The Trainer #73: Automotive Electrical Fundamentals - Improve Your Electrical Troubleshooting Skills

Main Ideas

The video presents electrical troubleshooting as a skill built on three foundations:

1. Read wiring diagrams: Understand not only where wires run, but also what each component does and what readings to expect.
2. Use electrical test equipment effectively: The digital multimeter is the main tool discussed.
3. Understand circuit fundamentals: Knowing how voltage, current, and resistance interact makes test results easier to interpret.

Basic Circuit Components

A working circuit needs:

• A load: The component that performs the work, such as a light bulb.
• A source: The presenter uses the battery as the main testing reference, even when the engine is running.
• A path: The positive-side path carries power to the load; the ground-side path returns it to the source.
• A control device: A switch, relay, or electronic control unit may control the circuit. A component can serve different roles in different circuits.
• Circuit protection: A fuse, fusible link, or circuit breaker protects against overloads and is placed on the positive side.

A relay illustrates why it is important to identify a component’s role in a specific diagram: its energized coil acts as a load, while its contacts act as a switch.

Circuit Patterns and Troubleshooting Focus

The presenter says the key practical question is often not whether a circuit is series or parallel, but which parts are shared and which are unique:

• If several loads fail together, look for a fault in a part of the circuit they share.
• If only one load fails, focus on the wiring, connectors, or controls unique to that load.

Voltage, Current, and Resistance

• Voltage (E): Electromotive potential—the electrical “push” between two points. A meter in voltage mode measures the potential difference between its leads. A fully charged battery is described as reading about 12.6 volts.
• Current (I): The flow of electrons through the circuit. In most circuits, current is the same at different points in the path.
• Resistance (R): Anything that hinders current flow. Loads are intended sources of resistance, but wires, connectors, and switches also have some resistance. Unwanted resistance in those parts can disrupt circuit operation.

The video emphasizes that many electrical faults are caused by a change in resistance. An open circuit has extremely high resistance; a short to ground can sharply reduce circuit resistance. Unwanted resistance may result from corrosion, damaged or overheated connectors, compromised wiring, moisture, or improper repairs. It can cause a load to stop working, work slowly, or operate dimly.

In a series circuit, voltage is used to overcome resistance. When there are multiple sources of resistance, the voltage is divided among them in proportion to their resistance. That is why unwanted resistance can “take” some of the voltage the load needs.

Why Voltage-Drop Testing Matters

A resistance check made with the circuit off is a static measurement. It may fail to reveal a connection that cannot carry current properly when the circuit is operating. The presenter uses a starter cable as an example: it can show low resistance on an ohmmeter even though a damaged cable would fail under the starter’s high current demand.

A voltage-drop test is dynamic: the circuit must be operating so current is flowing. It measures voltage lost across a portion of the circuit and helps locate unwanted resistance. The presenter recommends practicing with a battery, wires, and bulbs before applying the technique to vehicle faults.

Voltage-Drop Testing Procedure

Basic Method

1. Set the meter to DC volts.
2. Put the negative meter lead on the negative battery post and leave it there.
   • Do not substitute a nearby body or frame ground, because that could exclude the ground-path section where the fault lies.
   • A long test lead may be needed to reach distant parts of the vehicle.
3. Make sure the circuit is on and carrying current. For example, someone must press the horn button, step on the brake, or run the engine if the system operates only while the engine is running.
4. Measure and note the battery voltage.
5. With the positive meter lead, measure at the positive side of the load, as close to the load as practical.
6. Measure at the ground side of the load, again as close as practical.
7. Account for any portion of the path that was not included between the test points; it could still contain the fault.

Interpreting Readings

• The positive side of the load should be close to battery voltage. A substantial reduction indicates unwanted resistance or an open on the positive side.
• The ground side should normally be close to zero relative to battery negative. A reading appreciably above zero indicates a problem on the ground side.
• A near-zero reading at the positive side can indicate an open on the positive side. The presenter also notes that, depending on the readings and test setup, an open may be between the test points or within the load.
• If both sides appear to show battery voltage, the circuit may not be operating, or the ground path may be open.
• If the positive and ground paths test normally but the component still does not function, consider a mechanical failure in the load rather than an electrical fault.

The presenter cautions against getting distracted by a few tenths of a volt: in his experience, significant unwanted resistance usually produces a more noticeable change. The acceptable drop depends partly on how much current the circuit carries.

Narrowing Down the Fault

Once testing identifies which side of the circuit is problematic:

1. Use the wiring diagram to identify accessible points along that path.
2. Test at an intermediate point between the original test points.
3. If the abnormal reading remains, the fault is still between the new point and the other test point.
4. If the reading returns to normal, the fault lies on the portion just passed.
5. Repeat, moving between points as needed, until the fault is isolated.

Start with accessible connectors and likely trouble spots rather than immediately dismantling the vehicle. In the video’s example, testing and tracing the unique part of a failed rear backup lamp circuit locates a broken wire in the trunk-lid harness.

Alternative Direct Voltage-Drop Setup

To measure voltage drop more directly:

• Positive-side test: Put both meter leads on the positive side of the circuit—one on battery positive and the other at the load’s positive feed.
• Ground-side test: Put both leads on the negative side—one on battery negative and the other at the load’s ground.
• Keep the circuit operating during either test.

The presenter suggests roughly 0.5 volts as a practical go/no-go reference and notes that some meters may have limited millivolt ranges. The central principle is to compare points on the same side of the circuit while current is flowing.

Speakers and Sources Featured

• Pete: The presenter and instructor, identified in the subtitles as “Pete.”
• Motor Age Training: Named as the episode’s sponsor; no separate sponsor speaker is featured.