Detonation sensors (knock sensors) on Chrysler cars

by Bob O’Neill and Bob Lincoln

This sensor is not present on the 4-cylinder TBI engines.

When the engine is running too lean or some other factor such as too much boost causes the engine to begin detonating (igniting the fuel before it is supposed to be ignited), the detonation sensor signals the ECU. This sensor is a piezo-electric unit designed to generate a voltage when it detects a vibration.  The ECU, then, assumes there is detonation when the sensor sends a voltage above .08v when the engine is running at 2000 RPM, or when the sensor sends a voltage above 4.0v at 4000 RPM or above.

For 1984 models the ECU reduces the maximum advance ignition timing for all cylinders by 2 degrees when the sensor indicates detonation occurs.  If the detonation signal continues, the maximum advance is reduced again by 14 degrees.  After the timing have been reduced to limit detonation and detonation stops the ECU restores the 14 degrees of maximum advance.

For 1985 and later models, the ECU reduces the amount of timing (from the usual setting) based on the severity and persistence of the detonation.  The ECU identifies which cylinder is detonating by checking the position of the distributor based on the signal from the Hall Effect pickup, and reduces timing only for that cylinder. 

If detonation occurs at part throttle and the turbo is producing no boost, timing is reduced by 4.2 degrees or up to 8.2 degrees when detonation occurs.  When the turbo is producing boost  and detonation occurs, the reduction is 2.1 degrees or up to a maximum of 15 degrees. 

The ECU will also reduce boost when the engine detonates while in boost.  If detonation continues and happens more than once every 5.5 seconds, the ECU reduces the boost until the MAP sensor output is reduced up to 8% of what it was.  If detonation continues and happens more than once every 17 seconds, but less than once every 5.5 seconds, boost is reduced, based on how often detonation has occurred up, to a maximum of 8%.

Detonation that occurs once every 18 seconds or more is not considered by the ECU and no reduction in boost or timing is required.

LINKS: Turbo engines | MAP sensor

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