Testing procedure is thump it with the handle of a screwdriver and see if it starts working again! When it fails the ABS light will always come on and the engine may run rough, stall when D is selected or become hard to start when hot.

But seriously, you can open them up and re-solder all the pcb joints - it worked for me and the car ran fine for another 2 years before I sold it.

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Mick J
'08 Chrysler 300CRD (MB OM 642 engine)
'95 E220 estate
'89 230TE (R.I.P.)