ASE A6 Practice Test — Electrical/Electronic Systems

In a series circuit, voltage divides proportionally across each resistor based on its resistance. Total resistance is 3 + 6 = 9 ohms. Total current is V/R = 12/9 = 1.33 amps. Voltage across the 6-ohm resistor is I x R = 1.33 x 6 = 8 volts. The remaining 4 volts drops across the 3-ohm resistor (1.33 x 3 = 4). Total voltage drops equal source voltage: 8 + 4 = 12V. This is the voltage divider principle, and it is fundamental to understanding why unwanted resistance (corrosion, loose connections) in a circuit steals voltage from the component you are trying to power. If a corroded connector adds 2 ohms of resistance to a 4-ohm load, the load only gets 8 volts instead of 12. That is why voltage drop testing catches problems that ohmmeter testing misses.

Multiple unrelated electrical systems acting up intermittently points to a common power or ground issue — not to individual component failures. Battery cable connections and ground straps are the shared electrical foundation for every system in the vehicle. A loose or corroded battery terminal or ground strap creates intermittent high resistance in the path that feeds everything. When resistance increases momentarily, voltage drops to multiple circuits simultaneously, causing flickering, resets, and dropouts. Always start with the basics — battery terminals, cable ends, and body ground straps. Clean them, tighten them, and retest. An alternator with excessive AC ripple (A) is a valid concern but is less common than connection issues. Replacing a BCM (C) or ignition switch (D) before checking connections is throwing parts at a problem. The simplest, most common cause always gets checked first.

Both technicians are correct. The CAN bus uses two wires — CAN high and CAN low — that carry differential signals. A short to ground on the CAN high wire (Technician A) collapses the signal voltage and prevents proper communication between all modules on that bus segment. Every module loses the ability to talk, resulting in multiple U-codes (communication DTCs) across the network. A faulty terminating resistor (Technician B) causes signal reflections on the CAN bus wires. CAN bus networks use 120-ohm terminating resistors at each end of the bus to absorb the signal and prevent it from bouncing back. If a resistor opens or shorts, the reflections corrupt the data, causing communication errors across the network. When you see multiple U-codes from unrelated modules, measure the CAN bus termination resistance — you should see 60 ohms between CAN high and CAN low (two 120-ohm resistors in parallel). Any other reading points to a network-level problem.

A starter relay with welded contacts would cause the opposite problem — the starter would stay engaged or crank constantly because the welded contacts create a permanent connection. Welded relay contacts mean the circuit is always closed, sending power to the starter even when the key is released. This is a stuck-cranking or continuous-cranking condition, not slow crank. A low battery (A) reduces the voltage and current available to the starter. High resistance in the ground circuit (B) reduces current flow by creating an unwanted voltage drop. A corroded or undersized positive cable (D) restricts current flow to the starter. All three reduce the energy reaching the starter motor, making it turn slowly. Slow crank is always an energy delivery problem — not enough voltage, not enough current, or both. Welded contacts are a control problem, not an energy problem.

Maximum acceptable voltage drop on the positive side of the starter circuit is 0.5 volts (some specs say 0.2V for the cable alone). A reading of 0.8V means excessive resistance exists somewhere in the positive cable path — at the battery terminal, along the cable, at a junction, or at the starter connection. That 0.8 volts is being wasted as heat in the resistance instead of reaching the starter. At 200 amps of cranking current, 0.8V of drop means 160 watts of energy being turned into heat in the cable instead of turning the starter. The fix is to clean all connections, inspect the cable for corrosion under the insulation, and retest. If the voltage drop persists after cleaning, the cable has internal corrosion and must be replaced. Voltage drop testing is the only way to find this — the cable may look fine on the outside while corroded internally.

Both technicians are correct. With the engine running at 2,000 RPM, battery voltage should be 13.5-14.5V. A reading of 12.4V means the alternator is not charging — the battery is supplying the entire electrical load without help from the alternator, and it is slowly discharging. Technician A correctly identifies the no-charge condition. Technician B correctly identifies one of the most common causes — a broken, loose, or slipping drive belt prevents the alternator from spinning, so it cannot generate output. Before condemning the alternator itself, always check the belt, the belt tensioner, and the alternator wiring (B+ connection, field circuit, ground). A broken belt is a five-second visual check. A loose or glazed belt may look fine but slip under load. If the belt is good and tight, then check the alternator field circuit and internal components. Work from simple to complex.

In a parallel circuit, total current equals the sum of all branch currents. Three branches at 2 amps each: 2 + 2 + 2 = 6 amps total. This is Kirchhoff's Current Law — the current flowing into a junction equals the current flowing out. Each branch sees the full source voltage independently, and each draws current based on its own resistance. The source must supply the combined total. This is why adding accessories to a vehicle (more parallel branches) increases the total current draw from the battery and alternator. It is also why a short circuit is dangerous — a near-zero-resistance branch draws enormous current that the wiring was not designed to handle. Fuses protect against this by opening the circuit when current exceeds the wire rating. Understanding parallel circuit current is essential for diagnosing excessive current draw and fuse sizing.

Technician A is correct. A dim headlight on one side with a normal headlight on the other is a classic voltage drop scenario. The dim side is not receiving full system voltage because resistance in the ground circuit (or power supply circuit) is consuming part of the voltage. A voltage drop test on the ground wire from the headlight connector to the battery negative will reveal the resistance. Common culprits are corroded ground connectors, damaged ground wires, or loose ground bolt connections. Technician B is incorrect because headlight bulbs of the same part number have consistent filament resistance from the factory — they do not vary enough to cause a noticeable brightness difference. If you replace the bulb and the new one is also dim, the circuit is the problem, not the component. Always test the circuit before replacing the part.

A fuse that blows immediately upon installation — before you even try to operate the window — means there is a dead short to ground somewhere in the circuit downstream of the fuse. As soon as the fuse completes the circuit, the short draws massive current and the fuse blows to protect the wiring. This is typically caused by a pinched or chafed wire where the insulation has been worn through and the conductor touches metal body panels. Common locations are where wiring passes through door jambs, under seats, or through grommets in the firewall. A motor drawing slightly more current (A) would blow the fuse during operation, not instantly. An incorrect fuse (C) is worth verifying but does not explain an instant blow. A weak battery (D) would cause slow operation, not a blown fuse. Instant fuse failure equals dead short — trace the wiring to find the contact point.

Air-fuel ratio calculation and fuel injector control are functions of the powertrain control module (PCM) or engine control module (ECM) — not the body control module. The BCM manages body electrical functions: lighting (A), door locks and keyless entry (B), chimes and warnings (D), wipers, power windows, and other convenience features. The PCM handles everything related to engine and transmission operation — fuel delivery, ignition timing, emissions, and transmission shift control. These are separate modules that communicate over the CAN bus network. On the ASE test, knowing which module controls which function is critical. If a question describes a lighting or comfort system issue, think BCM. If it describes an engine performance or drivability issue, think PCM. If multiple unrelated systems from both categories are failing, think network communication.