Powertrain Controls - ECM/PCM
CODE 42 ELECTRONIC SPARK TIMING (EST) CIRCUITCode Chart 42:
Ignition Wiring Diagram:
CIRCUIT DESCRIPTION:
The ignition module sends a reference signal (YEL/RED wire) to the ECM, when the engine is cranking. While the engine speed is under 400 rpm, the ignition module controls ignition timing. When the engine speed exceeds 400 rpm, the ECM applies 5.0 volts to the "bypass" line, (YEL/GRN wire) to switch the timing to ECM control (EST YEL wire). When the system is running on the ignition module, that is, no voltage on the by-pass line, the ignition module grounds the EST signal. The ECM expects to see no voltage on the EST line during this condition. If it senses a voltage, it sets Code 42 and will not go into the EST mode. When the rpm for EST is reached (about 400 rpm), and by-pass voltage applied, the EST should no longer be grounded in the ignition module so the EST voltage should be varying. If the by-pass line is open or grounded, the ignition module will not switch to EST mode so the EST voltage will be low and Code 42 will be set. If the EST line is grounded, the ignition module will switch to EST, but because the line is grounded there will be no EST signal. A Code 42 will be set.
TEST DESCRIPTION: The numbers below refer to circled numbers on the diagnostic chart.
1. A Code 42 means the ECM has seen an open or short to ground in the EST or bypass circuits. This test confirms Code 42 and that the fault causing the code is present.
2. Checks for a normal EST ground path through the ignition module. An EST YEL wire, shorted to ground, will also read less than 500 ohms. This will be checked later.
3. As the test light voltage touches YEL/GRN wire, the module should switch causing the ohmmeter to "overrange" if the meter is in the 1000-2000 ohm position. Selecting 10-20,000 ohm position will indicate greater than 5000 ohms. The module should "switch."
4. If the module did not switch, this step checks for:
a. EST YEL wire shorted to ground.
b. Bypass YEL/GRN wire open.
c. Faulty ignition module connection or module.
5. Confirms that Code 42 is caused by a faulty ECM and not an intermittent in YEL wire or YEL/GRN wire.