Theory of Operation
Knock is the spontaneous auto-ignition of the remaining fuel/air mixture in the engine combustion chamber that
occurs after normal combustion has started. It can occur under extreme vehicle operating conditions such as high
engine temperature, high MAP, low humidity, and heavy loads to the engine. Knock is caused by excessive spark
advance for the given engine operating conditions. Severe, continuous knock may be caused by carbon deposits,
bad gasoline, and/or low octane fuel. Avoiding light audible knock is important for customer satisfaction while pre-
venting excessive knock is important to protect engine components. The output voltage from the knock circuit rep-
resents the strength of the engine knock and is read by the engine controller. The knock system output voltage is
not zero due to engine background noise, even when knock is not present. When the engine is operated under high
load conditions where knock is possible, the knock voltage is tested to decide if it exceeds the knock voltage thresh-
old. Knock has occurred when the knock voltage is at or above this knock threshold. When knock is detected a
calibrated short term knock spark retard to be subtracted from the spark advance is calculated. The amount of
retarded spark advance is based off a calibrated severity of the knock event. This retarded spark advance is used
in the next ignition event to prevent further knock events. If knock continues, an additional amount of short term
spark advance retard is added. When knock stops, short term knock spark retard is eliminated, the long term knock
spark retard is reduced by a calibrated amount to recover some previously retarded spark advance. This decreases
spark retard to improve engine performance.
•
When Monitored:
This monitor runs above 2000 rpm, under open throttle conditions. The Knock diagnostic does not run at idle
or during decelerations. The high voltage test runs all the times the engine is running.
•
Set Condition:
The High voltage fault will set at 4.8 volts. Two Trip Fault. Three good trips to turn off the MIL.
Possible Causes
(K242) KNOCK SENSOR NO.2 SIGNAL CIRCUIT SHORTED TO BATTERY VOLTAGE
(K242) KNOCK SENSOR NO.2 SIGNAL CIRCUIT OPEN
(K924) KNOCK SENSOR NO.2 RETURN CIRCUIT OPEN
(K242) KNOCK SENSOR NO.2 SIGNAL CIRCUIT SHORTED TO THE (K924) KNOCK SENSOR NO.2 RETURN
CIRCUIT
(K242) KNOCK SENSOR NO.2 SIGNAL CIRCUIT SHORTED TO GROUND
KNOCK SENSOR
PCM
Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 9 - ENGINE -
DIAGNOSIS AND TESTING).
Diagnostic Test
1.
ACTIVE DTC
Ignition on, engine not running.
With a scan tool, read DTCs.
Is the DTC active at this time?
Yes
>> Go To 2
No
>> Refer to the INTERMITTENT CONDITION Diagnostic Procedure.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
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ENGINE ELECTRICAL DIAGNOSTICS
KJ