Roy, the TBI gets a fused 12V straight to the injector, and grounds through the quad drivers in the ECM. Injector pulse width is set at a 50% on-time default setting and the amplifiers use the sensor inputs and the O2 sensor signal to add or subtract from that point to fine-tune the mixture. The correction is applied anywhere from 10 to 90 times per second, depending on the clock frequency. Much more accurate than a carburetor!
Jerry
