unsure on the air gap and i havent got abs so i cant check for you but to test the sensor,
Unplug the sensor, put your mulitimeter on the pins and set it to vAC and slowly turn the wheel. Everytime a raised part of the ring on the hub passes the sensor, you should get a small voltage. if not or if you just have a constant voltage, they are fooked and need replacing.
i suppose you can play with the air gap till you get a decent reading or no reading if you'd like but most cars ive worked on dont have adjustable height sensors
Unplug the sensor, put your mulitimeter on the pins and set it to vAC and slowly turn the wheel. Everytime a raised part of the ring on the hub passes the sensor, you should get a small voltage. if not or if you just have a constant voltage, they are fooked and need replacing.
i suppose you can play with the air gap till you get a decent reading or no reading if you'd like but most cars ive worked on dont have adjustable height sensors