My 91 Accord SE with 140K miles has an Antilock Brake System problem. The ABS light comes on after approximately 1 mile of driving. The fluid level in both reservoirs is good. I've measured the resistance of each wheel sensor. The values are as follows:

Front Right 3.25K Ohms (3250 Ohms)
Front Left 3.25K Ohms (3250 Ohms)

Rear Right 10.65K Ohms
Rear Left 11.95K Ohms

Do these values tell me anything? Should all the readings be the same? Should both rear wheel sensors be the same as one another like the front wheels are?

I also noticed all the sensors and the ridges on the hubs are very dirty. I haven't cleaned them yet but plan to. How do thy work? Are they Hall effect, magnetic, laser or something else?

After checking the resistance of the sensors I forgot to reconnect the rear left and found something interesting. In that condition the ABS light came on after driving only about 40 feet versus 1 mile. Is there any value in disconnecting each sensor one by one and driving to see if I can disconnect one and not get a change in symptoms - similar to the way you'd do a power balance test by pulling spark plug wires one by one to find a misfiring cylinder? After reconnecting the disconnected rear left sensor the symptoms of ABS light illuminating after 1 mile driving returned.

New wheel sensors sell for around $100 to $200 each. Each sensor has a different part number, so shotgnuning parts is out of the question as is buying one sensor and trying it on each wheel until I get lucky. The dealer computer diagnostic check is $75. Sensors at the dealer are $200, so at $300+ installed I'm not sure the repair is worth it on a $2000 car.

Any ideas?