On my car I replaced the ignition wires. After that my ABS/tracs was screwy.
Never thinking the ignition wires would cause this I went straight to the abs control unit diagnosing / checking wires and so on.
Finally before thinking about sending the module in I changed out the ignition wires with the orignals and everything was back to normal.
Ask you customer if they replaced the ignition wires recently (or around the time when things were failing)

Just had Vic Rocha rebuild my ABS control module and when I put it back on the ABS light was still on. The local Volvo dealer had confirmed that the wheel sensors were all good prior to the control module rebuild. Long story short, the power wires to the ABS pump had defective insulation and were intermittently shorting out and blowing fuse 9 (if I remember correctly). I wasn't looking for that fuse with the ABS as that is on fuse 15. (I'm sure if the fuse numbers are incorrect someone will correct me.) I replaced just the pump from a junk yard find and after replacing the proper fuse the ABS warning light has not been back on, and the system is working as designed.

Since a plastic cover covers the pump, it was difficult to determine that the wiring insulation was defective. If you can get the cover off without destroying it, check the insulation on the wiring.

Good hunting.


Oliver

64 122S
66 17800S
86 245DL
96 850GLT (With good ABS)

Have you monitored the offending sensor with an oscilloscope? It has been awhile but I have seen them in the past where the pulse wheel was damaged and caused a screwy wave form. Take a look at the "picture" that your sensors are making and see if you can make a diagnosis from there.