I need help with ABS

The ABS light went on. Even if I shut down and restart the engine it stays on. Sometimes, in the past, when I drove through a hard bump the light went on, but by turning the power off the ABS would come back again and the light went off after startup as it should. but now it does not go off
When I bought the car I realized that the previous owner had put a different tyre size on the left back tyre . After I bought I changed all the tyres for some new Yokohama C-Drive. Could running the car with one tyre of bigger diameter could have spoiled a something that finished its time while I'm driving now? any ideas on how can I check if there is a faulty sensor and which one is it?
thanx
chris

Hi chris

The abs unit has a self diagnosis function that you can envoke.
send me a PM.

my abs light will not go off the brakes where checked they are fine please help can't sell car with theses on

The first check is dead easy. Have a look at the 10A fuse located on its own behind the cover immediately to the left of the glovebox. (At least that's where it is on a RIGHT HAND DRIVE 164)! Undo the little plastic fastener at the bottom with a small coin and slide the panel downwards and off. Behind it will be the big Bosch ABS relay and the 10A ABS fuse. If you’re very lucky, the fault will be no more than a fuse.

The next test is pretty easy too. Disconnect each of the sensors in turn and measure their resistance. The front sensor connectors are dead easy to find – they’re on top of the inner wings near the strut tops. The connectors are beige cylinders on the ends of thick black wires. Pull one apart and stick a multimeter set to measure resistances of about 1000 Ohms across the two terminals inside the plug leading to the sensor. If all is well, you should see a reading of about 1000 Ohms (1K). The manual allows anything between 600 and 1400 ohms. Try this with all four sensors in turn. The rear connectors are a bit harder to find. One is located under the battery and the other is located in a similar position on the offside of the boot floor. In both cases, the boot floor and side carpet needs to be pulled away to allow access.

Assuming all four sensors read normally, the next stage is a bit harder. You need to check the signal being fed out of each sensor. Set a multimeter to read the smallest AC voltage scale you can. Mine was about 200 volts AC – which is really way too big. Go for a little drive with it connected across the two pins of each sensor in turn. It is extremely difficult to connect a multimeter to the connector -the simplest way is to have a scrap plug with two bare wires if you can get one. Failing that, you can try to probe the pins on the back of the control unit under the dash.

Pins 4 and 5 (Brown and White wires) are for the Left Hand front wheel. Pins 21 and 23 (Brown and Greeen wires) are for the Right Hand front wheel. Pins 9 and 7 (Brown and Yellow wires) are for the Left Hand rear wheel and Pins 26 and 24 (Brown and Red wires) are for the right hand rear wheel. Above about 30 or 40 MPH, the sensor should generate a big enough voltage for the meter to be able to read – possibly anything between 0.5 and a couple of volts. The signal gets bigger with speed. Check that each pair of pins corresponding to each sensor are sending a signal to the computer by repeating this test for each of them. If they are, it’s bad news because whatever’s wrong with your system it ain’t the wheel sensors! If they’re not (or any one of them isn’t) that’s likely to be the problem. Either the sensor is faulty, or the gap between it and the toothed wheel on the hub is too big.