What you should do

Step 1:  Identify and understand the suspect circuit(s)

Having completed steps 1 through 4 of the Strategy Based Diagnostics chart, you should have enough information to identify the system(s) or sub-system(s) involved. Using the service information, you should determine and investigate the following circuit characteristics:

• Electrical:
  • How is the circuit powered (power distribution charts and/or fuse block details)?
  • How is the circuit grounded (ground distribution charts)?
  • How is the circuit controlled or sensed (theory of operation):
    • If it is a switched circuit, is it normally open or normally closed?
    • Is the power switched or is the ground switched?
    • Is it a variable resistance circuit (ECT sensor or TP sensor, for example)?
    • Is it a signal generating device (MAP sensor of VSS, for example)?
    • Does it rely on some mechanical/vacuum device to operate?
  • Physical:
  • Where are the circuit components (component locators and wire harness routing diagrams):
    • Are there areas where wires could be chafed or pinched (brackets or frames)?
    • Are there areas subjected to extreme temperatures?
    • Are there areas subjected to vibration or movement (engine, transmission or suspension)?
    • Are there areas exposed to moisture, road salt or other corrosives (battery acid, oil or other fluids)?
    • Are there common mounting areas with other systems/components?
  • Have previous repairs been performed to wiring,
  • Connectors, components or mounting areas (causing pinched wires between panels and drivetrain or suspension components without causing and immediate problem)?
  • Does the vehicle have aftermarket or dealer-installed equipment (radios, telephone, etc.)

Step 2:  Isolate the problem

At this point, you should have a good idea of what could cause the present condition, as well as could not cause the condition. Actions to take include the following:

• Divide (and separate, where possible) the system or circuit into smaller sections
• Confine the problem to a smaller area of the vehicle (start with main harness connections while removing panels and trim as necessary in order to eliminate large vehicle sections from further investigation)
• For two or more circuits that do not share a common power or ground, concentrate on areas where harnesses are routed together or connectors are shared (refer to the following hints)

Hints: Though the symptoms may vary, basic electrical failures are generally caused by:

• Loose connections:
  • Open/high resistance in terminals, splices, connectors or grounds
  • Incorrect connector/harness routing (usually in new vehicles or after a repair has been made):
  • Open/high resistance in terminals, splices, connectors of grounds
• Corrosion and wire damage:
  • Open/high resistance in terminals, splices, connectors of grounds
  • Component failure:
  • Opens/short and high resistance in relays, modules, switches or loads
• Aftermarket equipment affecting normal operation of other systems

You may isolate circuits by:

• Unplugging connectors or removing a fuse to separate one part of the circuit from another part
• Operating shared circuits and eliminating those that function normally from the suspect circuit
• If only one component fails to operate, begin testing at the component
• If a number of components do no operate, begin tests at the area of commonality (such as power sources, ground circuits, switches or major connectors)