Steve,

I was able to produce symptoms similar to what you describe by inserting a 1.5 V flashlight battery into the CAS return wire such that the CAS end of the wire was 1.5 V more negative than the CAS return wire going to the EC2:  
CAS ----- battery ++++ EC2

The opposite polarity didn't cause problems.  With the battery installed, there are one or two sparks when starting to turn the CAS and then no more sparks until the CAS sensor is turning significantly faster than would be the case when turning the engine with the starter.

It seems that you may have the CAS return grounded in addition to the connection to the EC2.  If this is the case, then the voltage drop from the starter current may shift the CAS return voltage similar to what I did with the test setup.  If I disconnect the CAS from the EC2 at the EC2 connector, all the CAS wires show infinite resistance to ground.  It might be useful to see if your wiring is similar.

Steve Boese