Engine Speed Sensor
The engine speed signal comes from the Hall Effect Unit mounted above the distributor on the 2.5L engine and from terminal "R" of the conventional HEI module in the distributor on all other models. It is not a sensor in that it is not a separate component.
Pulses from the distributor are sent to the ECM where the time between these pulses is used to calculate the engine speed. The ECM adds spark advance modifications to the signal and sends the signal back to the distributor.
NOTE:
More sensors are used by the ECM to control engine performance and other systems. Refer to GENERAL MOTORS COMPUTER COMMAND CONTROL article in COMPUTERIZED ENGINE CONTROL section for more information.