Brake Fluid Pressure Sensor/Switch: Description and Operation

PRESSURE DIFFERENTIAL WARNING VALVE





The warning valve is installed only in cars without ABS.
Made of aluminum alloy, the valve is incorporated in the brake pipe manifold mounted on the inner wheel housing Its function is to warn the driver of excessive difference in pressure between the two braking circuits (e.g. in the event of leakage).
On models from 1989 on, the warning valve was replaced by a level switch in the brake fluid reservoir, a feature already installed on cars with ABS.
The warning valve actuating pin rests on the mid-section of a movable piston in the chamber between the primary and secondary circuits of the manifold.
When the brakes are applied normally i.e. when the pressure in both circuits is approximately equal, the piston is in the position shown in figure A.





A. Equal pressure in both circuits
If an excessive drop in pressure (due to leakage or air in the system) occurs in one of the circuits, the piston will be displaced laterally by the higher pressure in the other circuit (figure B).
The valve actuating pin will then ground the warning lamp on the instrument panel and the lamp will light.
The piston must be returned to the idle position when the fault has been corrected. This is achieved by exerting moderate pressure on the brake pedal.
B. Pressure drop in one circuit (right-hand)