Jeep Engines - Oljeep

4.2L IN-LINE 6 (IGNITION SYSTEM) 327SOLID STATE IGNITION SYSTEMIgnitionA Solid State Ignition (SSI) system is used on the 4.2L sixcylinderengine. The SSI system is controlled by the engineMicro-Computer Unit. The SSI system consists of thefollowing components:Ignition moduleIgnition coilResistance wireDistributor0 Knock sensorDistributor cap and rotorSpark plugs and wiresSystem OperationThe ignition module is activated by the MCU when theignition switch is in the START or ON position. Theprimary circuit is closed and current flows through the coilprimary winding. When the engine begins turning thedistributor, the trigger wheel teeth rotate past the pickup,coil assembly. As each tooth aligns with the pickup coil, theresulting pulse triggers the MCU which in turn signals theignition module which closes the primary circuit. A highvoltage is then induced in the coil secondary winding andcurrent flows to the distributor cap and rotor. The rotorconnects the high voltage to the proper spark plug. Thetiming of the ignition is constantly changed by the MCU,and vacuum and centrifugal advance mechanismsaccording to engine operation.Ignition AdvanceCentrifugal (mechanical) advance is controlled by enginespeed. Flyweights connected to the distributor shaft arethrown outward by centrifugal force. Higher engine rpmthrows the weights further out. Calibrated rate springs areused to control this movement. The outward motion of theflyweights causes the rotor and trigger wheel to beadvanced on the distributor shaft several degrees in thedirection of normal rotation. This is referred to ascentrifugal ignition advance.When the engine is operating under light load, thecarburetor throttle plates restrict airflow, causing arelatively lean mixture to enter the combustion chambers.Ignition must occur earlier because the lean mixturerequires a longer time to bum. The vacuum ignitionadvance mechanism is used for this purpose. Whencarburetor ported or manifold vacuum is high, the vacuumadvance mechanism moves the pickup coil assemblyseveral degrees opposite to the direction the distributor isSystemrotating. This causes the pickup coil to react to the presenceof the trigger wheel teeth earlier. This process is referred toas vacuum ignition advance. With low vacuum operatingconditions, such as wide open throttle acceleration, a springin the vacuum advance mechanism pushes the pickup coilback to a position of zero advance.IGNITION SYSTEM COMPONENTSMicro-Computer Unit (MCU)The 4.2L engine’s ignition system is controlled by theMicro-Computer Unit (MCU). The MCU monitors variousengine sensors that indicate engine operating conditions.One of these inputs originates from the distributor andrepresents engine speed which is used to determine ignitiontiming. The MCU processes ignition signals and then sendsthem to the ignition module. By modifying the signal itsends to the ignition module, the MCU can retard ignitiontiming in response to the various inputs it receives. TheMCU also controls air-fuel ratio and the pulse air system.The MCU is a completely sealed module which is locatedbehind the glove box next to the radio. It is not repairableand must be replaced as a unit if service is required.Inputs:Outputs:Coolant temperature switchThermal electric switchFour and ten inch Hg vacuum switchesOxygen (02) sensorWide open throttle (W.O.T.) switchKnock sensorDistributor0 Stepper motorIdle relaySole-vac throttle positionerUpstream and downstream solenoidsPCV solenoidIgnition ModuleThe electronic ignition module is located in the enginecompartment behind the bottom of the radiator overflowbottle. It is a permanently sealed, solid state module that is notrepairable and must be replaced as a unit if service is required.