Warning Signals and Low Air Pressure
What the exam tests
Warning-signal questions focus on the pressure at which each device acts and the correct response, not just the name of the light.
Pressure thresholds
- Low air pressure warning: a buzzer and/or light must activate before pressure drops below 60 psi. This is your first warning that you are losing air.
- Spring brakes: apply automatically at 20–45 psi as pressure continues to fall. This is a fail-safe stop, not a normal brake application.
- Gauges: supply pressure gauges show reservoir pressure; an application pressure gauge shows how much pressure you are applying.
- Parking brake indicator: shows the spring brakes are applied; it should go out when the control is pushed in and pressure is up.
- ABS light: a steady light after the self-test indicates an ABS fault; normal brakes remain, but wheel lock protection may be reduced.
Worked example
A driver notices the gauge falling, hears the buzzer at about 58 psi, and keeps driving to the next exit. Reasoning: the warning activated correctly, but continuing to drive is wrong. The spring brakes may apply at 20–45 psi, possibly in traffic. The correct action is to stop as soon as it is safe, secure the vehicle, and find the leak.
Traps that cost marks
- Saying the warning comes on at 20 psi; 20–45 psi is the spring brake range.
- Confusing the low-pressure warning with the spring brake application.
- Treating the ABS light as a reason to stop immediately; it is not a low-pressure warning.
- Assuming a warning that resets means the leak is fixed.
- Forgetting that the spring brakes apply automatically, even if the yellow knob is pushed in.