Vehicle Electrical Diagnostics Flashcards

A technician is performing a parasitic draw test on a modern vehicle. After connecting a DVOM in series with the negative battery cable, the initial reading is 2.5 amps. After 45 minutes, the reading stabilizes at 85 milliamps (0.085A). What is the MOST likely cause for this reading?

Answer: One or more control modules have failed to enter 'sleep mode'.

A normal parasitic draw on a modern vehicle should be less than 50 milliamps after all modules have entered their low-power 'sleep mode'. A reading of 85mA, while lower than the initial draw, is still excessive and indicates that a component or control module is still active and drawing too much current.

When performing a voltage drop test on the ground side of a starter motor circuit, a technician connects the red lead of a DVOM to the starter motor housing and the black lead to the negative battery terminal. While cranking the engine, the meter reads 0.9 volts. What does this reading indicate?

Answer: There is excessive resistance in the ground circuit between the starter and the battery.

A voltage drop test measures the voltage lost due to resistance in a circuit. When testing the ground side, the DVOM is measuring the resistance between the component's ground and the battery's negative terminal. A reading above 0.2 to 0.5 volts during high-load operation (like cranking) indicates a poor connection, such as a corroded ground strap or loose fastener.

A technician is diagnosing a communication issue on a high-speed CAN bus network. Using a multimeter with the battery disconnected, what should the resistance be when measuring between the CAN High (Pin 6) and CAN Low (Pin 14) terminals at the DLC?

Answer: Approximately 60 ohms.

A standard high-speed CAN bus network is terminated at each end by a 120-ohm resistor. When measuring the resistance of the entire network in parallel (as is done at the DLC), the total resistance should be approximately 60 ohms. This reading confirms the integrity of the two terminating resistors and the circuit wiring.

Which of the following measurements requires the Digital Volt-Ohm Meter (DVOM) to be connected in series with the circuit being tested?

Answer: Amperage (Current)

To measure amperage (current), the entire flow of electricity must pass through the meter. This requires disconnecting the circuit and placing the meter leads in series, making the meter part of the circuit itself. Voltage and voltage drop are measured in parallel, and resistance is measured with the circuit de-energized.

A vehicle comes in with an inoperative right front headlight (low beam). The technician has confirmed the bulb and fuse are good. The next step is to check for power at the headlight connector. With the headlight switch on, the technician measures the voltage between the power wire at the connector and a known good chassis ground. The DVOM reads 12.4 volts. What is the most likely cause of the problem?

Answer: A faulty or missing ground connection for the headlight bulb.

The technician has confirmed that there is proper voltage (12.4V) available at the power side of the circuit. Since the bulb and fuse are known to be good, the only remaining possibility for the circuit to be incomplete is a problem on the ground side. An open or high-resistance ground path will prevent current from flowing through the bulb, keeping it from illuminating.

When using a DVOM to test for an open circuit (infinite resistance) in a section of wire, what is the proper procedure?

Answer: Isolate the wire from the circuit, set the DVOM to Ohms (Ω), and connect the leads to each end of the wire.

To test for resistance or continuity, the component or wire must be isolated and de-energized to prevent damage to the meter and get an accurate reading. Setting the DVOM to Ohms and placing a lead on each end of the wire will measure the resistance between those two points. An 'OL' or infinite reading indicates an open circuit (a break in the wire).