Description, L-Jetronic

�x Fig. 53b Bosch L-Jetronic Diagram (Typical):

Bosch L-Jetronic/Air Flow Controlled (AFC) fuel injection, Fig. 53, is an electronically controlled pulse time system which uses measurements of intake air flow to regulate the amount of fuel injected into the engine. The system is controlled by an Electronic Control Module (ECM) which monitors intake air flow, exhaust gas oxygen content and other engine and vehicle operating conditions through a group of sensors. The ECM computes the amount of fuel necessary to maintain ideal air/fuel mixture ratios based on these sensor signals and its internal memory, and controls operation of the fuel injectors and electric supply pump in order to maintain ideal air/fuel mixtures. Separate injectors for each cylinder consist of solenoid operated valves which spray fuel into the intake manifold near each intake valve. The injectors are activated by voltage pulses from the ECM, and injectors are energized once each crankshaft revolution, delivering 1/2 the amount of the fuel necessary for proper combustion each time they are energized. The ECM controls the amount of fuel delivered during each crankshaft revolution, and thereby the air/fuel mixtures, by varying the amount of time the injectors are energized during each pulse.
The ECM monitors engine coolant and intake air temperature, engine RPM and vehicle speed, throttle position, and on some models barometric pressure and engine vacuum in order to determine engine and vehicle operating conditions. Air/fuel mixtures entering the engine are computed by monitoring the exhaust gas oxygen content, as the amount of oxygen remaining in unburned exhaust gas is directly proportional to the incoming air/fuel mixture ratio. The ECM determines the amount of fuel necessary to maintain ideal air/fuel mixtures necessary for operating conditions by monitoring signals from the air flow sensor, then tailors injector operation to maintain the ideal mixture based on signals from the exhaust gas oxygen sensor. In addition, the ECM controls operation of a cold start system and an electronic idle speed control system.
The cold start system consists of a cold start injector, thermo time switch and an auxiliary air (throttle bypass) regulator valve. When energized, the cold start injector sprays fuel into the intake manifold distribution chamber to provide mixture enrichment for cold starting. Voltage is applied to the cold start valve and thermo time switch during cranking, and the valve solenoid ground circuit is completed through the thermo time switch. The thermo time switch is heated both by applied current and engine coolant, and allows the cold start valve to operate for 3 - 10 seconds during cranking when coolant temperature is below 95°F.
The auxiliary air (throttle bypass) regulator valve allows air to bypass the closed throttle plate during cold engine operation in order to provide a fast idle and prevent engine stalling. When the engine is started cold the valve is open, air flows around the closed throttle plate through a bypass channel and the ECM, sensing increased air flow, provides additional mixture enrichment to compensate for cold engine operating conditions. As the engine runs, engine heat and current applied to a heating coil within the valve heat a bi-metal element, the valve closes, and idle speed is maintained by air flowing through the normal idle bypass channel.
Curb idle speed is controlled by the basic throttle position adjustment and by air flow through the idle air bypass channel. The idle bypass channel allows sufficient air to bypass the closed throttle plate to maintain a stable curb idle speed and compensate for accessory loading. Air flow through the idle bypass channel is controlled by an electronically operated Idle Air Control (IAC) valve. The IAC valve is controlled by a separate idle control module.