Although bus and splice charts are entirely different, I’ll lump them together for the sake of editorial simplicity. Simply, a bus communications chart identifies the wires by which the various on-board computers and modules communicate with each other. In most cases, bus wires can be identified as pairs of wires twisted together which typically carry pulsed, low-voltage signals of about 2.5 volts. In any case, it’s important to not confuse a bus communications wire with a wire that transmits battery voltage.
As for splice charts, it’s important to be able to verify the location and integrity of various splices in the electrical wiring. In some cases, reduced voltage in an electrical circuit may be the fault of a bad splice rather than a bad contact in the ignition switch.
VOLT, OHM & AMP METERS
Because it’s necessary to measure very subtle differences in voltage, resistance and amperage throughout an electrical system, it’s extremely important to use professional-grade, 10 meg-ohm impedance meters. Meters that don’t meet this standard will often provide inaccurate and misleading readings, which will cause a technician to make major errors in ignition switch diagnostics.
DIAGNOSTIC SCENARIOS
Worn ignition switches may be indicated by improbable occurrences such as the Malfunction Indicator Light (MIL) being illuminated and multiple diagnostic trouble codes (DTCs) being retrieved from a powertrain control module’s (PCM’s) diagnostic memory. Instead of the multiple DTCs being caused by multiple component failure, the problem may be caused by the ignition switch not supplying adequate amperage to the operating system itself.
A good illustration might be six or more DTCs contained within a specific system like an electronic automatic transmission or four-wheel-drive transfer case. It’s highly improbable, for example, that six shift solenoids contained in an automatic transmission would fail simultaneously. The most likely place to start a diagnosis would be at the ignition switch terminal supplying voltage and amperage to the transmission solenoid circuit.
VOLTAGE VARIATIONS
Continuing this scenario, the worst procedure is to disturb electrical connections, wiring or components in the system. Instead, the safest procedure is to measure the voltage available at the fuse that supplies the transmission solenoid circuits. By connecting our voltmeter to a good ground and measuring the available voltage at the fuse, we can determine if high resistance in the ignition switch is reducing amperage flow to the fuse and the components which it powers.
If a battery voltage of 13.5 volts is available at other fuses and only 9.5 volts is available at the transmission solenoid fuse, we might conclude that resistance has developed in the ignition switch and is starving each of the transmission solenoids of electrical amperage. Since the PCM can detect a solenoid or solenoid circuit failure by measuring the amperage flow through the ground connection in the PCM, the PCM may store a DTC for each of the affected solenoids.
Although this is only one diagnostic scenario, the procedure is similar for diagnosing many other electrical systems supplied by the ignition switch. It’s important to remember any diagnostic scenario is dictated by the vehicle’s component configuration as illustrated in a wiring schematic or power distribution chart. The remainder relies primarily on the individual technician’s experience, knowledge and diagnostic ingenuity.
