Mercury MerCruiser MCM 454 Mag MPI Service Manual page 731

TROUBLE CODE DIAGNOSTICS - 454/502 MAG MPI & 8.2L MPI
Code 43 (1 of 3): Knock Sensor (KS) Circuit (Scan)
CIRCUIT DESCRIPTION:
Sensing engine detonation or spark knock is accomplished with a module that sends a volt-
age signal to the ECM. As the knock sensor detects engine knock, the voltage from the KS
module to the ECM drops, and this signals the ECM to retard timing. The ECM will retard
the timing when knock is detected and rpm is above a certain value.
DIAGNOSTIC AIDS:
If CKT 496 is routed too close to secondary ignition wires, the KS module may see the inter-
ference as a knock signal, resulting in false retard.
An intermittent problem may be caused by a poor or corroded connection, rubbed through
wire connection, a wire that is broken inside the insulation, or a corroded wire.
Any circuitry that is suspected as causing the intermittent complaint should be thoroughly
checked for backed-out terminals, improper mating, broken locks, improperly formed or
damaged terminals, poor terminal-to-wiring connections, corroded terminals and/or wiring,
or physical damage to the wiring harness. After repairs, clear codes following "Clearing
Codes" at the front of this section. Failure to do so may result in codes not properly being
cleared.
TEST DESCRIPTION:
Number(s) below refer to step number(s) on the diagnostic table.
Step 2. This step determines if there is a problem in the circuit. When an KS circuit fails, the
ECM will switch to a default value of about 3.2 degrees of timing retard.
Step 3. This step checks if there is a voltage source to the knock sensor from the KS module.
Step 4. This step will determine if the knock sensor is faulty.
Index
Page 5F-34
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a
a - Knock Sensor Module
b - To System Relay
c - Starboard Knock Sensor
d - Port Knock Sensor
e - Knock Sensor Signal
SERVICE MANUAL NUMBER 23
FUSE 15A
b
485 GRN
PUR
c
e
d
90-861326--1 MARCH 1999