GM Bonneville/Eighty Eight/LeSabre 1986-1999

Knock Sensor



Varying octane levels in today's gasoline engines may cause detonation in some engines. Detonation is caused by an uncontrolled explosion (burn) in the combustion chamber. This uncontrolled explosion can produce a flame front opposite that of a normal flame front produced by the spark plug. The rattling sound normally associated with detonation is the result of 2 or more opposing pressures (flame fronts) colliding within the combustion chamber. Though light detonation is sometimes considered normal, heavy detonation could cause engine damage.

The 3.8L (VIN K) engine uses 2 knock sensors.

The Knock Sensor (KS) is mounted in the engine block, where it detects abnormal engine vibration, such as detonation (spark knock). When a knock is detected, the KS produces an AC voltage signal that is sent to the PCM. The system is designed to retard spark 10-15° to counteract the effects of varying levels of octane in gasoline. This allows the engine to use maximum spark advance to improve driveability and fuel economy.


See Figures 1, 2 and 3

  1. Visually check the connector, making sure it is connected properly and that all of the terminals are straight, tight and free of corrosion.
  3. Using a DVOM, check for an AC voltage signal between the sensor terminal and ground, while tapping on the engine block near the sensor. If voltage is not detected, check the resistance of the KS sensor between the sensor terminal and sensor body. If the resistance is more than 80-110 kilohms, the sensor is faulty.

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Fig. Fig. 1: Knock Sensor (KS) circuit - except 3.8L (VIN K) engine

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Fig. Fig. 2: Knock Sensor (KS) circuits - 1995-97 3.8L (VIN K) engine

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Fig. Fig. 3: Knock Sensor (KS) circuits - 1998-99 3.8L (VIN K) engine

  1. If voltage is seen, check the circuit continuity back to the PCM.
  3. If the sensor and circuits are functional, the PCM may be faulty.


See Figure 4

  1. Disconnect the negative battery cable.
  3. Raise and safely support the vehicle.
  5. If necessary, remove the splash shield.
  7. Position a suitable drain pan under the vehicle, then drain the engine coolant.
  9. If necessary, remove the knock sensor heat shield mounting bolt, located under the freeze plug (or block heater, if equipped).
  11. Disconnect the knock sensor wiring harness. On the 3.8L (VIN K) engine, you must detach the connectors from the bank 1 and bank 2 knock sensors.
  13. Remove the knock sensor from the engine block.

The knock sensor is mounted in the engine block cooling passage. Engine coolant in the block will drain when the sensor is removed.

Click image to see an enlarged view

Fig. Fig. 4: Exploded view of the knock sensor, electrical connector and heat shield

  1. Installation is the reverse of removal. Tighten the sensor to 14 ft. lbs. (19 Nm). Do NOT apply any type of sealant to the knock sensor threads.