A6 · Electrical / Electronic Systems
A6 Circuit Key
Symbols, signals & diagnostic logic
If you don't know what a symbol means, you can't read the schematic. Use this as your bench reference. Drill the flashcards until it's muscle memory.
Core electrical
| Symbol | Meaning | Tech use |
|---|---|---|
| + / B+ | Battery positive / supply | Constant or feed voltage; usually B+ is available all the time. |
| − | Negative / ground | Return path to battery negative or chassis ground. |
| IGN | Ignition-switched power | Hot only with key in ON/RUN. |
| ACC | Accessory power | Hot in accessory position only. |
| START | Crank signal | Active only while cranking — drops out after. |
| V / VDC | DC voltage | Most automotive circuit voltage testing. |
| VAC | AC voltage | Alternator ripple, speed sensors, AC waveform testing. |
| A / mA | Amps / milliamps | Current flow. mA for parasitic draw. |
| Ω / kΩ / MΩ | Ohms / kilo / mega | Resistance. Power off before testing. |
| W | Watts | Electrical power / load rating. |
| Hz | Frequency | Cycles per second — speed sensors, PWM. |
| mV | Millivolts | Small sensor signals; O2 sensors live here. |
| OL / ∞ | Over limit / infinite | On the meter = open circuit, no continuity. |
Power sources
| Symbol | Meaning | Tech use |
|---|---|---|
| 12V | Nominal system voltage | Passenger vehicle baseline. Engine off ~12.6V healthy. |
| 5V REF / VREF | Sensor reference voltage | ECM/PCM-supplied. Always test before condemning a sensor. |
| VBAT / VPWR | Module battery feed | Module's direct power supply. |
| KAM | Keep Alive Memory | Constant power retaining module memory and adaptations. |
| CHG / ALT / GEN | Charging system | Alternator/generator output circuit. |
| Fusible link | High-current protection | Protects major power feeds — replace with same gauge only. |
| PDC / UBEC / BEC | Power distribution center | Underhood or body electrical center. |
Grounds
| Symbol | Meaning | Tech use |
|---|---|---|
| GND | Generic ground | Generic return path. Could be chassis, sensor, or splice. |
| Chassis ground | Body/frame ground | Common return path for most loads. |
| Engine ground | Engine block ground | Critical for starter, charging, sensors. |
| Sensor ground | Module-supplied ground | Low-current ground supplied by ECM/PCM. |
| SIG RTN | Signal return | Sensor return path to module — not chassis ground. |
| Low reference | GM-style sensor ground | Same idea as SIG RTN — module-side ground. |
| G101, G102... | Ground location ID | Physical ground point reference — find it in the body diagram. |
| Ground splice | Shared ground | Multiple circuits join before grounding. |
| Ground strap | Braided bond | Engine/body/frame bonding path. |
Diagnostic note
Grounds aren't just "grounds." A bad ground causes high sensor voltage, dim lamps, communication faults, false DTCs, no-crank/no-start, and voltage drop failures. Test voltage drop ground-side under load.
Wires & circuits
| Symbol | Meaning | Tech use |
|---|---|---|
| Straight line | Conductor / wire | Electrical path between components. |
| Cross, no dot | Wires cross, no connection | Just passing over each other on the diagram. |
| Cross with dot | Connected / spliced | Junction point. Current can flow both directions. |
| Dashed line | Shielded, optional, related | Check the diagram legend — meaning varies. |
| Thick line | High-current circuit | Battery, starter, alternator feeds. |
| Thin line | Low-current / signal | Signal or control circuit. |
| Twisted pair | Data network wires | CAN, LIN, FlexRay — twist suppresses EMI. |
| Shielded wire | EMI shielding | Sensors and low-voltage signals. |
| Drain wire | Shield ground path | Grounds the shield — keep it intact. |
| Circuit number | Wire ID | Trace the same wire across diagrams. |
Wire color codes
| Abbreviation | Color |
|---|---|
| BK / BLK | Black |
| WH / WHT | White |
| RD / RED | Red |
| BU / BLU / DB / LB | Blue (dark / light) |
| GN / GRN / DG / LG | Green (dark / light) |
| YE / YEL | Yellow |
| BR / BRN | Brown |
| GY / GRY | Gray |
| OR / ORN | Orange |
| PK / PNK | Pink |
| VT / VIO / PPL | Violet / Purple |
| TN / TAN | Tan |
Stripe code reads base/stripe. Example: BK/WH = black wire with white stripe.
