The light only tells you that the system isn't happy and has probably shut down all or part of the system. With the light on you need to assume no ABS function and a reversion to full non-ABS braking. It might turn off the front circuit just as the wheel goes into deep slip. You'll only know for sure when picking gravel out of your face.
An intermittent fault is usually one of two things, a sensor whose air gap varies (seen as impossibly varying speed by the ECU) or a bad electrical connection telling it a sensor or modulator is no longer connected. The fastest and easiest way to find out which is to put the bike in diagnostic mode and see what the flash code tells you.
Do not let the grease monkeys start parts swapping. If they can't get an answer from their PC get them buzzing out the loom. I spent 15 years working on ABS and for every ECU or modulator accepted under warranty there were ten sensors and a thousand chopped wires or dodgy connectors.
If you want to check an ABS sensor (it's just a loop of wire with an induced current) you can get a fair bit of info with a multi meter. Pull the ECU plug (work from the back if you can to avoid pushing pins out) and find the in and out for each sensor. They should have a resistance (15 Ohms is typical) and no connection on either pin to ground. Set the meter to AC (yes Alternating Current, not DC, the sine wave is the tooth count that tells it what the wheel is doing) and spin the wheel. At 30 rpm the sensor should make a couple of volts and a nice steady reading. Open circuits sensors are kaput, ones that make zero or wildly fluctuating voltages are probably at the wrong air gap or facing damaged polewheels or dodgy bearings.
Voltage fluctuations on the supply (bad regulator or battery) will cause an intermittent shut down. Car ABS gets unhappy below 9V or above 18. The Bavarian B*****ds though turn off at 11.9 and 14, It all makes work for the working man to do.
Andy
