Marty --

The early D-jet injection system used an electrical switch to sense the throttle position. This switch is mounted on the intake manifold, right at the throttle shaft, and is a black plastic box about 2"x2"x1/2". It also has an electrical cable attached.

This switch, or sensor, has a zig-zag or sawtooth pattern on a printed circuit board, creating a pattern of electrical contacts. As you open the throttle a wiper rotates with the throttle shaft, crossing these contacts and generating electrical pulses. The FI ecu counts the pulses (crossings) and can determine throttle position.

When the throttle returns to idle, a different switch contact closes, telling the ecu that the throttle is back at idle -- and resetting the counter to zero.