Troubleshooting With A Digital Multimeter

NOTE: Do not insert a DMM test probe into any connector or fuse block terminal The diameter of the DMM test probe will deform most terminals. A deformed terminal can cause a poor connection between the device, which can result in system failures. Always use the J 35616-A Connect Test Adapter Kit when using a DMM in order to probe terminals.

Important: Circuits which include any solid state control modules, such as the PCM, should be tested only with a 10 megohm or higher impedance J 39200 digital multimeter.

The J 39200 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 reference during new procedures.
A DMM should be used instead of a test light to check for voltage. While a test light shows whether voltage is present, a DMM indicates how much voltage is present.
When testing for voltage or continuity at the connection, it is not necessary to separate the two halves of the connector. Unless testing a Weather Pack �, a Metri Pack � or other sealed system, always probe the connector from the back. Always check both sides of the connector. An accumulation of dirt and corrosion between contact surfaces is sometimes a cause of electrical problems.
The ohmmeter function on a DMM shows how much resistance exists between two points along a circuit. Low resistance in a circuit means good continuity.

Important: Disconnect the battery 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 checked:
- Backprobe 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.
- Disconnect the harness at both ends of the suspected circuit where it connects either to a component or to other harnesses.
Use the J 35616-A Connector Test Adapter Kit in order to connect the DMM to the circuit.
- 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 checking multiple circuits quickly.