Connectors & terminals
| Symbol | Meaning | Tech use |
|---|---|---|
| C101, C102... | Connector ID | Find it in the connector location diagram. |
| Pin / cavity # | Terminal position | Where to back-probe or measure. |
| Male terminal | Pin side | Protruding contact. |
| Female terminal | Socket side | Receiving contact. |
| Face view | Looking at connector face | Critical — pinout flips if you read the wrong side. |
| Harness side | Wiring harness end | Where you usually back-probe. |
| Component side | Module / device end | Opposite side of the connector. |
| TPA | Terminal Position Assurance | Secondary lock preventing terminal push-out. |
| CPA | Connector Position Assurance | Lock that secures the connector itself. |
| Back-probe | Test from rear | Tests circuit without disconnecting. |
| Breakout box | Test adapter | Test circuit without damaging terminals. |
| Inline connector | Harness-to-harness | Common diagnostic separation point. |
| Bulkhead | Firewall connector | Links engine bay to cabin circuits. |
Diagnostic note
Most wrong-pin diagnoses come from reading the connector view backward. Confirm "harness side vs. component side" before you probe anything.
Splices & distribution
| Symbol | Meaning | Tech use |
|---|---|---|
| S101, S102... | Splice ID | Multiple wires joined together. |
| Splice dot | Electrically joined | Shared feed or ground node. |
| Bus bar | Shared conductor bar | Multiple connections on same rail. |
| Star connector | Network branch hub | Common in communication networks. |
| Common feed | Shared voltage supply | One power source feeds multiple components. |
| Common ground | Shared ground path | One bad ground = multiple symptoms. |
Circuit protection
| Symbol | Meaning | Tech use |
|---|---|---|
| Fuse | Overcurrent protection | Opens when current exceeds rating. |
| 10A / 15A / 20A... | Fuse rating | Max current before fuse opens. |
| Fusible link | Slow-blow high-current | Major power feed protection. |
| Maxi fuse | High-current fuse | Power distribution protection. |
| Mini / micro fuse | Lower-current fuse | Common in fuse panels. |
| Circuit breaker | Resettable protection | Opens then resets auto or manual. |
| PTC fuse | Positive temp coefficient | Resistance increases with heat. |
| Thermal limiter | Opens with heat | Protects heated circuits. |
Diagnostic note
A blown fuse is evidence, not a diagnosis. Ask: overload, short to ground, wrong rating, component failure, or harness damage?
