Checking Circuits by Back-Probing a Connector

Back-probing should only be employed where a diagnostic step requires a circuit to be tested under actual operating conditions. Back-probing is a risky testing method due to the uncertainty of the probe connection and the possibility of damaging terminals

• Back-probing may be used where a circuit must be analyzed with the voltage-drop method. All voltage-drop tests will measure a small amount of voltage (expect less than 5 percent of circuit operating voltage).
  • A zero-volt result indicates incorrect test conditions (no current flow or bad back-probe connections).

• Occasionally, module failure mode behavior will change the operation of a circuit when it is opened for testing. Back-probing allows testing without altering normal module function.

Back-probing may be employed for circuit analysis if the following cautions are carefully observed:

• Do not force test leads or other probes into connectors. Adequate care must be exercised to avoid connector terminal damage while ensuring good electrical contact is made with the circuit or terminal. Failure to follow these instructions may cause damage to wiring, terminals, or connectors, and subsequent electrical faults.
  • Use back probes specifically designed for the purpose to assist in making a good test connection and to prevent connector or terminal damage during back-probing.

• Back-probing is the wrong test for a single point test for presence of voltage. When zero volts is a possible result, you cannot tell the difference between a bad probe contact and a zero-volt result. Disconnect the circuit and test normally.
• Back-probing is the wrong test for circuit continuity or opens (using an ohmmeter) between two points. You cannot tell the difference between bad probe contacts and an open circuit. Disconnect and isolate the circuit, and test normally.