Powertrain Controls - ECM/PCM

Diagnostic Trouble Code 43:

Knock Sensor Circuit:

CIRCUIT DESCRIPTION
The knock sensor system is used to detect engine detonation. The Engine Control Module (ECM) will retard the spark timing based on a signal being received from the knock sensor. The knock sensor produces an Alternating Current (AC) voltage which is sent to the ECM. The amount of AC voltage produced by the knock sensor is determined by the amount of knock. The circuitry within the knock sensor causes the ECM's 5 volts to be pulled down so that under a no knock condition, CKT 496 would measure about 2.5 volts.

There are two internal ECM tests run on this circuit to determine it is operating correctly. If either of the tests fail, a Diagnostic Trouble Code (DTC) 43 will be set.

Test # 1
If CKT 496 becomes open or shorted to ground, the voltage will either go above 3.62 volts or below 1.44 volts. If either of these conditions are met for about 16 seconds, a DTC 43 will be stored.

Test # 2
This system also performs a functional test to determine if the knock sensor is responding to engine detonation. To perform this test, the ECM will advance the spark under certain load conditions and look for a knock signal response. If knock is detected before conditions are met to run the test, the test is bypassed, but if the test is run and no knock detected, the DTC 43 will be set.

This test is performed when:
^ Engine running longer than 4 minutes.
^ Engine coolant temperature is over 90°C (194°F).
^ High engine load based on air flow.

TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.

1. If the conditions for the first test, as described above, have been met, the Tech 1 scanner will always indicate "NO" when the knock signal position is selected. If an audible knock is heard from the engine repair, the internal engine problem, as normally no knock should be detected at idle.

2. The ECM supplies 5 volts on CKT 496 which should be present at the knock sensor terminal.

3. This test determines if the knock sensor is faulty or if the Knock Sensor (KS) portion of the Programmable Read Only Memory (PROM) is faulty.

DIAGNOSTIC AIDS
Check CKT 496 for a potential open or short to ground.

Also check for proper installation of PROM.

Refer to Intermittents in Diagnosis By Symptom. - Intermittent Malfunctions

If the customer's complaint is the Malfunction Indicator Lamp (MIL) (Service Engine Soon) comes "ON" when in acceleration, the second portion of the DTC is failing. There is a possibility that the Distributor Ignition (DI) control module was in bypass mode when the 43 test was run. An intermittent open in the IC circuit will put the DI module in bypass which will not allow the spark to be advanced, so the 2nd test would fail. If ECM also had a DTC 42 stored, then the IC circuit is likely the cause of the DTC 43.