Switches
| Symbol | Meaning | Tech use |
|---|---|---|
| Open switch | Off — no current | Visible gap in symbol = circuit not flowing. |
| Closed switch | On — current flowing | No gap = continuity. |
| NO | Normally open | Closes when activated. |
| NC | Normally closed | Opens when activated. |
| Momentary | Active only while pressed | Horn, start button, window switch. |
| Pressure switch | Pressure-triggered | A/C, oil pressure circuits. |
| Temperature switch | Temp-triggered | Cooling fan, thermal controls. |
| Brake switch | Pedal position input | Brake lamps, cruise cancel, shift interlock. |
| Clutch switch | Clutch position input | Start enable / cruise cancel. |
| Park/neutral switch | Trans range input | Start circuit enable. |
| Limit switch | Position limit | Seats, windows, liftgates. |
| Multi-position | Selector switch | Ignition switch, multifunction switch. |
| Reed switch | Magnetic switch | Level, position, security sensors. |
Relay symbols & terminals
| Symbol | Meaning | Tech use |
|---|---|---|
| Relay coil | Electromagnet | Control side of relay. |
| Relay contacts | Switched side | Load side circuit. |
| NO contact | Normally open | Closes when relay energizes. |
| NC contact | Normally closed | Opens when relay energizes. |
| Terminal 30 | Common power feed | B+ feed into relay. |
| Terminal 87 | Normally open output | Power out when energized. |
| Terminal 87a | Normally closed output | Power out when not energized. |
| Terminal 85 | Coil ground side | Control ground. |
| Terminal 86 | Coil power side | Control power. |
| Relay diode | Coil suppression | Protects driver module from spike. |
| Solid-state relay | Electronic switch | No mechanical contacts. |
Trap rule
Don't just swap the relay. Test both sides: control (coil command + ground) AND load (power feed + output + voltage drop under load).
Resistors
| Symbol | Meaning | Tech use |
|---|---|---|
| Fixed resistor | Set resistance | Limits current, drops voltage. |
| Variable resistor | Adjustable | Used in older controls. |
| Potentiometer | 3-wire variable | TPS, pedal position sensors. |
| Rheostat | 2-wire variable | Dimmer circuits. |
| Pull-up resistor | Holds circuit high | Module input default state. |
| Pull-down resistor | Holds circuit low | Module input default state. |
| Terminating resistor | Network resistance | CAN typically uses 120Ω at each end. |
| Heater resistor | Voltage drop for fan | Blower motor speed control. |
Diodes & semiconductors
| Symbol | Meaning | Tech use |
|---|---|---|
| Diode | One-way current | Prevents backfeed. |
| Zener diode | Voltage limiter | Voltage regulation / protection. |
| LED | Light-emitting diode | Indicators, modern lighting — polarity matters. |
| Photodiode | Light-sensitive diode | Optical sensors. |
| Transistor | Electronic switch | Inside modules — drivers. |
| NPN transistor | Low-side driver | Switches the ground side. |
| PNP transistor | High-side driver | Switches the power side. |
| MOSFET | Solid-state switch | Fan, injector, motor PWM control. |
| IGBT | High-power switch | Hybrid/EV inverter stage. |
| Rectifier / diode bridge | AC → DC conversion | Alternator output stage. |
Diagnostic note
Diodes are tested in diode mode on the meter, not Ω. A healthy diode conducts one direction (~0.5–0.7V drop), blocks the other (OL).
Capacitors & coils
| Symbol | Meaning | Tech use |
|---|---|---|
| Capacitor | Stores charge | Noise suppression, filtering. |
| Polarized cap | +/− oriented | Install correct polarity only. |
| Inductor / coil | Magnetic-field storage | Relay coil, ignition coil, solenoid. |
| Ignition coil | Step-up transformer | Primary + secondary patterns to test. |
| Solenoid | Electromagnetic actuator | Starter, EVAP purge, shift solenoid. |
| Choke coil | Filters noise | EMI suppression. |
Motors & actuators
| Symbol | Meaning | Tech use |
|---|---|---|
| DC motor | Direct-current motor | Most vehicle motors. |
| Stepper motor | Step-by-step movement | Idle air, HVAC doors, gauges. |
| Servo motor | Position-controlled | Blend doors, electronic throttle. |
| Pump motor | Electric pump | Fuel, washer, ABS pump. |
| Fan motor | Cooling / blower | Radiator fan, HVAC blower. |
