Troubleshooting with a Digital Multimeter

Troubleshooting with a Digital Multimeter

Special Tools

EL-39200 - Digital Multimeter (DMM)

For equivalent regional tools, refer to Special Tools Tools and Equipment.

Caution: Refer to Test Probe Caution Test Probe Caution.

Digital Multimeter (DMM) Basic Requirements

Service information is validated using digital multimeters that meet or exceed the following requirements. Using a multimeter that does not meet these basic requirements may give inaccurate readings which could lead to an incorrect diagnosis.

Display and Settings

1. Four digits displayed

2. Voltage, current, resistance, frequency, diode test settings, and min-max function

3. Display positive and negative values

Voltage

1. Have at least 10 Mohms input impedance

2. Test a DC voltage range of 0.1-1000 V.

3. Test an AC voltage range of 0.1-1000 V.

Current

1. Test a DC current range of 0.1 micro-10 A.

2. Test an AC current range of 0.1 micro-10 A.

Resistance

Test a resistance range of 0.1 ohms-40 Mohms and displays infinite for a value greater than 40 Mohms (O.L. (Over-load))

Frequency

Test for a frequency range of 0.5 Hz-199 k Hz

Diode Test

1. Apply at least 1 mA when performing the diode bias test

2. Display the break over voltage drop (0-3 V) of a forward biased diode

3. Display infinite (O.L. (Over-load)) when a diode is reverse biased

Min-Max Readings

1. Can update at a 25 ms sample rate

2. Saves and displays minimum and maximum values

Note: Circuits which include any solid state control modules, such as the engine control module (ECM), should only be tested with a 10 MOhms or higher impedance digital multimeter such as the EL-39200.

The DMM instruction manual is a good source of information and should be read thoroughly upon receipt of the DMM as well as kept on hand for future reference.

A DMM should be used instead of a test lamp in order to test for voltage in high impedance circuits. While a test lamp shows whether voltage is present if the impedance is low enough, a DMM indicates how much voltage is present. In other words, if there is not enough current, the test lamp will not illuminate even though voltage is present.

The ohmmeter function on a DMM shows how much resistance exists between 2 points along a circuit. Low resistance in a circuit means good continuity.

Note: Disconnect the power feed from the suspect circuit when measuring resistance with a DMM. This prevents incorrect readings. DMMs apply such a small voltage to measure resistance that the presence of voltages can upset a resistance reading.

Diodes and solid state components in a circuit can cause a DMM to display a false reading. To find out if a component is affecting a measurement take a reading once, then reverse the leads and take a second reading. If the readings differ the solid state component is affecting the measurement.

Following are examples of the various methods of connecting the DMM to the circuit to be tested:

* Probe both ends of the connector and either hold the leads in place while manipulating the connector or tape the leads to the harness for continuous monitoring while you perform other operations or test driving. Refer to Probing Electrical Connectors Probing Electrical Connectors.

* Disconnect the harness at both ends of the suspected circuit where it connects either to a component or to other harnesses.

* If the system that is being diagnosed has a specified pinout or breakout box, it may be used in order to simplify connecting the DMM to the circuit or for testing multiple circuits quickly.