ITI Mechanic Auto Electrical & Electronics (AITT Theory) flashcards

33 cards. Read the term and the definition here. The deck at the top of this page is optional.

Ohm's Law

Voltage equals current multiplied by resistance (V = I × R). Used to calculate expected current in a circuit or to find resistance from measured voltage and current.

Kirchhoff's Current Law

The total current entering a junction equals the total current leaving it. Applied to diagnose short circuits and parallel branch currents in vehicle wiring.

Least Count of Vernier

The smallest measurement a vernier caliper can resolve, equal to one main-scale division divided by the number of vernier divisions. A 1 mm scale with 50 divisions gives 0.02 mm.

Voltage Drop Test

Measuring voltage lost across a cable, connector, or switch while current flows. More than 0.5 V on a starter feed indicates excessive resistance and a poor connection.

CAT III Rating

A multimeter safety category for measurements on fixed installations and distribution circuits. Required for testing traction batteries and high-energy vehicle systems safely.

Insulation Resistance Test

Applying a DC test voltage with a megger to measure resistance between a conductor and earth. Readings below about 0.5 MΩ indicate moisture or insulation breakdown.

Lead-Acid Cell Voltage

A fully charged lead-acid cell has a nominal voltage of about 2.1 V. Six cells connected in series produce the standard 12.6 V automotive battery.

Cold Cranking Amps

The current a battery delivers at minus 18 degrees Celsius for 30 seconds while maintaining at least 7.2 V. It indicates starting capability in cold conditions.

Specific Gravity of Electrolyte

A hydrometer reading that indicates battery state of charge. About 1.265 means fully charged and 1.120 means discharged, corrected for temperature.

Parasitic Drain

Current drawn from the battery when the ignition is off and modules have slept. A healthy vehicle typically draws under about 50 milliamps.

Alternator Ripple Voltage

The small AC voltage superimposed on charging output, caused by rectification. Excessive ripple, above roughly 0.5 V AC, indicates a failed rectifier diode.

Starter Solenoid Windings

The pull-in winding draws high current to engage the drive, while the hold-in winding draws less to keep it engaged. A click with no crank suggests low supply current.

Blade Fuse Rating

The continuous current a blade fuse carries without blowing. It is chosen above normal circuit current but below the cable's current capacity.

Relay Terminal 30

The battery positive feed terminal on a standard automotive relay. Terminal 87 feeds the load, and terminals 85 and 86 connect the coil.

Earth Voltage Drop

Voltage measured between a component's earth point and battery negative while the circuit is loaded. More than 0.1 V indicates a poor chassis connection.

Ignition Coil Resistance

Primary winding typically measures 0.5 to 2 ohms and secondary 5 to 15 kilo-ohms. A coil may test normal cold but fail when hot.

Inductive Crank Sensor

A two-wire sensor that generates an AC voltage as reluctor teeth pass its coil. Resistance is typically 500 to 1500 ohms and signal rises with engine speed.

Zirconia Oxygen Sensor

A sensor producing a voltage between 0.1 and 0.9 V that switches around 0.45 V at stoichiometric mixture. Its heater draws one to three amps.

MAP Sensor Output

A voltage between roughly 0.5 and 4.5 V that falls at high vacuum and rises near atmospheric pressure. Used by the ECU to calculate engine load.

Indicator Flash Rate

Normal indicator flashing is about 60 to 120 times per minute. A faster flash usually means a bulb has failed and circuit current has dropped.

Halogen Bulb Handling

Halogen bulbs must not be touched on the glass because skin oils create hot spots. Contamination causes premature filament failure and uneven light output.

Diode Forward Voltage

The voltage lost across a conducting diode, about 0.7 V for silicon and 0.3 V for germanium. A much higher reading indicates an open diode.

NPN Transistor Switching

An NPN transistor conducts between collector and emitter when its base is about 0.7 V above the emitter. Commonly used as a low-side driver in ECUs.

Resistor Colour Code

A marking system where each colour represents a digit from zero for black to nine for white. Gold and silver bands indicate tolerance of five and ten percent.

CAN Bus Termination

Two 120 ohm resistors at the ends of a high-speed CAN network. A healthy bus measures about 60 ohms across CAN-H and CAN-L with power removed.

CAN Differential Signal

CAN-H and CAN-L carry opposite signals around 2.5 V. A dominant bit drives CAN-H toward 3.5 V and CAN-L toward 1.5 V, giving about 2 V difference.

LIN Bus

A single-wire, low-speed network running at 1 to 20 kbit/s. Used for body components such as switches, sensors, and door modules under a master node.

OBD-II Connector Pins

Pin 16 is battery positive, pins 4 and 5 are earths, and pins 6 and 14 are CAN-H and CAN-L. Pins 7 and 15 carry older ISO serial lines.

Freeze-Frame Data

A snapshot of sensor readings and operating conditions stored when a diagnostic trouble code sets. Helps reproduce intermittent faults under the same conditions.

HV Isolation Test

Measuring resistance between high-voltage conductors and chassis to confirm electrical isolation. A common pass threshold is above 100 ohms per volt of system voltage.

Service Disconnect Procedure

Power off, remove the service plug, wait the specified discharge time, then verify zero volts at the inverter with a CAT III meter before working.

Regenerative Braking

The motor acts as a generator during deceleration, converting kinetic energy to electrical energy. The inverter rectifies this to charge the high-voltage battery.

High-Voltage Cable Colour

Orange identifies high-voltage cables on hybrid and electric vehicles. These cables must never be cut, probed, or repaired and are replaced as assemblies.