| Window motor | Reversible polarity | Direction = polarity swap. |
| Wiper motor | Wiper + park circuit | Park position diagnosis often needed. |
| Door lock actuator | Lock motor / solenoid | Pulsed or reversible polarity. |
| Fuel injector | Solenoid valve | PCM-controlled, ground-side. |
| EVAP purge | Vapor control | Duty-cycle PWM control. |
| VVT solenoid | Cam timing oil control | PWM actuator. |
| Shift solenoid | Trans hydraulic control | TCM/PCM-controlled. |
| ABS solenoid | Brake pressure control | Inside ABS modulator. |
| Electronic throttle | Motorized throttle | Dual TPS + motor control. |
Lamps, indicators & loads
| Symbol | Meaning | Tech use |
|---|---|---|
| Lamp / bulb | Light load | Headlamp, marker, interior. |
| LED lamp | Diode-based light | Polarity-sensitive. |
| Indicator lamp | Warning / status | Dash warning or telltale. |
| Backlight | Illumination circuit | Switch / cluster backlight. |
| Heater element | Resistive heat load | Defogger, seat heater, O2 heater. |
| Grid heater | High-current heater | Diesel intake heater. |
| Horn / buzzer | Sound device | Relay-controlled load. |
| Load | Anything consuming current | Motor, lamp, solenoid, module. |
Sensors
| Symbol | Meaning | Tech use |
|---|---|---|
| Thermistor (NTC) | Temp-sensitive resistor | Resistance ↓ as temp ↑. ECT, IAT, ambient. |
| PTC thermistor | Inverse temp resistor | Resistance ↑ with temp. |
| Potentiometer | Variable voltage | TPS, APP, position sensors. |
| Hall-effect | Digital speed/position | CKP, CMP, wheel speed. |
| Magnetic pickup | AC-generating sensor | Older crank / wheel speed. |
| MAP | Manifold pressure | Engine load input. |
| MAF | Mass airflow | Airflow into engine. |
| O2 sensor | Oxygen sensor | Exhaust mixture feedback. |
| A/F sensor | Wideband mixture | Wideband fuel control. |
| Knock sensor | Piezo vibration | Detonation detection. |
| WSS | Wheel speed sensor | ABS / TCS / stability. |
| Yaw rate | Rotation sensor | Stability control. |
| Steering angle | Steering position | Stability / ADAS. |
| Accelerometer | G-force sensor | Airbag / stability. |
| Current sensor | Measures current | Battery management, EV. |
| NOx / PM / DEF | Diesel emission sensors | SCR, DPF, aftertreatment. |
Control modules
| Label | Meaning |
|---|---|
| ECM | Engine Control Module |
| PCM | Powertrain Control Module (engine + trans) |
| TCM | Transmission Control Module |
| BCM | Body Control Module |
| EBCM / ABS module | Electronic Brake / Anti-lock Brake module |
| SRS / SDM | Airbag / Sensing Diagnostic Module |
| IPC / Cluster | Instrument panel cluster |
| EPS | Electric power steering |
| HVAC module | Climate control module |
| Gateway | Routes data between networks |
| ADAS module | Driver assistance controller |
| BPCM / BECM | Battery Pack / Energy Control Module (hybrid/EV) |
| HPCM | Hybrid Powertrain Control Module |
| OBCM | Onboard Charger Module (EV) |
Trap rule
A module DTC does not mean a bad module. Verify power, ground, communication, inputs, and outputs before condemning any module.
Communication networks
| Symbol | Meaning | Tech use |
|---|---|---|
| CAN | Controller Area Network | Main vehicle data network. |
| CAN-H / CAN-L | CAN high / low | Differential pair. Both required for traffic. |
| HS-CAN | High-speed CAN | Powertrain / chassis network. |
| MS-CAN | Medium-speed CAN | Body network. |
| LS-CAN | Low-speed CAN | Lower-priority body systems. |
| LIN | Local Interconnect Network | Single-wire sub-network. |
| FlexRay | High-speed network | Chassis / ADAS. |
| MOST | Media-oriented network | Infotainment / audio. |
| Ethernet | High-speed vehicle bus | ADAS, cameras, gateways. |
| K-Line | Legacy diagnostic | Older scan-tool comm. |
| DLC / OBD-II | Data link connector | Standard scan-tool port. |
| Terminating resistor | Network end resistance | CAN: one 120Ω at each end. |
| 60Ω at DLC | Healthy CAN reading | Two 120Ω in parallel = 60Ω. |
| U-code | Network communication DTC | Module comm fault. |
CAN diagnostic order
1) Battery voltage → 2) Powers / grounds → 3) DLC comm → 4) CAN resistance → 5) CAN-H / CAN-L voltages → 6) Scope waveform.
