DIAGNOSE – BRAKE WARNING LIGHT IS ON
by Lance Wright


SYMPTOM SUMMARY
The red brake warning lamp remains on after the vehicle is started.

USUAL CAUSE
The brake system is equipped with a red brake warning lamp that will illuminate under certain conditions. Some vehicles feature a float located in the master cylinder that will illuminate the brake warning lamp when the fluid level becomes too low. The light will also come on if hydraulic pressure is lost in one of the front or rear brake channels. Usually the proportioning valve contains the circuit that will turn the light on during hydraulic pressure failure. It will also automatically prevent any additional fluid to be routed to the leaking channel. This ensures there is an adequate brake fluid and pressure supply to the working channel. The light usually functions as the parking brake indicator lamp as well.

DIAGNOSIS
The first step in diagnosing this symptom is a visual inspection. Check the brake fluid level in the master cylinder to ensure it is within specifications. If the brake fluid level is low, you will need to inspect the system for leaks. You may refer to the “Brake Fluid Consumption” symptom for further details on the diagnosis of brake fluid loss. Ensure the emergency brake is completely released. On most vehicles, the emergency brake circuit will illuminate the bulb at a different intensity than the fluid level or pressure sensors. If the brakes function normally and have been inspected and found to be leak free, you should suspect a malfunction in the emergency brake light circuit. Apply and release the emergency brake and look for a change in the bulbs intensity. If no change occurs, this further confirms a malfunction in the emergency brake light circuit. This switch is usually located on the emergency brake mechanism and should be inspected for damage.

PRECAUTIONS, TIPS, and NOTES
Always use the proper brake fluid DOT rating when refilling or adding to the brake fluid master cylinder. Do not add more than 1 oz of fluid to the master cylinder without inspecting the system for leaks.

(Lance owned his own auto repair shop for 30 years before retiring in 2006.)