This ABS light is driving me crazy

Hello members. A few months ago, on a rainy day in the French 'black mountains' the abs light on my 2001 MGF came on. At first intermittent, but after a few weeks it became steady. Water + light, an electrical issue at first sight. I didn't care much because for me abs is an option, the car is braking fine. The problem: the car won't pass the french "controle technique" because if the option is there, it has to work. So I changed the four sensors, I checked the lines from the abs connector to the modulator connector. Current passes, no short circuit (+ or mass), test lamp on all lines OK. Ohm on the new sensors OK. Fuses OK. I disconnected the battery for a while. Took the car for a 200 kms spin. ABS still on. Bought pscan. On the abs interface, live data interval shows, speed on four wheels shows, spinning the wheels with ignition on gives a speed on all the wheels. But: no readings for brake light switch, valve relay, pump relay, valve relay voltage and circuit condition. No error codes. The 'information' button comes up with a blank screen. Tried abs 5.0 and 5.3, only ignition on, with engine on, but no changes. What is strange: when I connected pscan for the first time, I'm sure the 'brake light switch' gave a reading on/off, and the system gave the error 'front left sensor open line' or something like that. The only thing I can think of is changing the brake fluid (level is OK, but the fluid is old). Does anyone has an idea? Maybe changing the modulator but Rimmer indicates you have to reprogram using testbook (can psacn do this??). Thanks, Sypko (dutchman living in France, not many MGF's on the road here).

These ECUs only support four live data parameters at a time.
If you want to monitor brake pedal switch (for example) then you need to not monitor one of the wheel speed sensors.
If the light comes on often enough then the answer is probably to start a log file, monitor all four speed sensors and drive it.
Then plot the wheel speed sensors on a graph using excel or libreoffice (or whatever)  and hopefully you will see one sensor behaving differently.
Some ABS ECUs throw you out of the diagnostic session at a certain speed (I can't remember if ABS5.0/5.3 is one of those.
If this happens you might have to limit your speed to below 30kph or so whilst you are logging it.

I understand, thanks. Will try this as soon as I can. Hope that there is light at the end of the tunnel. Also I found the topic 'logging data to a file' on the forum. In the mems3 screen you can check all live data boxes at the same time. I assumed it was the same for the abs ecu. Is it normal that the information button at abs gives nothing?

bit of red herring but have you cleaned the back of the sensors ? they are magnetic and during some recent metal work mine attracted shavings which knocked one out of range. 

Worth giving them a clean, easy job. might of picked up a flake of rust from a lesser vehicle. 

Mat 

The information button is for things like serial numbers and VIN numbers.
Some ECUs don't have these, and others have not been fully implemented by pscan.

Thanks Mat for the suggestion. The sensors are brand new and clean. I also checked the sensor rings for damage or debris. Front disc brakes are new too.

OK, great. I'm learning as I move on. Thanks for your reply and the info.

There is a set clearance between the sensor and the ribs on the disc, check that this is correct on all sensors, too large a gap can cause problems. I dunno what the spec is but it will be found hiding somewhere in the workshop manual!

Thanks Cobber for the suggestion. I checked the sensor gaps after renewal and found that the front left was very close about 0.3 mil. The others about 0.8 mil. I put a washer to the front left, to get about 0,8 mil. What if the gap is too big, how can I reduce it in this case? Bend the bracket?? Miracle: yesterday blocked in a trafic jam and suddenly abs light out! All the way home light out. Got pscan, no errors, brake switch OK open/closed, circuit condition 'no interruption', valve relay 'closed', pump relay 'open', valve relay voltage 12,3 V. Start the car, abs light on! Take it for a spin, 2 kms light out, 5 kms light on, this morning start light out, 1 kms light on. Connected pscan once again, everything ok, no errors. Anyway, I must bleed the brakes because the fluid is far too old. Maybe a brake fluid pressure problem because the fluid is contaminated with water ? Didn't do the log file yet. Will keep you posted!

Yeah you bend the bracket if necessary.
Too large a gap and to wont register the magnetic field of the teeth going past, and too close and it may not see the gaps properly.
It works by counting the teeth against time, so the more teeth it counts going past over a set period of time the faster the wheel is going. 
0.8mm sounds OK, generally these things require a gap of @ 1mm thereabouts. 0.3mm is way to close. There is a bit of leeway allowed in that it doesn't have to be precise, the specification will be lurking in the workshop manual somewhere.
But if it works OK at 0.8mm then I'd leave well enough alone!

have you considered dropping the ABS electrical plug off and cleaning the contacts ?

Thank you Stubbs, yes I did.