Voltage Drop Test
Voltage drop is the quantity of voltage dropped or lost as it passes through a component or resistance in a circuit. In order to run a voltage drop test, voltage must be present in the circuit. Connect the tester leads in parallel with the circuit being tested.
Use a digital voltmeter to find a poor or corroded ground connection. Turn the ignition key on KOEO (key on engine off). Use the service manual to find the key points to probe and set the meter to 1 volt. Connect the negative probe to the battery's negative terminal and back probe the PCM connection at the proper ground wire 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 the circuit is required. ECM's are extremely sensitive, always consult the manufacturer's specifications, because testing and voltage tolerance varies 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, sensors, and pin locations.