ABS brakes

i have a 98 lumina and just recently the abs brakes have been playing up. when i brake i can hear the abs motors running. i took it into a mechanic to test and they put it on a tester and said it was a sensor in the wheels or something like , so i left it there to be tested and i have just found out there test didn't go any further. they don't have a tester to check the sensors at the wheels or anything else.
Now i'm looking for a way to test the sensors at the wheels myself, is there away? if i spin the wheels can i read a voltage or is there a resistants that i can test for on the sensors. is there some other wa to test them. is there any infoermation i can get of the web about abs brakes that might help me.
Any information will be greatfully appreciated
dan

Thats a very good question, and I don't have a definative answer, because that isn't one of my strong areas. But I can tell you that the sensors work on the same principle as a crank position sensor, basically uses a change in magnetic field to sense the presence of metal teeth going past on a reluctor. These toothed sensor rings are part of the brake rotor (usually), and the system uses the signals to compare wheel speed by counting teeth going by on all wheels. If one count is lower than another in any given period of time, it knows that wheel is beginning to lock, and will relieve brake pressure to that one.

Do a visual check to see if any sensor rings are damaged, teeth missing, etc. and make sure the gap between the sensor and the ring appears the same on all sensors (there is probably a spec for this, but I don't know it). Check that no wiring is damaged and that the sensors aren't packed with crud or anything.

You could probably test them with an ohmmeter set on the proper range to see the spikes in resistance when the teeth pass by as the wheel is turned, and see if you get a bad reading.

Other than that, I'm lost too.

Jeff is right on the money as far as operation goes; the reluctor rings excite the sensor which produces AC voltage (I don't remember the exact ratio, I'll try to remember and look it up when I go in tomorrow for you). When the EBCM detects a wheel is slowing down, close to lockup, the EBCM commands the motor pack related to that wheel to relieve pressure. On these cars, the reluctor ring is on the outer end of the CV axle. Usually I spray some brakeclean on the ring to break loose any road grime or caked dirt, as both will affect the reading somewhat.

thanks for your help, i don't have the car with me at the moment, but will have a chance this week end to look into it. Thanks for your help Jeff and cadgear.