Checking Warning Device Operation
FIGURE 39-38
Typical warning lights illuminated.

Warning devices are installed on the vehicle to provide the driver with information about critical vehicle operating and safety systems, including the battery, supplemental restraint system (SRS), anti-lock brake system (ABS), park brake, oil pressure, and engine temperature Figure 39-38. In some cases, the systems use sensors to relay information to the warning device, such as low oil pressure. In this case, the oil pressure may need to be measured with an external oil pressure gauge. If the oil pressure is OK, then the oil pressure switch may be faulty and will need to be replaced. In other circuits, such as the ABS system, the PCM may send a signal through each wheel speed sensor to measure its continuity. In this case, you will need to measure the resistance of the sensor or the continuity of the wiring harness.

The warning devices may be controlled by an electronic control unit (ECU). SRS systems are very dangerous; for example, they control the deployment of airbags by using pyrotechnical charges. Check the manufacturer’s procedures and properly disable the SRS system before working on it. Check vehicle faults codes and PIDs for possible faults. A DVOM or oscilloscope may be used to check signal voltages from sensors used on the vehicle warning devices.

To check warning device operation, follow the steps in Skill Drill 39-6:

1
Identify the circuit to be tested. Research the correct test procedure from the manufacturer’s information.
2
Check fault codes and circuit PIDs using a scan tool. Inspect the sensors, wires, and connectors.
3
Test the affected circuit and any sensors with a DVOM or oscilloscope to determine the correct output.
4
If sensors or devices test OK, check the individual circuits for continuity.
5
Identify the circuit to be tested. Research the correct test procedure from the manufacturer’s information.