Liberty, Wrangler 2006-2007

Knock Sensor



Two knock sensors are used on the 3.7L V6 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.

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 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.
  3. Remove intake manifold.
  5. Remove sensor mounting bolts. Remove sensors from engine.
    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.


To install:

Click image to see an enlarged view

Fig. Knock sensor locations (1) and mounting bolts (2)-3.7L engine

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 sensor bolt is relatively light for an 8mm bolt.

  1. Thoroughly clean knock sensor mounting holes.
  3. Install sensors into cylinder block.
  5. Install and tighten mounting bolts to 15 ft. lbs. (20 Nm).
  7. Install intake manifold.
  9. Connect knock sensor wiring harness to engine harness at rear of intake manifold.


  1. Start the engine and allow it to reach normal operating temperature. Using a diagnostic scan tool, check for the presence of any Diagnostic Trouble Codes (DTCs). Record and address these codes as necessary.
  3. Refer to any Technical Service Bulletins (TSBs) that may apply.
  5. Review the scan tool Freeze Frame information. If possible, try to duplicate the conditions under which the DTC set.
  7. With the engine running at normal operating temperature, monitor the scan tool parameters related to the DTC while wiggling the wire harness. Look for parameter values to change and/or a DTC to set. Turn the ignition off.
  9. Visually inspect the related wire harness. Disconnect all the related harness connectors. Look for any chafed, pierced, pinched, partially broken wires and broken, bent, pushed out, or corroded terminals. Perform a voltage drop test on the related circuits between the suspected inoperative component and the ECM.
    Do not probe the ECM harness connectors. Probing the ECM harness connectors will damage the ECM terminals resulting in poor terminal to pin connection. Install Miller Special Tool #8815 to perform diagnosis.

  11. Inspect and clean all ECM, engine, and chassis grounds that are related to the most current DTC.
  13. If numerous trouble codes were set, use a wire schematic and look for any common ground or supply circuits.
  15. For any Relay DTCs, actuate the Relay with the scan tool and wiggle the related wire harness to try to interrupt the actuation.
  17. For intermittent Evaporative Emission trouble codes perform a visual and physical inspection of the related parts including hoses and the Fuel Filler cap.
  19. Use the scan tool to perform a System Test if one applies to failing component. A co-pilot, data recorder, and/or lab scope should be used to help diagnose intermittent conditions.