The pic in the service manual (and pasted above) is incorrect on several points.
A is the spark timing output, it sits very close to 0v most of the time. To fire a spark it switches to ~5v for a few mSec, then back to 0. The spark fires at the end of that pulse.
B is the Bypass control. Engine off it stays at 0, the ignition module is in bypass mode, and the coils are controlled by the module alone. Sparks are fired at 10 degrees BTDC
After the engine starts, the ECM looks to see if there are pulses on A, there should be none because the ignition module shorts A to ground in Bypass mode. If pulses are detected it presumes the timing signal wire is open circuit, and triggers a 42.
If this is ok, the ECM activates Bypass, the module ungrounds the timing signal and switches to run mode. The ECM then expects to detect pulses on A, if it doesn't it triggers a 42 and switches Bypass back off. (puts the system back in bypass mode) This is how it detects an open or shorted Bypass wire.
Complex diagnostic, but effective to diagnose the 2 wires.
Bob