ASE Practice Test (A6: Electrical/Electronic Systems) 2 — Questions and Answers

Question 1: A technician measures 14.1 volts at the battery with the engine running at 2,000 RPM. The headlights and other accessories are on. Is this reading normal?

No, the alternator is undercharging — voltage should be above 15 volts with load
Yes, 13.5-14.5 volts is the normal charging voltage range (Correct answer)
No, voltage should drop below 12.6 volts when accessories are on
Yes, but only if the battery is fully charged; otherwise it indicates a weak alternator

Correct answer: Yes, 13.5-14.5 volts is the normal charging voltage range

A charging system voltage of 13.5-14.5 volts (some manufacturers specify up to 14.8V) with accessories on and the engine running is normal. This voltage is sufficient to charge the battery and power all vehicle systems without overcharging.

The vehicle's charging system maintains battery charge and powers all electrical systems while the engine is running. The alternator (or generator in newer terminology) converts mechanical energy from the drive belt into electrical energy. A voltage regulator, either internal to the alternator or controlled by the ECM in modern vehicles, maintains output voltage within a safe range. The normal charging voltage range is 13.5-14.5 volts (with 14.8 volts acceptable in some designs). This range ensures the battery remains charged (a fully charged 12V battery sits at approximately 12.6-12.8V at rest) without overcharging (which would damage the battery and electrical components). Voltage above 14.8-15.0V indicates overcharging from a failing regulator — this can damage the battery, cause electrolyte boil-off in flooded lead-acid batteries, and damage sensitive electronics. Voltage below 13.5V with engine running and accessories on indicates an undercharging condition — the battery will eventually discharge. The 14.1V reading with accessories running is squarely within the normal range. Modern charging systems with battery state-of-charge monitoring (BSOC) or battery current sensors may vary output dynamically, sometimes showing lower voltage when the battery is fully charged and higher voltage immediately after a deep discharge cycle.

Question 2: A technician performs a voltage drop test across a ground connection and measures 0.4 volts. The specification is less than 0.1 volt. What does this indicate?

The connection has excessive resistance causing voltage drop and reduced circuit performance (Correct answer)
The circuit is drawing too much current and needs a larger fuse
The voltage drop is within the acceptable range for a main ground connection
The measurement indicates a good clean connection with normal resistance

Correct answer: The connection has excessive resistance causing voltage drop and reduced circuit performance

A voltage drop of 0.4 volts across a ground connection exceeds the maximum 0.1-volt specification, indicating excessive resistance at that connection. This resistance reduces effective circuit voltage, causes components to underperform, and can cause overheating at the connection point.

Voltage drop testing is the preferred method for identifying resistance problems in electrical circuits, particularly in connections, grounds, and conductors. Ohm's Law dictates that any resistance in a circuit will cause a voltage drop (V = I × R) proportional to the current flowing through it. For a ground connection, which should be a direct, low-impedance path to the battery negative terminal, the ideal voltage drop is zero. In practice, a small amount of resistance is unavoidable, so the acceptable limit is typically 0.1 volt or less (some specifications allow 0.2V for longer ground paths). A 0.4-volt drop across a single ground connection indicates significant resistance at that point — dirty, loose, or corroded connection, damaged wire, or a poor ground strap attachment to the body. This excessive resistance has real consequences: it reduces the effective voltage available to the component on that circuit. An interior light, for example, with 0.4V dropped in its ground circuit has only 11.8V available (12.2 - 0.4) instead of the full 12.2V, causing it to be dimmer. For sensitive electronic components (sensors, modules), excessive ground voltage drop can cause erratic operation, false sensor readings, and fault codes. The repair is to clean, tighten, or replace the ground connection and retest. Voltage drop test points include both the ground wire ends and any intermediate connection.

Question 3: A technician is using an oscilloscope to test a crankshaft position sensor (CKP). The waveform shows the expected pattern but is missing one group of pulses at regular intervals. What does the missing pulse group indicate?

A defective oscilloscope probe connection
A reference window or timing notch in the reluctor ring that the CKP uses as a TDC reference signal (Correct answer)
A failing CKP sensor that intermittently drops signal
Electrical noise in the ignition system interfering with the signal

Correct answer: A reference window or timing notch in the reluctor ring that the CKP uses as a TDC reference signal

Most CKP sensor reluctor rings have a gap (missing tooth or teeth) that creates a missing pulse or pulse gap in the sensor output. The ECM uses this 'missing tooth' reference to identify a specific crank position, typically used as a cylinder identification or TDC reference point.

Crankshaft position sensors (CKP) monitor the rotation of a reluctor ring (tone wheel) attached to the crankshaft. The reluctor ring is a toothed wheel whose teeth pass the sensor, generating electrical pulses. Each tooth passage generates one pulse, so the pulse frequency is proportional to engine speed. Most modern engines use a reluctor ring with a deliberate gap — a group of missing teeth (commonly called the 'missing tooth' or 'sync gap') at a specific crank position. The most common designs use 36-minus-2 teeth (36-2) or 60-minus-2 teeth (60-2), meaning a wheel that would have 36 or 60 evenly spaced teeth, but with 2 consecutive teeth removed. When the gap passes the sensor, the expected pulses are absent. The ECM recognizes this abnormally long gap (where pulses should be but aren't) as the index mark. From this reference, the ECM knows the absolute crankshaft position and can determine engine timing events for fuel injection and ignition with precision. This is essential for sequential fuel injection and individual cylinder ignition control. Seeing a regular, periodic missing group of pulses in the oscilloscope waveform is entirely normal and expected — it is the designed-in reference gap. The CKP waveform should be evaluated for: consistent pulse heights, clean transitions, absence of extra noise pulses, and the presence of the reference gap at the expected interval.

