DTC 42

Diagnostic Chart





Circuit Diagram

Circuit Description
The electronic ignition module sends a reference signal (YEL/RED wire) to the engine control module, when the engine is cranking. While the engine speed is under 500 revolution per minute, the ignition control module will control ignition timing. When the engine speed exceeds 500 revolution per minute, the engine control module applies 5.0 volts to the bypass line, to switch the timing to engine control module control.
When the system is running on the electronic ignition module, that is, no voltage on the bypass line, the electronic ignition module grounds the ignition control signal. The engine control module expects to sense or detect no voltage on the ignition control line during this condition. If it senses or detects a voltage, it sets diagnostic trouble code 42 and will not go into the ignition control mode.

When the revolution per minute for ignition control is reached (about 500 revolution per minute), voltage will he applied to the bypass line. The ignition control should no longer be grounded in the electronic ignition module, so the ignition control voltage should be varying. If the bypass line is open or grounded, the electronic ignition module will not switch to ignition control mode, so the ignition control voltage will be low and diagnostic trouble code 42 will be set.
If the ignition control line is grounded, the electronic ignition module will switch to ignition control but, because the line is grounded, there will be no ignition control signal. A diagnostic trouble code 42 will be set.

Test Description
The number(s) below refer to the circled number(s) on the diagnostic chart.
1. Diagnostic trouble code 42 means the engine control module has detected an open or short to ground in the ignition control or bypass circuits. This test confirms diagnostic trouble code 42 and that the fault causing the diagnostic trouble code is present.
2. Checks for a normal ignition control ground path through the ignition control module. An ignition control circuit wire, shorted to ground, will also read less than 500 ohms, however, this will be checked later.
3. As the test light voltage touches bypass circuit wire, the module should switch, causing the ohmmeter to "over-range" if the meter is in the 1000-2000 ohms position. Selecting the 10-20,000 ohms position will indicate above 5000 ohms. The important thing is that the module "switched."
4. The module did not switch and this step checks for:
- ignition control circuit wire shorted to ground.
- Bypass circuit wire open.
- Faulty ignition control module connection or module.
5. Confirms that diagnostic trouble code 42 is a faulty engine control module and not an intermittent in circuit wire or bypass circuit wire.