Diagnosis, Testing and Fault Finding
Diagnosis, Testing and Fault Finding
This domain tests a logical diagnostic process, use of multimeters, test lamps, oscilloscopes and current clamps, voltage-drop testing and interpretation of readings.
Core rules
- Follow a sequence: verify the complaint, gather information, visual inspection, check for DTCs, study the wiring diagram, test the circuit, confirm the repair.
- Measure voltage with the circuit loaded. Voltage drop across a connection should be measured while current flows.
- Resistance tests are valid only on an isolated, dead circuit. An ohmmeter supplies its own small current.
- A test lamp proves power and earth but draws current; a high-impedance multimeter can show a ghost voltage through a poor connection.
- Current clamps measure current without breaking the circuit. For parasitic drain, measure with everything off and modules asleep.
- Compare readings with specifications and test the earth return as carefully as the feed.
Worked example
A headlamp is dim. Battery voltage is 12.4 V. At the lamp feed, measured to a good earth, 9.8 V is recorded. Feed voltage drop is 12.4 − 9.8 = 2.6 V. Measuring from the lamp earth terminal to battery negative gives 0.9 V; the feed side therefore accounts for 1.7 V. The total 2.6 V is excessive. Inspect and voltage-drop test the fuse, switch, relay contacts and connectors on the feed, and the earth strap and body bonding on the return. Repair the highest drop first.
Traps
Using an ohmmeter on a live circuit; condemning a component because a DTC points to it; measuring voltage without a load and missing a high-resistance connection; probing connector faces and spreading terminals; forgetting that a good earth is part of the circuit; replacing parts without confirming the fault.