Scan tool & DTC
| Term | Meaning |
|---|---|
| DTC | Diagnostic trouble code |
| MIL / CEL | Malfunction indicator / check engine light |
| Pending code | Fault detected but not matured |
| Current code | Active fault right now |
| History code | Past fault |
| Permanent code | Stored until monitor confirms repair |
| Freeze frame | Snapshot of data when DTC set |
| PID | Parameter ID — a single data value |
| Mode $06 | Onboard monitor test results |
| KOEO / KOER | Key on engine off / engine running |
| Closed / Open loop | With / without O2 feedback fuel control |
| WOT | Wide open throttle |
| Bi-directional | Scan tool commanding components |
Meter symbols
| Symbol | Meaning | Tech use |
|---|---|---|
| V⎓ / VDC | DC voltage | Battery / circuit voltage. |
| V~ / VAC | AC voltage | Ripple, AC signals. |
| Ω | Resistance | Component / circuit. Power OFF first. |
| ))) Continuity | Audible continuity | Quick check for complete circuit. |
| Diode test | Forward / reverse drop | Tests diode + junctions. |
| A / mA | Current | In-line. Inrush + sustained. |
| Clamp meter | Non-intrusive current | No need to break circuit. |
| Hz | Frequency | Speed sensors, PWM rate. |
| Duty % | Duty cycle | % on-time of PWM signal. |
| Min/Max | Captures extremes | Intermittent fault hunting. |
| LoZ | Low-impedance voltage | Kills ghost voltage on capacitive lines. |
| REL | Relative / zero | Cancels test lead resistance. |
Oscilloscope
| Term | Meaning |
|---|---|
| Time / div | Horizontal time scale per grid division |
| Volts / div | Vertical voltage scale per grid division |
| Trigger | Signal point that stabilizes the waveform |
| Rising / falling edge | Trigger on signal up- or down-stroke |
| Square wave | Digital on/off signal |
| Sine wave | Smooth AC waveform |
| Pulse train / PWM | Repeated digital pulses, variable duty |
| Amplitude | Signal height / voltage |
| Period | Time for one complete cycle |
| Glitch / dropout | Abnormal signal event / loss |
| Ripple | AC variation on DC voltage (alternator) |
| Clipping | Signal cut off / limited |
| Current ramp | Rise pattern — tests motors, coils, injectors |
| Primary / secondary ignition | Coil waveform patterns |
Common electrical faults
| Term | Meaning | What it tells you |
|---|---|---|
| Open circuit | Path broken | No current flow. OL on the meter. |
| Short to ground | Power touches ground | Blown fuse, low voltage, excess current. |
| Short to power | Circuit hit by B+ | Unexpected voltage where it shouldn't be. |
| High resistance | Extra resistance in path | Voltage drop, dim/weak operation. |
| Voltage drop | V lost across resistance | Test UNDER LOAD only. <0.2V on grounds is healthy. |
| Intermittent open | Circuit opens randomly | Vibration- or heat-sensitive. |
| Intermittent short | Circuit shorts randomly | Random fuses, random codes. |
| Backfeed | Voltage from wrong path | Ghost voltage. Use LoZ. |
| Parasitic draw | Current with key off | Battery drain. 50mA or less typical. |
| Excessive draw | Too much current | Overloaded circuit / failing component. |
| No-load voltage | V without current demand | Can mislead — always re-test under load. |
| Loaded voltage | V while circuit operates | Real number for diagnosis. |
| Floating ground | Unstable ground reference | Strange sensor / module behavior. |
| Poor terminal tension | Loose connector contact | Intermittent faults. |
| Corrosion | High-resistance contact | Voltage drop, opens. |
| Terminal push-out | Terminal not seated | Intermittent / dead pin. |
Diagnostic logic
