Code 42 - Est (Electronic Spark Timing)
Ignition module sends reference signal (circuit No. 430) to ECM when engine is cranking. When engine speed is less than 400 RPM, the ECM applies 5 volts to by-pass line (circuit No. 424) to switch the timing to be controlled by ECM.
When system is running on ignition module (no voltage on by-pass line), ignition module grounds the Electronic Spark Timing (EST) signal. The ECM expects to see no voltage on the EST line. If ECM sees a voltage signal, Code 42 will set and system will not go into EST mode.
When RPM for EST is reached (approximately 400 RPM) and by-pass voltage is applied, EST should no longer be grounded in the ignition module, so EST voltage will vary. If by-pass line is open or grounded, ignition module will not switch to EST, so EST voltage will be low and Code 42 will be set.
If EST line is grounded, the ignition module will switch to EST, but because line is grounded, there will be no EST signal and a Code 42 will be set.
- Code 42 indicates ECM has detected an open or short to ground in EST or by-pass circuit. This confirms that Code 42 and the fault causing the code are present.
- This checks for normal EST ground path through ignition module. An EST circuit No. 423 shorted to ground will also read less than 500 ohms.
- As test light voltage touches circuit No. 424, the module should switch, causing ohmmeter to overrange if ohmmeter is adjusted to the 1000-2000 ohms range. Selecting the 10,000-20,000 ohms range will indicate greater than 5000 ohms. The important thing is that module is switched.
- If module did not switch, this checks for shorted EST circuit No. 423, open in circuit No. 424, faulty ignition module connection or faulty ignition module.
- This step confirms that Code 42 is caused by a faulty ECM and not an intermittent in circuits No. 423 and 424.