Electrical and electronics review

Electrical was the block the study group feared most and needed most. On the first practice exam, the three of them together got fewer than half the network questions right. Marty did not start with networks. He drew a battery, a fuse, a switch, a bulb and a ground on the whiteboard and asked where a voltmeter would read 12 volts, where it would read zero, and where it would read something in between, with the switch closed and a corroded ground. "Every electrical question on the exam, including the network ones, is this drawing with more boxes in it."

Electrical/Electronic Systems, A6, has 50 scored questions on general electrical diagnosis, batteries, starting and charging, lighting, instruments and driver information, body electrical systems including power accessories and supplemental restraints, and networks. A6 is widely considered the hardest A-series test, and it is the one candidates most often need to retake.

The review follows AUTO-220 (Electrical and Electronic Systems), with module programming from AUTO-330 lesson 11.

Ohm's law and circuit faults

Ohm's law, voltage equals current times resistance, and the power equation, power equals voltage times current, are in every electrical question, sometimes as arithmetic and more often as reasoning. In a series circuit, the current is the same everywhere and the voltages across the loads add up to the source voltage; in a parallel circuit, each branch has the source voltage and the currents add. An open circuit stops current; a short to ground bypasses the load and usually blows the fuse; a short to power keeps a load on when it should be off; high resistance, from corrosion or a loose terminal, reduces current and makes a load dim or slow.

A voltage drop test, from AUTO-220 lesson 3, measures the voltage across part of a circuit while current flows. It is the single most useful electrical test, and the exam returns to it constantly. A good connection or a short length of wire drops very little, typically under 0.1 V per connection and under about 0.5 V across a whole side of a circuit under load; a starter cable circuit may be allowed slightly more by the maker. A reading near source voltage across a closed switch means the switch is open inside; a reading of near zero across a load that does not work means no current is flowing, so the open is elsewhere.

# Voltage drop survey around a lamp circuit (readings with the circuit on)
source = 12.1
drops = {"fuse": 0.05, "switch": 0.08, "power wire": 0.06, "lamp": 10.81, "ground wire and connection": 1.10}
print(f"sum of drops: {sum(drops.values()):.2f} V against source {source:.2f} V")
for part, v in drops.items():
    if part == "lamp":
        print(f"{part:<28}{v:6.2f} V  the load")
    else:
        print(f"{part:<28}{v:6.2f} V  {'too high' if v > 0.2 else 'ok'}")
lamp_r = 2.2
print(f"lamp power now about {10.81 ** 2 / lamp_r:.0f} W, against {11.9 ** 2 / lamp_r:.0f} W with a good ground")

sum of drops: 12.10 V against source 12.10 V
fuse                          0.05 V  ok
switch                        0.08 V  ok
power wire                    0.06 V  ok
lamp                         10.81 V  the load
ground wire and connection    1.10 V  too high
lamp power now about 53 W, against 64 W with a good ground

Meters and testing

A digital multimeter set to volts is connected in parallel across what is being measured; set to amps it is connected in series, in the circuit; set to ohms it must be connected to a circuit with no power. Measuring resistance through a powered circuit gives false readings and can damage the meter. A test light shows voltage but not how much; a powered test light can damage electronic circuits and must never be used on airbag or module circuits. Fused jumper leads bypass a suspect switch or wire safely. Parasitic draw, the current the vehicle uses with everything off, is measured with an ammeter or a clamp-on probe after the modules have gone to sleep; AUTO-220's capstone found a telematics box keeping the network awake. Many makers allow around 20 to 50 milliamps after the sleep time, which can be 45 minutes or more.

Batteries, starting and charging

A battery's state of charge is read from its open-circuit voltage after the surface charge is removed: about 12.6 V is fully charged and about 12.2 V is half charged for a flooded battery, slightly higher for AGM. Capacity is tested with a load test or a conductance tester. Cold cranking amps is the current a battery can deliver at 0 °F for 30 seconds while staying above 7.2 V. A replaced battery on many vehicles must be registered with the battery management system so the charging strategy matches it.

Starting system questions combine Ohm's law with voltage drop: a slow crank with a good battery points to high resistance in the cables, connections or the starter itself, found by voltage drop on the positive and ground sides while cranking, or to an engine that is hard to turn. A click with no crank points to the solenoid, its feed, or a large voltage drop. Charging system questions cover alternator output, measured under load against the rating; ripple from a failed diode, measured on AC volts or with a scope, as on courier van 1 in AUTO-220; and smart charging, where the module varies the alternator's target voltage, so a reading of 12.8 V at idle may be normal.

Wiring diagrams and body electrical

Exam questions often include a wiring diagram and ask where to test next or what fault matches the symptoms. Read the diagram the way AUTO-220 lesson 5 taught: find the power source and fuse, follow the path through switches and connectors to the load, then to ground, and note what else shares each fuse, connector and ground. A fault that affects several circuits at once points to what they share, usually a fuse, a ground or a module. Lighting and body questions cover headlamps and their switching, often through a body module, turn signals that flash fast when a bulb is out or when LEDs replace bulbs, power windows and locks, and instrument cluster faults that come from the network rather than the gauge.

Modules and networks

Modules communicate on networks. The controller area network, CAN, uses two twisted wires, CAN high and CAN low. A high-speed CAN bus at rest sits at about 2.5 V on both wires; when active, CAN high rises to about 3.5 V and CAN low falls to about 1.5 V. Each end of the bus has a 120 ohm terminating resistor, so with the battery disconnected, the resistance between CAN high and CAN low at the diagnostic connector should read about 60 ohms. A reading of about 120 ohms means one terminator is missing or the bus is open on one side; near zero means the wires are shorted together. LIN buses are single-wire, slower subnetworks for things like mirrors and switches, run by a master module. A gateway module routes messages between networks and, on many newer vehicles, controls scan tool access, which is why secure gateways require an authorized login.

Network diagnosis starts with a full scan to see which modules respond and which have lost communication codes. A module that does not respond while the others do points to that module's power, ground or network connection. Several modules losing communication with one point to a shared fault or a module dragging the bus down; disconnecting modules one at a time while watching the bus finds it. The minivan's damaged connector terminal after a bumper repair, and courier van 3's chafed CAN low wire in AUTO-220's capstone, were the examples.

Module programming, from AUTO-330 lesson 11, follows the maker's procedure through J2534 or the maker's tool, with a battery maintainer holding a stable voltage, a stable internet connection and the correct calibration file. Security-related functions need the technician's registered access. Programming is legitimate repair only within what the maker releases: no changes to emissions, odometer or immobilizer functions.

Traps in this area

Electrical questions love two traps. The first is static readings: a voltage reading taken with no current flowing looks perfect even with a badly corroded connection, because the corrosion only drops voltage when current flows. The second is the shared component: when several things fail together, the answer is what they share. And remember A6's reputation: read every electrical stem twice.

More practice in this area

Network questions on the exam rarely ask you to decode a message. They ask what to measure and what the measurement means: the bus resistance with the battery off, the bus voltages with the key on, which modules respond to a scan, and what a lost communication code in one module says about another. They also test module replacement: a new module usually needs programming or configuration, and some need a security procedure to be paired with the vehicle.

Kiran's network gap closed on the training board. After two evenings measuring terminating resistance, opening and shorting the bus wires on purpose and watching which modules dropped off, the questions that had looked impossible turned into the same circuit questions as everything else: a pair of wires, two resistors and some modules that need power and ground.