Voltage Drop Testing
Voltage drop is the quantity of voltage dropped or lost as it passes through a component or through resistance in a circuit. In order to run a voltage drop test, voltage must be present in a circuit. The voltage drop in a circuit is the difference between voltage at the source or battery and the voltage at the component to which the voltage is flowing. Whenever working with sensitive components like a PCM use the recommended settings on the voltage meter. Be careful not to short one pin to another and when using any meter make sure the tool has the proper impedance. An analog vs. digital meter question may show up on your test. Analog meters are not recommended for use with ECM's.
Use a digital voltmeter to find a poor or corroded ground connection. Turn the ignition key on KOEO (key on engine off). Use a service manual to find the key points to be probed and set the meter to 1volt. Connect the negative probe to the battery's negative terminal and back probe the PCM connection at the proper ground wires with the positive probe to identify any corroded, faulty, or poor ground wires and connections. The typical voltage drop for this type of circuit is below or between 1 and 2 volts. Anything over 3 volts is considered unacceptable and further inspection of this ground circuit is required. Always consult manufacturers specifications as voltage tolerance may vary from one model to another.
As you can see a voltage drop test is a great way to find excessive resistance in a circuit. This test is just a typical example and can be performed on many of the PCM's sensor circuits as well. These tests require the appropriate service manual for procedures, specifications, sensor, and pin locations.
