Couldn't workout which forum to use, please move if i've posted in the wrong one.

Hi there, I'm trying to find some answers to wheel a wheel sensor may go overvoltage.
The problem is in my brothers ford 1993 eb2 falcon fairmont (I think equivilant to Taurus).
The abs light came on roughly a minute or so after driving, a friend hooked up a scanner and told me there is a fault with the left front wheel sensor.
I took it out, cleaned it up, it looked o.k so i put it back in and connected an ac voltmeter up to it and turned the wheel by hand, both sides.
The front right sensor output ranges 0 - 1v ac average 0.6v.
While the left front sensor can reach upto 3v ac.
My question is, can the wrong airgap or fine cracks in the sensor cause this much variation in the voltage output?
Does anyone know what the air gap should be?
Has anyone got any other ideas?
Thanks