ABS problem

[paulXK8]

The ABS warning light on my 2005 XK8 has come on. I've made sure the battery is fully charged - light still on. Relevant fuses are protected by orange covers to prevent removal so an't be easily checked. Code reader has 3 fault codes:
C1236 Rear left wheel speed signal comparison fault
C1145 Front right wheel input circuit failure
C1155 From left wheel input circuit failure
Each of the above wheel sensors measures 800 ohms across its contacts - but only in one polarity i.e. if you reverse the contacts they all read zero - why if it is a simple coil? Also no measurable AC output when spinning the wheels in any polarity.

So what is up? Is this a passive or active system? If it's passive as it looks like it is from the driver manual I should be able to read some AC output but can't. The 3 fault codes cannot be 3 simultaneous faults - they must be related to one primary fault. All the sensors have the same resistance. The workshop manual directs me to pin point tests.

Any thoughts appreciated.

[Astromorg]

My understanding is that post MY2003 with DSC, the old passive ABS wheel sensors were upgraded to more precise two-wire, active Hall effect sensors. Such sensors have an electronic circuit built into them and the semiconductors involved would explain your one-way resistance measurement.

These sensors require the ABS/DSC module to provide power through the non-ground wire to the sensor and the voltage of that power to be modulated by the sensor to a pulse rate dependent on wheel speed. Jaguar state that the pulse rate is 46 per wheel revolution which presumably equates to the number of teeth on the ABS/DSC.

Testing in situ will probably require a meter (with pulse measurement -not AC - as a specific feature) connected in parallel with the sensor, ignition to be on (maybe engine running?) and the wheel spun as required! (Easier with a scanner that can read sensor output directly)

Having said all that, I agree with your thought that three simultaneous, new ABS/DSC faults is pushing coincidence a bit! My suspicions would fall onto the Module being the most obvious common point of all. Then, before spending cash, I would have a close look at the state of all the pins in the 25 way connector to the DSC module.

And I'm sure you won't have forgotten the essential requirement before chasing any sort of electrical problem that is to have good battery that is well charged. A battery disconnect for 5 minutes and, while the ground lead is disconnected from the battery, touch the lead to the positive battery post briefly; always worth a try!