DMM Resistance Checks

Electrical Circuits

  • Vehicle electrical circuits supply power and ground to components that allow the vehicle to operate.

  • Some circuits include resistors to limit current flow through electrical components.

  • In some cases, resistance may increase or decrease unexpectedly, leading to malfunctions or failures.

  • A digital multimeter (DMM) can be used to diagnose the cause and locate points of abnormal resistance in these circuits.

Reasons to Check Circuits for Resistance

Dome Light

  • There are several reasons to troubleshoot a circuit and test resistance, such as:

    • Inoperative electrical components

    • Poor circuit or device performance

    • Diagnosing sensor concerns

    • Ensuring components are working properly

  • One example of an inoperative component is a dome light that doesn’t turn on or is too dim when the door is opened.

Power Mirrors

  • Another type of inoperative electrical component that may need troubleshooting is the power mirror.

  • The issue may involve the mirrors not moving in all directions or moving slowly in the desired direction.

Headlight Bulbs

  • A common circuit or electrical device to test for poor performance is the headlight system.

  • This can occur when headlight bulbs appear dimmer than normal, or when one headlight is dimmer than the other.

Blower Motor

  • Another circuit or electrical device that may need testing for poor performance is the blower motor.

  • If the blower motor spins slower than normal, it results in reduced airflow through the vents, creating a customer concern about weak air output.

Thermistor

  • You may need to troubleshoot a circuit and test resistance when diagnosing sensor concerns.

  • For example, checking the resistance range of a thermistor helps troubleshoot a temperature sensor issue.

Fuel Injector

  • Finally, you may need to troubleshoot a circuit and test its resistance to ensure proper component function.

  • For example, checking the resistance of a fuel injector can help diagnose a misfire or a trouble code related to improper fuel injector operation.

Avoiding Damage

  • To prevent damage to the DMM or the vehicle’s electrical components during a resistance test, always disconnect power from the circuit before measuring resistance.

  • Always follow the manufacturer’s test procedures provided in the service manual when testing circuits.