Knock Sensor (KS) & Module

Mefi 1

The knock module is used in the MEFI 1 systems. During no knockconditions it sends an 8-10 volt signal to the ECM. When knock is detected it will remove the signal from the ECM. The ECM will then retard the timing as much as 10%. The knock system was fully incorporated into the ECM with the introduction of the “WT” models equipped with the MEFI 3 system. All MEFI 3 and 4 systems have a knock system built into the ECM.

A summary of the knock system follows. There are 2 different systems for modifying spark advance based off of knock sensor input for MEFI 4 engines. The first is the knock system. It is active above idle after a minimum temperature and run timehave been met. It can retard timing up to approximately 10 degrees based on input fromthe knock sensor. The other system is the octane rating system which only operates in the upper 1/3 of the power band. The octane rating system fine tunes the spark advance under high RPM high load conditions. The octane rating system works as follows.

The MEFI 4 system has 2 spark tables, one for high octane (approx 93) and one for low octane (approx 87). Once the engine is above a certain temp, a run timer has timed out and the engine is in the upper 1/3 of RPM and load rating for the engine, the octane rating system will fine tune the spark advance to obtain the optimum spark timing for that particular operating condition. The 2 tables are based on RPM and MAP.

As an example, one engine’s high octane table may show 29 degrees of spark advance at 4600 rpm with 3”of vacuum in the intake. The low octane table for the engine’s same operating condition may show 26 degrees of spark advance. Once the octane rating system is active, as the knock sensors show that the cylinders are knocking, the ECM will step timing down from 29 towards 26 degrees to make the knock go away. After the knock is gone, the system will start advancing the timing back towards 29degrees until the knock sensor just starts to indicate knock. The system will always tryto maximize spark advance to where knock just starts to be indicated.

All of this shows up on the scan tool and Diacom under octane rating. 100% means that the value from the high octane table is being used and 0% means that value from the low octane table is being used. Obviously, ratings between 0 and 100% are using values between the high and low table values.