Digital Multimeter (Dmm)

A Digital Multimeter (DMM) can be used instead of a test light to check for voltage. While a test light shows whether or not voltage is present, a DMM indicates how much voltage is present.

An increasing number of circuits include solid state control modules. One example is the Powertrain Control Module (PCM). Voltages in these circuits should be tested only with a 10-megohm or higher impedance J 39200 Digital Multimeter (DMM). Unless directed to within the diagnostics, never use a test light on circuits that contain solid state components, since damage to these components may result. 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(R) or a Metri Pack(R) connector 150 series and below, 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. A terminal contact checking procedure can be found later.

A Digital Multimeter can also be used instead of a self-powered test light to check for circuit continuity. The ohmmeter function on a DMM shows how much resistance there is between two points along a circuit. Low resistance means good continuity.

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.

When measuring resistance with a DMM, the vehicle's Battery should be disconnected. This will prevent 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 an Digital Multimeter to give a false reading. To find out if a component is affecting a measurement, take a reading once, reverse the leads and take a second reading. If the readings differ, the solid state component is affecting the measurement.