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    2007 Pontiac Grand Prix 3.8L SFI OHV 6cyl

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    Cherokee, Grand Cherokee, 1999-2005

    Knock Sensor

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    Operation



    The 2 knock sensors are bolted into the cylinder block under the intake manifold. The sensors are used only with the 3.7L engine.

    Two knock sensors are used on the 3.7L V-6 engine; one for each cylinder bank. When the knock sensor detects a knock in one of the cylinders on the corresponding bank, it sends an input signal to the Powertrain Control Module (PCM). In response, the PCM retards ignition timing for all cylinders by a scheduled amount.

    Knock sensors contain a piezoelectric material, which constantly vibrates and sends an input voltage (signal) to the PCM while the engine operates. As the intensity of the crystals vibration increases, the knock sensor output voltage also increases.

    The voltage signal produced by the knock sensor increases with the amplitude of vibration. The PCM receives the knock sensor voltage signal as an input. If the signal rises above a predetermined level, the PCM will store that value in memory and retard ignition timing to reduce engine knock. If the knock sensor voltage exceeds a preset value, the PCM retards ignition timing for all cylinders. It is not a selective cylinder retard.

    The PCM ignores knock sensor input during engine idle conditions. Once the engine speed exceeds a specified value, knock retard is allowed.

    Knock retard uses its own short-term and long-term memory program. Long-term memory stores previous detonation information in its battery-backed RAM. The maximum authority that long term memory has over timing retard can be calibrated. Short-term memory is allowed to retard timing up to a preset amount under all operating conditions (as long as rpm is above the minimum rpm) except at Wide Open Throttle (WOT). The PCM, using short-term memory, can respond quickly to retard timing when engine knock is detected. Short-term memory is lost any time the ignition key is turned OFF.


    NOTE
    Over or under tightening the sensor mounting bolts will affect knock sensor performance, possibly causing improper spark control. Always use the specified torque when installing the knock sensors.

    Removal & Installation



    The 2 knock sensors are bolted into the cylinder block under the intake manifold.


    NOTE
    The left sensor is identified by an identification tag (LEFT). It is also identified by a larger bolt head. The Powertrain Control Module (PCM) must have and know the correct sensor left/right positions. Do not mix the sensor locations.

    1. Disconnect knock sensor dual pigtail harness from engine wiring harness. This connection is made near rear of left valve cover.
    2.  
    3. Remove intake manifold.
    4.  
    5. Remove sensor mounting bolts. Remove sensors from engine.
      NOTE
      Note the foam strip on bolt threads. This foam is used only to retain the bolts to sensors for plant assembly. It is not used as a sealant. Do not apply any adhesive, sealant or thread locking compound to these bolts.

    6.  

    To Install:


    WARNING
    Over or under tightening the sensor mounting bolts will affect knock sensor performance, possibly causing improper spark control. Always use the specified torque when installing the knock sensors. The torque for the knock senor bolt is relatively light for an 8mm bolt.


    NOTE
    Note foam strip on bolt threads. This foam is used only to retain the bolts to sensors for plant assembly. It is not used as a sealant. Do not apply any adhesive, sealant or thread locking compound to these bolts.

    1. Thoroughly clean knock sensor mounting holes.
    2.  
    3. Install sensors into cylinder block.
    4.  
    5. Install and tighten mounting bolts.
    6.  
    7. Install intake manifold.
    8.  
    9. Connect knock sensor wiring harness to engine harness at rear of intake manifold.

      Click image to see an enlarged view

      Fig.

    10.  

     
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