Checking Digital Instrument Circuits

Instrument systems on modern vehicles are controlled by electronic circuits and sensors. Electronic control modules receive information from various sensors and send digital signals to various instruments to display the information. The data from sensors and instruments are sent and received on the CAN-bus vehicle network.

A diagnostic scan tool connected to the vehicle will greatly assist in diagnosing digital instruments. Use the scan tool to check fault codes and vehicle PIDs to check if sensors and instruments are operating correctly. A DVOM and oscilloscope can also be used to check sensor outputs and confirm correct operation. Research the diagnostic procedures and sensor outputs in the manufacturer’s information. Always disconnect sensors before conducting resistance checks. Automotive test lamps should not be used on digital circuits because they draw too much current and can overload the circuit. Be careful not to accidentally short wires or terminals to ground or power, as doing so may damage the circuits.

You should always inspect sensors, connectors, and wires for corrosion or damage. Connector plugs and pins should be tight with straight pins that have good connectivity. If back-probing wires or connectors is required, ensure that the pins are not damaged. If wire insulation is pierced, make sure it is reinsulated with tape or liquid insulation. Do not use room-temperature vulcanizing (RTV) silicone, as it can absorb moisture as it cures, which may promote corrosion.

To check digital instrument circuits, follow the steps in Skill Drill 39-5:

Identify the digital circuit to be tested. Research the correct test procedure from the manufacturer’s information.
Check fault codes and circuit PIDs using a scan tool.
Inspect the sensor, wires, and connectors.
Test the sensor with a DVOM or oscilloscope to determine correct output.
If the sensor tests OK, check the wiring harness for continuity back to the instrument panel, and determine necessary action.