Recently my 2004 XJ8 began throwing me error messages on the dashboard, something like "ABS FAULT", "DSC DISABLED", "RESTRICTED PERFORMANCE", "PARKBRAKE FAULT". The anti-lock brake system was offline, as were dynamic stability control and cruise control. I felt unsafe driving it, which is funny when you consider that what all those things collectively did was to turn my car into a 1990s car.

My iCarsoft LR II code scanner gave me more error codes, including "Rear L wheel speed signal missing", "left rear wheel speed sensor circuit fault", and "U3ff6, fault code not in DB". So I watched some YouTube videos on fixing the ABS speed sensors. I tried cleaning it, but that didn't work. So I bought a new speed sensor on ebay for $20. I tested it with an ohm-meter as Dennis suggested in the XK8 forum here, and got a reading of 637K ohms with 2 different ohm meters. That's way offbase from what it's supposed to be -- about 1K ohm -- but I was too dumb to know that, so I swapped it for the old one. (The old sensor gave a reading of 5.3M ohms.)

After that, cruise control worked, the red warning light had gone out, all the dash errors went away except PARKBRAKE FAULT, and all the speed sensor warning codes went away for good after I cleared them with my code reader. I'm still puzzled by the 637K ohms reading on 2 different ohm meters, but not curious enough to fiddle with it.

It seems that once cars had ABS speed sensors, it was cheaper to make other systems that wanted to know wheel speed, even the cruise control, take electronic readings from them, than to read some analog measurement of the axle's speed which I'm guessing the speedometer uses. (The speedometer kept working throughout.)

(Now I just have to diagnose and fix the "PARKING BRAKE FAULT", replace the headliner, replace the trunk latch mechanism, find and fix the short-circuits that are killing the battery, resurface the headlights, replace the front grille, and repair the chips in the paint!)

 

What brand of ohm meters are you using?
All of mine are the Harbor Freight volt, ohm meters.
Voltage readings are accurate enough, but not the ohms readings.

 

A Fluke and some cheap Chinese meter.
I didn't get a good connection, because the clips had hooks to catch wires from the side, and the sensor only exposed the tips of the wires.
(Alligator clips would've worked better.)
But the two readings were so close that I figured they must be correct.

 

The PARKBRAKE FAULT turned out to be due to needing a parkbrake reset after the battery is disconnected.
There's no mention of this reset procedure in the user manual, nor could I find any description of it on this X350 forum. I used the procedure given on this website for the 2003 S-type, and it worked on my 2004 XJ8:

1. Reset the EPB module:
   a. Switch off the ignition.
   b. Disconnect the battery for 30 seconds.
   c. Reconnect the battery.
   2. Start the engine.
   3. Firmly apply and release the footbrake five times.
   4. Confirm that the message APPLY PARK BRAKE [IIRC] is displayed on the instrument cluster message center. This indicates that the parking brake is in calibration mode.
   5. Lightly press the footbrake pedal.
   6. Apply the parking brake by pulling the EPB switch up.
   7. Release the parking brake by pushing the switch down.
   8. Release the footbrake.
   9. Confirm that the amber warning lamp is no longer illuminated on the instrument pack and the PARKBRAKE ERROR message is no longer displayed in the message center.
   10. Apply and release the EPB five times to ensure no error is present.