Problem: Red brake warning light on after service on vehicles equipped with pressure differential switch.
Cause: Pressure differential switch piston is offset causing pressure differential switch to remain in grounded position.
Solution: The solution applied will depend on what type of pressure differential switch the vehicle is equipped with. Most pressure differential switches have only one function – turn the red warning light on if a hydraulic failure occurs in either the primary or secondary circuits. Some others have an additional function of limiting fluid loss to the rear brake circuit in the event of a rear circuit failure.
Conventional pressure differential switch (fluid flow to rear brake):
Special purpose pressure differential switch (restricted fluid flow to rear brakes)
More Info: To better understand the fix an understanding of the pressure differential switch’s function is in order. Figure 51.1 is a cross sectional view of a stand-alone pressure differential switch. The pressure differential switch consists of a piston exposed to both primary and secondary circuit pressures and a normally open switch. The switch consists of the switch body,a spring- loaded plunger and a contact pin.
Special Purpose Pressure Differential Switches
Figure 51.3 Figure 51.4
If a rear circuit hydraulic failure occurs and the piston does not re-center the rear brakes will not be able to be bled until the piston is re-centered. See procedure under “Solution”.