Vehicle Speed Sensor

I replaced my motor with a ram jet 350. I disconnected the original car computer (ECM) only to learn that the speed is registered through the computer, via the sensor. The schematic says its a yellow and purple wire going into the ECM. The wiring diagram doesn't tell me which color wires go to the speedometer. Does anyone know? I don't want the computer to give me a check engine light. I want to use the computer only for speed. Any ideas?