I'm having problems with my 95Vauxhall Monterey ( UK rebadged Isuzu Trooper). The 4WD ABS system is kicking in during normal braking. The brake pedal goes firm, judders, and I am unable to stop the vehicle smoothly or quickly. The ABS light is working but does not light up to indicate a fault. If I remove the fuse and disable the ABS system then the brakes work fine (without ABS control of course!). Anyone have any clues as to where to start diagnosing this fault?

it looks like your problem is in the absmodule or a sensor.