Jeep Engines - Oljeep

4.01. POWER TECH IN-LINE 6 (IGNITION SYSTEM) 239Powertrain Control Module (PCM)The 4.0L engine ignition system is controlled by theengine’s Powertrain Control Module (PCM). The PCM is adigital microprocessor (computer). Air-fuel mixturecalibrations for various driving and atmospheric conditionsare pre-programmed into the PCM. The PCM monitors andanalyzes its various inputs, computes engine fuel andignition timing requirements based on these inputs, andcontrols fuel delivery and ignition timing accordingly. Asoperating conditions change, the PCM adjusts injectorpulse width and ignition timing for optimum performanceand fuel economy.Jeep High Performance ComputerMopar Performance Parts computer provides more sparkadvance and increased fuel flow for off road racingapplications. This computer will enhance other performancemodifications. For 1987-90, 4.0L engines only.P4529334Jeep high performance computer.Ignition Control Module (ICM)The Ignition Control Module (TCM) is mounted to theignition coil. Based on control system inputs, the PCMtriggers the ignition coil via the ICM. The PCM is able toadvance or retard ignition timing by controlling the ignitioncoil through the ICM.The ICM consists of a solid-state ignition circuit and anintegrated ignition coil that can be removed and servicedseparately if necessary.The PCM provides an input signal to the ICM. The ICMhas only two outputs:1. Tachometer signal to the tachometer and diagnosticconnector2.High voltage from the ignition coil to the distributorcapIgnition CoilThe ignition coil does not require special service other thanmaintaining the terminals and connectors.When an ignition coil is suspected of malfunctioning, test iton the vehicle. A coil may “break down” after the engine hasheated it to a high temperature. It is important that the coilbe at operating temperature when tested. Perform the testaccording to the test equipment manufacturer’s instructions.DistributorThe 4.0L engine distributor does not have built-incentrifugal or vacuum assisted advances. Ignition timingadvance is controlled by the Powertrain Control Module(PCM). The distributor is locked in place by an ear on thedistributor housing that a hold down bolt passes throughwhen the distributor is installed.For those of you who are contemplating an ignition systemmodification for racing, we will briefly discuss distributorsin more detail.There are two different electronic distributors, one forvacuum advance models and one for mechanical tach drivemodels. The vacuum advance model has a vacuum controlunit on the side. Interior parts (such as the reluctor) are notinterchangeable between the two distributors.Referring to the two types of distributors as being“mechanical” or “vacuum” is somewhat of a misnomersince the two distributors function basically the same. Themechanical distributor refers to the mechanical tach drivetake-off that this distributor features. It has no vacuumadvance unit. The mech-tach units are commonly referredto as “race” units. They also feature a more accurateregulator and a faster advance curve (full advance by 1,200rpm). The vacuum advance distributor has the vacuumadvance unit and must be used with an electrical tach(because it does not have the mech-tach drive feature).Vacuum advance units are less expensive and are designedfor dual purpose applications. The advance curve offersmore flexibility than the “race” unit, not being fullyadvanced until 2,000 rpm.The advance curve in the distributor is important to bothperformance and economy. There are two advance springs,one light and one heavy, in production units to reach thscurve. Some high performance vacuum advance models haveonly the light spring in the distributor and the advance curveis reshaped to give more advance at lower engine speed.As a general rule, a multi-purpose high performance machineshould have a fast advance curve. This means full centrifugaladvance by 2,000 rpm of engine speed. For a full-race dragcar such as a Pro Stock, a faster curve can be used.Note: At this time, Mopar Performance Parts does notoffer a vacuum advance distributor for the 4.0L engine.Since the mechanical distributor (stock on the 4.0L) is thepreferred racing piece, we recommend not modifying thesystem to a vacuum advance system, but rather thatemphasis be placed on obtaining a higher performancePowertrain Control Module (PCM) or Ignition ControlModule (ICM).