Problem:
This article will go over what to do when the red brake warning light comes on after service on vehicles equipped with pressure differential switch.
Red Brake Warning Light - Cause:
The pressure differential switch piston is offset, causing the pressure differential switch to remain in a grounded position.
How to Fix the Red Brake Warning Light:
The solution applied will depend on what type of pressure differential switch the vehicle has. 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 the additional function of limiting fluid loss to the rear brake circuit in a rear circuit failure.
Conventional pressure differential switch (fluid flow to rear brake):
Bleed the system using the manufacturer’s sequence until all the air is out of the system. If the pressure differential switch does not re-center itself during bleeding, it creates a pressure loss opposite the original hydraulic failure. For example, if the initial failure were a broken rear brake line, you would open either front bleeder to create this difference.
Once open, spike the brake pedal a couple of times while watching the red warning light (key on), using caution not to exceed ¾ pedal travel to prevent master cylinder damage. The pressure difference should push the piston back to the center.
Special purpose pressure differential switch (restricted fluid flow to rear brakes):
This type of pressure differential switch will not allow a system to bleed until the piston is re-centered. To re-center, bleed the system from the master cylinder down to the valve’s inlet until you can no longer see air. Accomplish this by cracking the line fitting at both the master outlet and valve inlet.
Once bled, spike the brake pedal a couple of times while watching the red warning light (key on), using caution not to exceed ¾ pedal travel to prevent master cylinder damage. The pressure difference should push the piston back to the center. Once re-centered, complete the repair by bleeding the rear brakes.
More Info on Red Brake Warning Light:
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 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.
51.1
The piston has what is called a detent in the center of it. The detent is a beveled indentation where the switch plunger rests. The switch plunger is held in place by the spring tension of the switch.
Each end of the piston is fitted with o-rings to form a seal from the primary and secondary circuit pressures. One end of the piston is exposed to primary circuit pressures, while the opposite end has exposure to secondary circuit pressures.
Regular system operation produces near equal pressures in both the primary and secondary hydraulic circuits. The spring-loaded plunger on the switch keeps the piston from moving under minor pressure differences. Typically it will prevent movement until the pressure difference reaches approximately 150 psi.
If a hydraulic failure occurs in either the primary or secondary circuits, it will create a pressure difference. The high pressure on one side of the piston will push the piston towards the failed side or low-pressure circuit (See Figure 51.2).
The movement of the piston will cause the open switch to go to a closed position which completes the ground to the red warning light. The light will remain on until the piston is re-centered.
Figure 51.2
Hydraulic Failure Causing Red Brake Warning Light
Most vehicles will never experience a hydraulic failure, and therefore the pressure differential piston will never move. This lack of movement is responsible for the piston “sticking” when offset due to a hydraulic failure. The piston bore corrodes and causes an interference fit.
Special Purpose Pressure Differential Switches
A group of specialized pressure differential switches has an additional function in a rear circuit failure. In addition to turning the red warning light on, they also restrict fluid flow to the rear brakes in the rear circuit hydraulic failure vent.
A typical method is in Figure 51.3. Figure 51.3 shows the piston in the neutral position. Fluid flow to the rear brakes is through the passages marked as such. Figure 51.4 shows what happens to these passages when a rear circuit hydraulic failure occurs. The piston offsets and covers the holes. As a result, fluid flow will be restricted to the rear circuit, limiting fluid loss.
51.3
51.4
If a rear circuit hydraulic failure occurs and the piston does not re-center, the rear brakes will not be bled until the piston is re-centered. See procedure under “Solution.”
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