CEL 9, Cylinder Position Sensor Check Engine Light

I have a 1990 Honda Prelude Si with a B21A1 engine. I recently had the engine light come on and it is showing 9 flashes, or the Cylinder Position Sensor. I have checked the resistance of the sensor and it came back within specs, according to the Factory Service Manual. I have run all diagnostic procedures according to the FSM, checking resistance of the sensor directly at the sensor and at the end of the wiring at the ECU, all checked out within specs. I have tried a different sensor and also a different ECU, and the CEL is still coming up. I have checked to make sure that the crank to cam timing is set correctly. The engine was recently rebuilt and all was running fine, the only work I had done right before the light came on was replaced the alternator. I am at a loss at this point after trying a new sensor, new ecu and checking the wire. Does anyone have any further suggestions? Most suggestions I have seen say to replace the distributor, but unlike most Hondas of this year, the Cylinder Position Sensor is not located in the distributor.