Engine Electrical and Electronic Systems Diagnosis and Repair
What the candidate must know
This domain covers the battery, starting system, charging system, sensors, actuators, wiring, grounds, and the ECM/PCM that manages the engine. Know how to perform a voltage drop test, how to check battery state of charge and load, how to test the starter circuit, how to evaluate charging output, and how to use scan data and codes to guide diagnosis. Understand sensor types (MAP, MAF, coolant temp, throttle position, O2) and how their failures affect driveability.
Traps in questions
- Cranking but no start vs. no crank: no crank points to battery, cables, starter, or neutral safety; crank-no-start points to fuel, spark, or compression. Do not mix these paths.
- Voltage drop testing is the correct way to find a bad cable or ground; measuring resistance with an ohmmeter can miss a cable that fails only under load. Questions often prefer the voltage drop method.
- Slow crank can be a weak battery, bad cable, or failing starter - test each with voltage drop, not by replacing the battery first.
- Charging system output should be checked at the battery with loads on; a reading at the alternator can be normal while the battery sees less due to cable resistance.
- Sensor codes do not always mean the sensor is bad; a coolant temp sensor reading implausibly cold can be wiring or the sensor, and it affects fuel trims and fan operation.
- Grounds are a frequent cause of multiple unrelated codes. Check grounds before chasing individual sensors.
Worked example
A truck cranks slowly and the battery tests good. A voltage drop test from the battery negative to the engine block shows 0.8 V while cranking (spec under 0.2 V). The ground cable/connection is the fault - not the starter.
Rule of thumb
No crank = starting circuit; crank no-start = fuel, spark, or compression. Use voltage drop, not ohmmeter resistance, to find high-resistance cables and grounds.