Chart C-5 Knock Sensor (KS) System Check

Chart C-5:

Knock Sensor Circuit:

CIRCUIT DESCRIPTION
The knock sensor (KS) is used to detect engine detonation and allows the Powertrain Control Module (PCM) to retard the ignition control timing based on the signal being received. The circuitry within the knock sensor causes the Powertrain Control Module (PCM) supplied 5 volts signal to be pulled down so that under a no knock condition CKT 496 measures about 2.5 volts. The knock sensor produces an AC signal which rides on the 2.5 volts DC voltage. The amplitude and frequency are dependent upon the knock level.
The Programmable Read Only Memory (PROM) used with this engine contains the functions which were part of the remotely mounted ESC modules used on past GM vehicles. The knock sensor portion of the PROM then sends a signal to other parts of the PCM which retards the spark timing to reduce detonation.

CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.

1. Tapping on engine block simulates a knock signal and determine if the knock sensor is capable of detecting detonation. If no knock is detected, try tapping on engine block closer to the knock sensor before replacing the sensor.
2. The knock sensor produces and AC voltage proportional in amplitude to the "loudness" of the knock condition.
3. The PCM uses the knock sensor information to estimate the octane level of the fuel being used. The PCM selects a spark calibration for either 87, 90 or 93 octane.
4. Fuel with an octane level below 87 may cause detonation even with a properly operating engine. Engine mechanical problems such as low oil pressure, worn rod or main bearings or valve train problems may cause a knock sensor to respond to internal mechanical failures.
5. If it is determined that the PROM is faulty, be sure that it is properly installed and latched into place. Refer to Powertrain Control Module.

DIAGNOSTIC AIDS
- Detonation is most likely to occur under high engine load conditions.
- If the knock sensor CKT 496 is routed too close to secondary ignition wires, it may induce a voltage and cause a false knock signal.
- Be sure that threads on knock sensor and engine block are clean and free of rust.
- Never use teflon tape, anti-seize compounds or sealants on knock sensor threads.
- Refer to Detonation/Spark Knock, in Diagnosis by Symptom. Detonation/Spark Knock/Pings