The notes do only relate to my own car, and are given in good faith but other cars may be similar. Naturally, the author cannot assume responsibility for what you decide to do to your own car!
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. 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 into the ABS computer. This is located in the centre of the car under the dashboard. To get to it, run the driver's seat fully back and take the two bolts out of the clutch footrest. A 10mm socket fits. Once this is out, the trim panel against the centre console in the driver's footwell can be slid downwards and out wards until clear of the centre console. The ABS computer is situated just above the floor towards the front bulkhead. With its connector facing you. There's no need to remove it at this stage. Disconnect the plug by releasing the spring clip on the end that the wires come out (seen here on the left of picture below) and then pulling this end away from the box. It is likely to be quite tight. Once this end is away, the plug will hinge at its other end because it has a tongue that hooks into a slot on the other end.
In the end of the plug opposite the wires, there is a small "Philips" screw. Remove this and the cover will slide off away from the end with the wires on. You may have to remove some insulation tape from round the end to do this. Once the back is off the plug, the backs of all the connector receptacles and their wires will be exposed. Carefully replace the plug. Hook the end with the tongue into position first and then lever the end with the wires down on to the pins of the ECU until the spring clip pops back into place. It is easy to bend either the pins or the receptacles in the plug if you're not careful!
The pins will probably be numbered (tiny numbers moulded into the plastic) but if they're not, here's the numbering system. Pin No. 1 is located closest to the end with the wires coming out on the row with the most pins. The numbering then carries on across the row until it gets to the end (Pin 18). Pin 19 is back at the same end as Pin 1 and then the numbers increase across the row such that Pin 35 is at the same end as Pin 18.
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By sticking the multimeter probes into the backs of the appropriately numbered receptacles on the ABS plug, it should be possible to verify that there is a resistance of between 1400 and 600 ohms between pins 4&5, 21&23, 9&7 and 26&24. Again, it pays to be gentle when inserting the multimeter probes so that you don't bend the backs of the receptacles. Normally, the plastic housing is moulded with dividers like an egg box so the probes will wedge happily between the plastic housing and the side of the receptacle. If you see the correct resistance between each pair of pins, you know there's no damage to the wiring between the ABS sensors and the computer. If you don't, then depending on what you see, you've probably found the fault.
A reading that is too low indicates a short circuit - either along one of the wires or in one of the connectors itself. The wires from the rear sensors run over the driver's side wheelarch and along the driver's side sill behind the carpet. Pull the plastic trim strip up that holds the door seals down and then pull the door seals off their flanges. The steel clips that retained the plastic trim will then have to be removed (there's a self-tapping screw in each of them) before the carpet can be pulled away from the floor. If the resistance is too high, this indicates a break in the wire or a fault in the connector.
If all the resistances are fine, there is still an outside chance that the fault is with one of the wheel sensors. For this next test, you need to secure the multimeter probes into the back of any one pair of receptacles and go for a drive. Obviously, the plug has to be connected to the ABS computer while you do this! It is also best to have an assistant who can see the multimeter in the car with you! Set the 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 and watch the meter. 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.Top Next Page Previous Page
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by Ian Hopley.
© Ian Hopley / alfisti.co.uk 2002