Electrical and Electronic Systems A6 Flashcards

A technician measures battery voltage at 12.4 volts before a load test, and the voltage drops to 10.1 volts during the load test (applying 50% of the cold cranking amp rating for 15 seconds). What does this indicate?

Answer: Battery has failed the load test and should be replaced

During a battery load test, voltage must remain above 9.6 volts (some specifications say 9.0 volts) for the duration of the test. A drop to 10.1 volts is within the passing range for most specifications; however, context matters — see the detailed explanation for full analysis.

A vehicle has an intermittent stalling problem. When the engine stalls, all warning lights illuminate. Attempts to restart immediately are sometimes successful and sometimes not. What does the symptom pattern (all warning lights on when stalling) indicate?

Answer: A complete loss of power to the PCM, causing all monitored systems to lose signal

When all warning lights illuminate simultaneously during a stall, it indicates a complete power loss to the instrument cluster and PCM — not multiple individual faults. When the PCM loses power, all of the warning lights (check engine, oil, battery, etc.) illuminate by default just as they do during the key-on bulb check.

An ECT (engine coolant temperature) sensor reads -40°F on the scan tool even though the engine is at normal operating temperature. What type of sensor failure does this indicate?

Answer: The sensor circuit is open, resulting in maximum resistance that the PCM interprets as extremely cold temperature

ECT sensors are negative temperature coefficient (NTC) thermistors — resistance decreases as temperature increases. An open circuit (infinite resistance) produces the same effect as an extremely cold sensor (very high resistance). The PCM interprets maximum resistance as the coldest possible temperature (-40°F).

A CAN bus communication fault is stored in multiple control modules. When measured with an oscilloscope, the CAN high and CAN low wires both show approximately 2.5 volts with no activity. What does this indicate?

Answer: A short circuit between the CAN high and CAN low wires, suppressing communication

On a properly functioning CAN bus, the idle (recessive) state shows both CAN H and CAN L at approximately 2.5 volts with no traffic. However, the problem here is 'no activity' — if modules are reporting communication faults, the bus should show data activity. Both wires stuck at 2.5V with no signal variation indicates a short between the two wires.

A technician is testing a starter motor circuit and finds 11.8 volts at the starter battery terminal during cranking, with 12.4 volts at the battery positive terminal during cranking. What calculation must be done and what does the result indicate?

Answer: Subtract to find 0.6 volt drop in the positive cable circuit — this exceeds the 0.5-volt maximum and indicates excessive resistance in the cable or connections

Subtracting gives 12.4 - 11.8 = 0.6 volts dropped across the positive cable circuit during cranking. This exceeds the typically accepted maximum of 0.5 volts (many manufacturers specify 0.2 volts or less), indicating excessive resistance in the positive cable, connections, or fusible link in that circuit.

A vehicle with a multiplexed body control module (BCM) system has several intermittent electrical faults: door locks operate randomly, interior lights flash, and the windows move unexpectedly. What is the MOST likely cause?

Answer: A poor BCM power or ground connection causing erratic module operation

Erratic, simultaneous operation of multiple BCM-controlled systems (locks, lights, windows) points to a problem with the BCM itself rather than individual components. A poor power or ground connection to the BCM causes erratic module behavior — the module intermittently malfunctions, commanding outputs randomly.