Quote Originally Posted by turbo_bu View Post
Looking at the TBI wiring diagrams, it looks like the very important crank sensor signal actual comes from the spark module. I think it is called "reference" on the TBI diagrams and comes in on pin B5. Does anyone know what this signal looks like? is it 5V? 12V? Square wave?
The attached picture is NOT a TBI computer! TBI computers use an identical ignition module system. The attached picture shows the ignition module converts the distributor pickup coil AC signal to a DC square wave. The voltage is likely less than 5 volts. The ECM is using the DC square wave FREQUENCY as a RPM reference.

gm-ign-module-schm.jpg