doing.

I saw them all at 0.00 and randomly 0.06 but I will check again tomorrow more carefully ...

Exactly what I saw on both the inlet and outlet valves. That would make sense when the brakes aren't applied but I would have expected them to change when the brake pedal is pressed. Nanocom document says:

• Inlet valve: This shows the voltage being applied to this valve by the ABS ECU. When driven the
voltage should be around 2.8 to 3.6 Volts and when not being driven should be around 0 to 0.5 Volts.
• Outlet valve: This shows the voltage being applied to this valve by the ABS ECU. When driven the
voltage should be around 2.8 to 3.6 Volts and when not being driven should be around 0 to 0.5 Volts.

and I'm assuming the valves will be driven when the brakes are applied......

"and I'm assuming the valves will be driven when the brakes are applied......"

Would the valves be driven if the wheel sensors report all is well?
ABS should not be actuated unless it thinks it's needed. As you pointed out, with the fuse removed, braking is perfect, just no ABS or TC.......

On an off topic note, I had to use the missus' 2000 HSK yesterday to bring a generator to a snowed in friend as it has some fairly aggressive 16 in M/S tyres.
His drive is steep, switched back, about 100 meters long and nominally graveled, but had 16 in of snow on it with ice below.
The TC worked it's guts out going up, and TC was very active coming down! Loads of fun!
Flawless performance, and he got lights and heat back!
He will officially stop referring to P-38s as "Cream Puffs"!
He used to race Baja 500, and regularly dis's Rangies!....Not any more.....

I've checked again ... 0.00 and/or 0.06 randomly, depressing the pedal makes no difference. With engine "on", same data, even depressing the pedal.
I've tried when on gear but I get a "ERROR connection" message, likely the ECU does not support movement after a certain speed ... maybe I should try in L1 so it goes very slow under whatever threshold was defined at Solihull ....

It a Wabco thing, the ECU stops communicating over 5mph.

Anyway, I've spent today on the car and thought I was going to declare Bolt the winner. But not.....

It needs an MoT and one thing I had noticed was that the RH front axle oil seal was leaking. Not a specific MoT fail I don't think, but I know the tester wouldn't be impressed so, ignoring the ABS for the time being, I set about changing that, It was pretty obvious that the hub had been out before and the dust shield around the ABS ring had been slotted rather than just having a hole so the ABS sensor doesn't need to be removed. Got the hub out and turning the hub it felt a bit like a worn bearing but didn't, if you know what I mean. I realised it was the ABS ring rubbing on something and then noticed this

a dent in the dust shield. Had a closer look at the ring and noticed some teeth had bits of rust on them

While others had almost merged into one with general crud

So, could it be that due to this the signal from the sensor was getting corrupted at higher speeds resulting in brakes that worked perfectly at low speed but the ABS was kicking in at higher speeds? Cleaned it all out and carried on with the job I had started. As I had suspicions about the brake light switch, I'd got a new, genuine one of those just to rule it out so fitted that too. Once all back together I took it for a test run.

ABS light goes out at 5 mph, no faults shown on the dash so apply the brakes at about 15mph, perfect. Go a bit faster and applying the brakes at 25mph, the ABS can be heard and felt operating just as before. As another test, as the RH rear sensor was such a nice fit I could pull it out with my fingers, I managed to get my arm down behind the wheel and pulled the sensor out. As soon as I started moving the dash beeped and came up with ABS Fault and Traction Failure, as you would expect when it doesn't see normal rotation on all 4 wheels. Drove off down the road, tried the brakes at 25mph and the ABS kicked in just as before. But, with a detected ABS fault, the ABS would normally be turned off? Shoved the RH rear sensor back in, drove a little further to confirm that everything was now back to normal and the fault message was no longer there. Then opened the bonnet and pulled fuse 38 that feeds the ABS ECU. Started up and got an immediate ABS Fault on the dash, drove off and applied the brakes at 25mph, perfect at 40mph, perfect and at 50mph still perfect.

This now all seems to point towards the ECU applying the ABS when it doesn't, and shouldn't, need to. It shouldn't do anything when it has detected a fault (or at least the earlier Wabco C doesn't, my other half used the Ascot with one duff wheel sensor, a dash full of warning lights and brakes that worked perfectly for 3 months last year) but it is doing. However, when I called my local tame dismantler (Avenger 4x4), the response when I asked if he had SRD100500 or 501 on the shelf, his reply was simply, rocking horse manure.....

There's a couple on eBay at the moment but not cheap and although the owner wants it sorting and is quite happy to pay £200 for an ECU, I'm not one to just throw bits at a car and hope they fix it, I want to know it will fix it first. However, I should have the opportunity to borrow one next week so will know once and for all.

As for the MoT, it's booked in tomorrow morning. As the brake test will be done at low speed with the deccelerometer, I'm hoping the tester doesn't go fast enough to notice.....

Courtesy of Garvin I picked up his known good spare SRD100501 ECU today and have just got in after trying it in the car. I can now say it isn't the ECU as it made no difference at all. That just leaves it losing the speed signal at speed on one or more wheels so I now have an excuse to treat myself to a scope so I can look at the output from each wheel at higher speeds than the Nano will display them. One halfshaft with the reluctor ring has been replaced already (LH rear), I'm still slightly suspicious of the RH front, I've had a look at the RH rear and that looked fine but no idea about the LH front. That may be in the same state as the other front one.

It did pass the MoT by the way as the tester used the decelerometer at 15mph and the brakes are fine at that speed, the problem only occurs at higher speeds.