Spark Knock Test
- If vehicle is not equipped with knock sensor or 3 connector ignition module, check ignition timing and reset as necessary. Testing is now complete. MCU system is functional and problem must be in some other area.
- If vehicle is equipped with knock sensor or 3-connector module, disconnect knock sensor and set timing. Disconnect 2-wire connector (Yel and Blk/Wht wires) and jumper wires together at module.
- Check ignition timing. If timing retards 16-20°, reconnect module and go to next step. If timing does not retard, replace module and repeat Spark Knock Test.
- With vehicle at normal operating temperature, set fast idle screw on cam step that will maintain engine speed above 1200 RPM. Read and record ignition timing.
- Locate zone vacuum switch, then remove and plug hose from it. Recheck timing, if timing retards more than 5 degrees and system has a knock sensor, go to step 7). If no knock sensor, testing is complete, MCU system okay. If timing does not retard 5 degrees, go to next step.
- While engine is still at 1200 RPM, disconnect 2-wire connector at ignition module. If timing retards, check Yellow wire for short to ground. If okay, replace MCU module and retest. If timing does not retard, check Yellow wire for open circuit. If okay, replace MCU module and retest.
- Reconnect knock sensor. Repeat MCU Running Test, but do not simulate knock by tapping rod on intake manifold. If service code 25 is obtained, MCU system is okay. If any other code is observed, replace knock sensor and retest.