Question 4: A customer's battery repeatedly goes dead overnight. The charging system tests normal. A current drain test shows 150 milliamps with all doors closed and the vehicle in sleep mode. The specification is less than 50 milliamps. What is the procedure to find the source of the parasitic drain?

Replace all fuses until the drain stops
Remove fuses one at a time while monitoring current until the drain drops to within specification (Correct answer)
Replace the battery since it is too small for the current requirements
Disconnect and reconnect the negative battery cable repeatedly to reset all modules

Correct answer: Remove fuses one at a time while monitoring current until the drain drops to within specification

To locate the circuit causing parasitic drain, remove fuses one at a time while monitoring total drain current with an ammeter in series with the battery. When removing a fuse causes the current to drop to within specification, the circuit protected by that fuse contains the source of the drain.

Parasitic drain (key-off battery drain) testing is performed when a battery loses charge without being used for an extended period and the charging system is normal. The test involves connecting an ammeter (set to milliamps) in series with the battery negative terminal and allowing all vehicle modules to enter sleep mode (typically 15-30 minutes after all doors are closed and all switches are off). With a baseline current measurement in hand (150 mA in this case, against a spec of less than 50 mA), the technician must identify which circuit is drawing the excess 100+ mA. This is done by selectively removing fuses from the fuse box while monitoring the ammeter. When a fuse is removed that eliminates all or most of the excess drain, the protected circuit is identified. The technician then systematically works through that circuit — checking the components, connectors, and wiring in that branch — to find the specific component that is not going into sleep mode or that has failed in a current-drawing state. Important precautions: allow modules to fully enter sleep mode before testing (some modules stay active for 20+ minutes after shutoff); use a fused jumper or a battery saver to prevent module memory loss during fuse removal; some fuses protect multiple circuits, requiring further subdivision; some drains are on the unfused battery cables (starter circuit, alternator, alarm systems).

Question 5: What does the term 'duty cycle' mean when describing the operation of a fuel injector or other solenoid controlled by the PCM?

The total number of times the component has been activated during its service life
The percentage of time the component is ON (energized) during each operating cycle (Correct answer)
The maximum voltage the component can tolerate without damage
The time delay between the PCM command and the actual component activation

Correct answer: The percentage of time the component is ON (energized) during each operating cycle

Duty cycle is the percentage of an operating cycle during which a component is energized (ON). A 50% duty cycle means the component is on for half and off for half of each cycle. Pulse-width modulation (PWM) varies duty cycle to control component output, such as fuel injector pulse width for fuel metering.

Pulse-width modulation (PWM) is a technique used extensively in automotive electronics to control the average output of a device by rapidly switching it on and off. The ratio of on-time to total cycle time is called the duty cycle and is expressed as a percentage. For example, a fuel injector operating at 50% duty cycle is energized for exactly half of each operating cycle. At 20% duty cycle, it is energized for 20% and off for 80% of the time. The PCM varies the injector duty cycle (the actual term is pulse width, measured in milliseconds) to control how much fuel is delivered per engine cycle based on sensor inputs (MAF, MAP, O2 sensors, etc.). Duty cycle measurements are also used for: idle air control valves (varying airflow to control idle speed), variable valve timing oil control solenoids (controlling phaser position), electronic throttle control motors, EVAP purge solenoids, and transmission solenoids. In each case, varying the duty cycle varies the average flow through the device. On an oscilloscope, a PWM signal appears as a square wave, and the duty cycle can be read directly as the proportion of time the signal is at the high (on) state versus the total period. A scan tool displaying duty cycle data (in percent) is useful for diagnosing issues with solenoid-controlled components — a solenoid showing 100% or 0% duty cycle when it should be varying indicates a control problem.

Question 6: A technician is diagnosing an intermittent no-start condition. The starter cranks the engine but it will not fire. A check with a test light shows the injectors are not being commanded during cranking. The PCM has a power and ground. What should be checked NEXT?

The fuel pressure regulator for proper operation
The crankshaft position sensor signal during cranking (Correct answer)
The throttle position sensor for a proper voltage signal
The engine coolant temperature sensor for correct readings

Correct answer: The crankshaft position sensor signal during cranking

The PCM will not command injector operation without receiving a valid crankshaft position signal. If the PCM has power and ground but the injectors are not activated during cranking, the most likely cause is a missing or invalid CKP sensor signal — the PCM cannot determine crank position to time injection.

Modern engine management systems use a 'confirm before commanding' approach: the PCM will not activate fuel injectors, coils, or other output devices until it has verified certain fundamental conditions. The crankshaft position sensor (CKP) is one of the most critical inputs — the PCM must receive a valid CKP signal before it will enable fuel injection and spark. This design serves as a safety feature: if the engine stops but a relay-powered fuel pump continues to run, fuel is not wasted; and in a crash situation where the CKP signal stops (engine not turning), fuel injection is disabled immediately. The PCM's strategy is: no CKP signal = engine not turning = no fuel, no spark. When the PCM has confirmed power and ground (it's receiving battery voltage and is connected to chassis ground) but still won't command injectors during cranking, the absence of a valid CKP signal is the most common cause. The CKP can fail to generate a signal due to: sensor failure, air gap too large, damaged reluctor ring teeth, broken wiring, or faulty connector. Diagnosis: connect an oscilloscope or lab scope to the CKP signal wire and crank the engine. A functioning CKP should produce a clear waveform during cranking. No signal or a weak signal confirms the CKP circuit as the fault. Also check for CKP-related fault codes even in no-start conditions — many systems can store codes from CKP faults even without the engine starting.

ASE Practice Test (A6: Electrical/Electronic Systems) 2

A technician measures 14.1 volts at the battery with the engine running at 2,000 RPM.
The headlights and other accessories are on.
Is this reading normal?