CIRCUIT DESCRIPTION:The Knock sensor is used to detect engine detonation and the ECM will retard the electronic spark timing (EST) base on the signal being received. The circuitry witin the knock sensor causes the ecm`s sopplied 5 volts signal to pulled down, so that under a no knock condition. the knock sensor circuit (dark blue) wire would measure about 2.5 volts. the knock sensor produces an AC signal which rides on the 2.5 volts DC voltage. The amplitude and signal frequency is dependent upon the knock level.
If the knock sensor circuit (bark blue wire) becomes open or shorted to ground, the voltage will either go above 3.5 volts or below 1.5 volts. If either of these conditions are met for 20 seconds, a code 43 will be stored.
TEST DESCRIPTION:Number(s) refer to circled number(s) on the diagnostic chart.
1. If a code 43 is detected, the ECM will retard spark timing by 10 degree. If an audible knock is heard from the engine, repair the internal engine problem, normally no knock should be detected at idle.
2. The ECM has a 5 volts pull-up resistor which should be present at the knock sensor terminal.
3. This test determines if the knock sensor is faulty or if the ESC portion of the MEM-CAl is faulty .
DIAGNOSTIC AIDS:Check the knock sensor circuit for a potential open or short to ground. Also check for proper installation of MEM-CAL. If the ESC circuit (drak blue wire) is routed to close to the secondary ignition wires it may induce a voltage and cause a false knock signal.
SOURCE: GM SERVICE MANUAL.
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