Brake Warning Light Switches

There are four basic types of brake warning light switches as shown, and usually form a common electrical circuit with the brake warning light.

When a pressure differential occurs between the front and rear brake systems, the halves will shuttle toward the side with the low pressure.

Fig. 4 Pressure differential valve and brake warning light switch:

As shown, movement of the differential valve forces the switch plunger upward over the tapered shoulder of the valve to close the switch contacts and light the dual brake warning lamp, signaling a brake system failure.

Fig. 5 Pressure differential valve and brake warning light switch:

The valve assembly consists of two valves in a common bore that are spring loaded toward the centered position. The spring-loaded switch contact plunger rests on top of the valves in the centered position (right view). When a pressure differential occurs between the front and rear brake systems, the valves will shuttle toward the side with the low pressure. The spring-loaded switch plunger is "triggered" and the ground circuit for the warning light is completed, lighting the lamp (left view).

Fig. 6 Pressure differential valve and brake warning light switch:

As pressure falls in one system, the other system's normal pressure forces the piston to the inoperative side, contacting the switch terminal, causing the warning light on the instrument panel to glow.

Fig. 7 Delco-Moraine aluminum dual master cylinder with built in warning light switch:

The switch mounted directly in the master cylinder assembly. Whenever there is a specified differential pressure, the switch piston will activate the brake failure warning switch and cause the brake warning light to glow.

Fig. 8 Typical pressure valve and brake warning light switch.:

TESTING WARNING LIGHT SYSTEM

If the parking brake light is connected into the service brake warning light system, the brake warning light will flash only when the parking brake is applied with the ignition turned ON. The same light will also glow should one of the two service brake systems fail when the brake pedal is applied.

To test the system, turn the ignition on and apply the parking brake. If the lamp fails to light, inspect for a burned out bulb, disconnected socket, a broken or disconnected wire at the switch.

Above is an exterior view of one of these switches. They are usually mounted on the left frame side rail or on the brake pedal bracket.

To test the brake warning system, raise the car and open a wheel bleeder valve while a helper depresses the brake pedal and observes the warning light on the instrument panel. If the bulb fails to light, inspect for a burned out bulb, disconnected socket, or a broken or disconnected wire at the switch. If the bulb is not burned out, and the wire continuity is proven, replace the brake warning switch.