What you should do

Step 1:  Acquire information

A thorough and comprehensive customer check sheet is critical to intermittent problem diagnosis. You should require this, since it will dictate the diagnostic starting point. The vehicle service history file is another source for accumulating information about the complaint.

Step 2:  Analyze the intermittent problem

Analyze the customer check sheet and service history file to determine conditions relevant to the suspect system (s).

Using service information, you must identify, trace and locate all electrical circuits related to the malfunctioning system(s). If there is more than one system failure, you should identify, trace and locate areas of commonality shared by the suspect circuits.

Step 3:  Simulate the symptom and isolate the problem

Simulate the symptom and isolate the system by reproducing all possible conditions suggested in Step 1 while monitoring suspected circuits/components/systems to isolate the problem symptom. Begin with the most logical circuit/component.

Isolate the circuit by dividing the suspect system into simpler circuits. Next, confine the problem into a smaller area of the system. Begin at the most logical point (or point of easiest access) and thoroughly check the isolated circuit for the fault, using basic circuit tests.

Hints: You can isolate a circuit by:

• Unplugging connectors or removing a fuse to separate one part of the circuit from another. If only component fails to operate, begin testing the component
• If a number of components do not operate, begin test at areas of commonality (such as power sources, ground circuits, switches, main connectors or major components)
• Substitute a known good part from the parts department or the vehicle system
• Try the suspect part in a known good vehicle

See Symptom Simulation Tests  for problem simulation procedures. Refer to Specifications  and LIGHTING SYSTEM for information about intermittent diagnosis . Follow procedures for basic circuit testing in LIGHTING SYSTEM .