Diagnostic Methods

Shorts to voltage are checked with a voltmeter Recommended practice: Turn ignition on (with battery connected) and measure the circuit for any voltage present (none should be present)

Checking Circuits By Back-Probing A Connector

Back-probing should be a testing method of last resort. It 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. Do not force test leads or other probes into connectors. Adequate care must be exercised to avoid connector terminal damage while ensuring that 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.

Do not test for the presence of voltage at a single point where zero volts is a possible result (you cannot tell the difference between a bad probe contact and a zero volt result). Do not test for continuity/opens (using an ohmmeter) between two points (you cannot tell the difference between bad probe contacts and an open circuit). Back-probing may be used where the circuit must be analyzed with the voltage-drop method (if the circuit carries greater than 5 amperes and no other means of testing will definitively eliminate circuit resistance as a possible fault). 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 is an acceptable remedy for these testing dilemmas.

Circuit Analysis Using Jumper Wires (Creating Substitute Circuits)

Jumper wires may be employed for circuit analysis.

Cautions:

Always use fused jumper wires — the recommended universal-testing jumper wire fuse is 2-5 amperes; larger fuse ratings should be used only in special circumstances. Use flex probes or equivalent to prevent connector terminal damage (flex probes are not intended to carry higher currents necessary to operate motors such as a cooling fan or blower motor). Follow workshop manual testing directions when using jumper wires to avoid component or harness damage due to incorrect jumper connections. Never repair a circuit by adding a new wire in parallel to the old one (overlaying the circuit) without fully understanding what caused the circuit to fail. Always find, examine, and repair the fault to correct the root cause and to repair any adjacent wiring that has been damaged.

Checking Modules

Generally, module failure rate is very low and therefore replacement modules usually do not resolve the root cause. Incorrect replacement of a module is often the result of inadequate testing.

Understand the correct module function. Make sure programmable parameters are set correctly for the function in question (Refer to 418-01 Module Configuration for more information). Resolve DTCs first — as directed by Diagnostic Routines. Test all inputs, both hard-wired and networked. Test outputs (see "Checking module switching circuits" below). Check applicable TSBs for module software changes (flash programming).

Checking module switching circuits.

Using the scan tool module-output command function (for example, IDS Output State Control) to activate components is a fast way to confirm an output is capable of being switched on by the module. Testing that reveals normal module-output function confirms the need to analyze the module inputs. Don't apply ground or power directly to module-switched components with jumper wires (unless directed by a workshop manual procedure), as the component could be damaged by a direct connection to ground/power.

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