| Concept | What the tech should think |
|---|---|
| Power → Load → Ground | Which side of the circuit is failing? Test each segment. |
| Source voltage | Is the circuit even being fed? |
| Voltage drop | Voltage eaten by unwanted resistance. Test under LOAD. |
| Command signal | Is the module asking the component to act? |
| Feedback signal | Did the system respond as expected? |
| Reference voltage | Is the 5V REF alive and stable? |
| Signal voltage | Does sensor signal match condition? |
| Low reference / sensor ground | Is the return path clean? |
| Load test | Test while circuit operates — avoid false-good readings. |
| Substitution test | Known-good swap. Use carefully; not a strategy by itself. |
| Wiggle test | Move harness while monitoring — finds intermittents. |
| Backprobe | Test without disconnecting — preserves circuit. |
| Pin drag test | Check terminal tension — finds loose pins. |
| Current ramp | Scope test motor/injector inrush patterns. |
| Control vs. Load side | Small control current vs. large load current — test both. |
High voltage / hybrid / EV
| Symbol | Meaning | Tech use |
|---|---|---|
| HV | High voltage | Anything above 60V DC — shock hazard. |
| Orange cable | HV circuit | Never cut. Never probe without PPE. |
| Service plug | HV disconnect | Must be removed before HV service. |
| HVIL | HV Interlock Loop | Safety loop — open = HV system off. |
| Inverter | DC → AC for motor | Drives traction motor. |
| DC-DC converter | HV → 12V | Charges 12V system from HV battery. |
| Traction motor | Electric drive motor | Main propulsion. |
| OBC | Onboard charger | AC plug-in → HV battery. |
| Isolation fault | HV leak to chassis | Critical safety code. |
| Insulation resistance | HV isolation value | MΩ scale — protects chassis. |
| Pre-charge circuit | Soft-start HV caps | Prevents inrush spike. |
| Contactor | HV relay | Connects/disconnects HV battery. |
| Ready mode | Propulsion active | Vehicle silent but live. |
Safety symbols
| Symbol | Meaning |
|---|---|
| ⚠ Warning | Hazard / caution |
| HV warning | Electric shock hazard |
| Battery warning | Acid / electrical hazard |
| Eye protection | Safety glasses required |
| Insulated gloves | Class 0 gloves required for HV service |
| No probe | Do not pierce/test at location |
| Airbag / SRS | Deployment hazard — wait time per OEM |
| Wait time | Capacitor / module discharge time |
| Disconnect battery | Remove battery power before service |
| ESD symbol | Static-sensitive electronics |
| Hot surface | Burn hazard |
A6 Trap Rules
The rules that separate techs who actually find the fault from techs who throw parts at it.
Rule 01
Voltage present does not mean the circuit can carry current.
Rule 02
Always test voltage drop under load.
Rule 03
A blown fuse is evidence, not the root cause.
Rule 04
A module DTC does not automatically mean a bad module.
Rule 05
A sensor signal can be wrong because its ground is bad.
Rule 06
A relay has two circuits — control and load. Test both.
Rule 07
Resistance tests on live circuits are garbage. Power off first.
Rule 08
Backfeed can make a dead circuit look alive. Use LoZ.
Rule 09
Connector pin views can be mirrored. Confirm orientation.
Rule 10
CAN faults usually start with powers, grounds, resistance, and termination.
Drill it
Make the symbols automatic
If you read a schematic and have to stop to think what a symbol means, you're not reading — you're translating. Translate enough times and it becomes automatic. That's what flashcards are for.
Start the Symbol drill →