Hand held diagnostic machine.

This is interesting .....
I found on the internet and copied it to here.


I've had a bit more of an investigation into the ABS system and the now perminant indicated ABS fault on my car, same model as that of the OP. A full explanation is below, if you're bored / interested, but to cut a long story short I'm now 99% certain that one of the front ABS sensors is to blame.

The ABS light in the instrument cluster is supplied with power whenever the ignition is on, and then is connected across the dashboard to the ABS ECU below under the passenger footwell. The ABS ECU then decides whether to ground this connection or not - in the case that all is working correctly the connection is not grounded and the circuit containing the ABS light is not completed.

The garage that looked at my car suggested that a dodgy ground was occuring that meant the ABS light circuit was completed regardless of what the ABS ECU thought. As far as I can see this can only occur after the ABS light and before the ABS ECU - ie, somewhere behind the dash. I pulled the passenger footwell apart to get at the ABS ECU (not an easy task because the cover over the ABS ECU is fastened with two shear bolts - no heads mean hammer and chisel required) and located the signal wire from the dash (pink / black). I then took all the driver's side of the dash apart to get at the connector and locate the other end of the signal wire. When the signal wire was cut, the ABS light did not illuminate, which proved that there was no grounding problem in the instrument cluster. I then tested the wire itself by connecting a new piece of wire in its place. This time the ABS light remained on, which proved that there was no grounding problem in the wire (or if there was there was also an ABS fault).

I then started to get a feeling that my initial suspicions about a wheel speed sensor being the problem were re-awakened. I then compared the resistances of the two front sensors. One was reading around 1800 Ohms, the other was right off the high end of the scale. As a quick check I ran a couple of pieces of wire across the car and wired the signal from the sensible sensor into both front sensor loom connections. This made the ABS light go out, which leads me to believe that the sensor with the massively high resistance is the one and only problem.

A few hours and some expletives later and the bad sensor was out of the upright and off the car (I hope mine isn't the only MX-5 where every suspension bolt is undone by first either rounding or shearing off the head and then drilling out the remains!?!?). I'm locating a replacement now - when I've fitted I'll let you know if the problem is cured.

Hope this helps with your ongoing diagnostics - although this kind of approach only works once an intermittant problem has turned into a perminant one!

PART 2 ..... The guys follow up .....




Just to complete the story of our saga, the problem was resolved by replacing the driver-side ABS sensor. How did I get to that? Well, it turns out that you CAN read the ABS ECU fault codes. I took the car up to Nottingham (5Speed) and he had somebody read the codes.


Regards.
Rich.