Hi,
I have new costly GMOEM brakes all the way around, now ocassionally the antilock light comes on for no foreseen reason, Once it you shut it off and re start and re drive it's gone ! Then sometimes it comes back shortly after being driven, it was off for about a week and last night it came back on again. Where should I look ? Why the on/off light?

 

They may have knicked or otherwise damaged the speed sensor wires when the brake job was performed...

 

Could this be because the rotors were not turned during the brake job? or the front was greased could some grease have gone on a sensor?

 

The rotors wouldnt cause this and its very unlikely that brake lube hit the sensor. I agree with Swart that most likely they knicked the ABS wire probably while getting the old pads out. They like to stick in there and at least mine I had to pry out. You can look at the wire going to the spindle on each wheel. If its cut open or badly pinched that is probably the reason. If I rememer correct you only need to check the fronts since the rear abs sensor is in the transmission.

 

ran into a similar prob like that last night replacing the sensors, basically one hub was factory and the other hub was aftermarket <- did not know this. here comes the fun part put bothe new sensors in and started the blazer no codes and no abs light, awesome you would think but NOPE. backed up and the abs light came on, so thinking wtf. went foward it went off. long story short a stock gm sensor does not fit into and aftermarket hub. compared the sensors we took out to the ones that we put in and sure enough one is shorter and caused the light to come on because it chewed up the sensor badly, and i had to blow out all the plastic while spinning the hub < not fun. so that being said just be real careful when replacing these, compare the sizes first.

yea so check the sensors and where they are plugged in, and if you have proper tools take them out and clean them, well of course if there not bad. ok im done rambling on lol