On the later XJ's (ChryCo, not Renix) the CPS is a "Hall effect" switch, actuated by the tone ring. Similar switches are used in Chevvy and Ford distributors, and work quite well. The nice thing is that a Hall effect will work even when dirty or wet, and do not require contact between the parts. Also, a Hall effect will work from only one side, instead of needing to be on both sides (like the optical interruptor used in the Mallory "Unilite" distributors")
The Renix version works by generating voltage (caused by the passage of metal past a magnetised sensor) and the ECM reads the pulses to determine the location of the crankshaft.
Either way, the end result is the same - a signal is generated that tells the ECM where the crankshaft is, a signal is generated by a similar sensor in the distributor to tell the ECM where the camshaft is, and both signals are used to generate pulses for ignition and fuel timing.
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