See Figures 1, 2 and 3
The Renix Multi-Point Fuel Injection (MPI) system, used on 4.0L engines through 1990, is controlled by a digital microprocessor called the Electronic Control Unit, or ECU. The ECU receives information from various input sensors. Based on this information, the ECU is programmed to provide a precise amount of fuel and the correct ignition timing to meet existing engine speed and load conditions.
The ECU also calculates ignition timing and operates the ignition control module. Ignition timing is modified by the ECU to meet any engine operating condition. Information such as air temperature, engine coolant temperature, engine speed, absolute pressure in the intake manifold, or the presence of spark knock is used by the ECU when calculating the correct ignition timing.
The ECU controls the engine by receiving input signals from the Manifold Absolute Pressure (MAP) sensor, the Crankshaft Position Sensor (CKP), knock sensor, oxygen sensor, Throttle Position Sensor (TPS), Coolant Temperature Sensor (CTS), Manifold Air Temperature (MAT) sensor and battery voltage. Based on the information received from the input sensors, the ECU then sends control (output) signals to the fuel pump relay, fuel injectors, idle speed control solenoid, ignition control module and the EGR valve.
As input signals to the ECU change, the ECU adjusts its signals to the output devices. For example, the ECU must calculate a different injector pulse width and ignition timing for idle than it calculates for Wide Open Throttle (WOT).