W124 - Wierd ABS light prob

Best guess? Worn brake pads. You're sitting still and light goes out as it should, so the ABS system is probably alright. Good thing as that is an expensive fix. You drive for a bit, and I am assuming you touch the brakes at some point, and the light comes on and remains on. It is probably telling you, in its' own way, that you need brakes. You drive again and when you apply brake, the light comes on again....I would check the wear on the brake pads. If it was the ABS system itself that was going bad, it wouldn't pass the diagnostics when starting, like you mentioned, and would stay on.

There may be a boatload of sensors and such that could be acting up, I wouldn't know about that, but the brake system is fairly straightforward, so I will hang in there with my "best guess". Let us know what it was when you find out.

This sounds more like an ABS pump valve or sensor rather than worn pads. I suggest that you find a good independent that will service ABS (some will not and tell you that ABS is only handled by the dealer) and see if they can diagnose the problem. *Sometimes* they can free up a stuck valve by exercising the unit via computer - I once had this done to a Honda for $75. It fixed the problem for about six months or so. I later found out that many Hondas had a known issue with the ABS valves. Another potential problem (and one that is relatively inexpensive to fix) is the sensors on the wheels themselves. On some cars they are optical and get dirty over time and simply need to be cleaned. Considering the season (winter) and your location (UK) it's quite possible that slush has pushed dirt into the brake rotor area which is where I believe that these sensors live.

Josh

sounds like a worn or dirty speed sensor lead .The fact that the abs pump pulsates the brake pedal at low speeds ( 5 mph ) indicates it has a weak signal and it cannot detect actual veh speed. The pulsation of pedal rules out brake pads .

I think it may be a faulty brake light switch. I'm having the same problem.If you figure out how to remove the panel under the dash, let me know.