i did this on the front, but if the rear sensors are similar (i bet they are) it will still work.
basically when the abs light comes on it means one of two things, no signal coming from the abs sensor or an unexpected signal. this can be due to one of 3 problems, a wiring fault, a sensor position problem or an internal sensor fault.
if you havent disturbed the sensor it should still be positioned correctly. however a quick check with a feeler gauge is worth while to see if the gap is within the recommended range (see haynes manual). the gap is crucial to the signal strength.
the wiring should then be checked by making sure the sensor harness is still connected to the main abs harness. if it all looks to be in tact and undamaged you should disconnect the sensor and then use a multimeter to test for a signal produced by the abs sensor. there will be 2 pins on the sensor's wiring. get a multimeter and set it to a low AC voltage setting, then touch the 2 multimeter terminals on the 2 pins on the connector. now rotate the wheel hub, doing this should produce a fluctuating AC current, usually only very small, +1V to -1V for example, however the multimeter will pick it up and at least show something on the screen when the hub is spun.
if you get a signal on this test the sensor is fine and you should suspect a problem with the wiring to the abs controller, or possibly the abs controller itself
if you get no signal measured on the multimeter it means the sensor's internal circuit is broken and the sensor isn't returning a signal to the abs controller. this is where the clever tinfoil trick is needed.
you simply remove the sensor's single allen bolt using an allen key. then you need to split the sensor in 2. this is easy if the sensor is stuck in position so tightly that you can simply pull the lid off using a big pair of pliers or similar. alternatively if the whole sensor comes off when you try to pull the lid off simply reomve it from the car and split it open by applying pressure to the sides with a big pair of pliers. once you split the sensor open you will have 2 halfs, one half still has the wire connected to it, and also has 2 small metal pins sticking out on the inside. the other half will have a black flat surface inside with 2 small slits. these slits are where the 2 pins usually sit. the problem with the sensor is that these 2 pins are no longer touching the metal contacts inside the slits.
to fix the sensor you need a very thin flat head screwdriver, an electrican's set of drivers will have a perfect one for the job. stick the screwdriver into each of the 2 small slits on the inside of the sensor, you need to make the slits bigger by moving the screwdriver back and forth while its in a slit. even tually you will be able to look in the slit and see the metal contact deep inside it. when both slits are big enough to see the contacts you can use the screwdriver to force tinfoil into the slits. the tinfoil needs to touch the metal contacts deep inside the slits. put enough tinfoil into each slit to ensure the pins on the other half of the sensor will definaely touch the tinfoil once it is put back together.
once you have enough tinfoil in each slit, push the 2 halves of the sensor back together, making sure you dont get the rubber waterproof ring caught or misplaced. with the 2 halves back together refit the sensor and check the gap with a feeler gauge (or if it didnt come off as a whole just stick the lid back on). then refit the allen bolt, reconnect the sensors wiring and everything should be ready to go. i've saved 2 of my sensors using this method and months later they still work fine, and i've tested my abs and it definately works.
