Current Flow Diagram Diagnosis

The ever increasing electrical and electronic content of modern vehicles can make diagnosis and repairs a real challenge.
We have seen a number of faults that without the correct approach, could lead technicians to at best take much longer to diagnose or at worse fail to or misdiagnose.
During training we encourage technicians to analyse how effective their diagnostic routine actually is.
Some have a set routine, others fly by the seat of their pants.
What ever technique you use, could you improve your diagnostic skills?
We have discovered that simple faults can fox even experienced technicians.
Take a simple central locking fault, the vehicle will lock and unlock all doors from the passenger door lock and the remote, but if the drivers door lock is used only the unlock function works.
This suggests to me,a fault with the door lock switch.
But how can this be proved quickly without stripping unnecessary trim from the vehicle.
The wiring or current flow diagram will often hold the key.
We recommend drawing your own diagram, it should contain only the detail you require to test the circuit.
This includes wires colours, pin numbers and what you expect to see on the voltmeter.

Take a look at the diagram below, where is the likely fault, where can you test easily, and what would you expect to see.

When the whole diagram is presented it can be difficult to see the wood from the trees.
However it can be easier with your own diagram. Like the one below.

Testing at the module means the door trim does not have to be removed, if the voltmeter reads 0.1Volts at pins 12, 13 & 9 when the switches make a path to ground then the circuits and switches are OK.
At pin 12 the voltmeter reads 0.1Volt when the passenger door switch is made. But remains at 12Volts when the drivers switch is made. The fault can only be between the switch and the join in the wiring.
As this join is inside the car it is possible to trace the wire, it was found to be broken in the door hinge area, a common failure due to the constant opening and closing of the door. A quick repair and normal operation was restored. All without ordering parts or stripping door trims.