ubergeeknz:clevedon:Klipspringer: If all 4 wheels are locked up you have no speed. hence no ABS....
Its the same as all 4 sensors not working. Take them out and see what happens.
Are you trying to not understand?
Wrong, if any wheel locks under braking that wheel sensor signals the ABS computer which in turn signals the ABS pump and activates the system. It could be on one wheel or four - it does not matter.
Actually Klipspringer is correct, if all four wheels lock up at the same time the computer only knows that you are "stopped", not that the wheels are slipping. It will not activate the pump.
The G sensors in a lot of vehicles will know the vehicle has momentum and if they are not fitted the speedo lets the ABS know the vehicle is in motion and activate the system.