7 Automotive Hydraulic Valves and Switches.pptx

Automotive Engineering

1. Automotive Chassis Systems Eighth Edition Chapter 07 Hydraulic Valves and Switches

2. Objectives Describe the operation of a pressure-differential switch and a brake fluid level sensor switch. Describe the operation of the proportioning valve. Discuss the need and use of a metering valve. List the components included in a combination valve. Describe how a brake light switch works.

3. Pressure-Differential Switch (Brake Warning Switch Purpose and Function A pressure-differential switch is used on vehicles built after 1967 with front/rear master cylinders to warn the driver of a loss of pressure in one of the two separate systems by lighting the dashboard red brake warning indicator lamp. This system was used until it was replaced with the brake fluid level sensors used on newer vehicles. The brake lines from both the front and the rear sections of the master cylinder are sent to this switch, which lights the brake warning indicator lamp in the event of a “difference in pressure” between the two sections. A failure in one part of the brake system does not result in a failure of the entire hydraulic system.

4. Pressure-Differential Switch (Brake Warning Switch Pressure-Differential Switch Reset After the hydraulic system has been repaired and bled, moderate pressure on the brake pedal centers the piston in the switch and turns off the warning lamp. If the lamp remains on, it may be necessary to do the following: Apply light pressure to the brake pedal. Momentarily open the bleeder valve on the side that did not have the failure. This procedure should center the pressure-differential switch valve in those vehicles that are not equipped with self-centering springs.

5. Brake Fluid Level Sensor Purpose and Function Since the late 1990s, master cylinders use a brake fluid level sensor or switch in the master cylinder reservoir. This sensor lights the red brake warning lamp (RBWL) on the dash if low brake fluid level is detected. A floattype sensor or a magnetic reed switch is commonly used and provides a complete electrical circuit when the brake fluid level is low.

6. Brake Fluid Level Sensor Diagnosing a “Red” Brake Warning Light The RBWL can be on for several reasons, including: Parking brake on . The same dash warning lamp is used to warn the driver that the parking brake is on. Low brake fluid . This condition causes the red dash warning lamp to come on if the vehicle is equipped with a master cylinder reservoir brake fluid level switch. Unequal brake pressure . The pressure-differential switch is used on older vehicles with a front/rear brake split system to warn the driver whenever there is low brake pressure to either the front or rear brakes. Worn brake pads . Vehicles equipped with electronic pad wear sensors can turn on the light when the pads wear out.

7. Brake Fluid Level Sensor Diagnosing a “Red” Brake Warning Light The most likely cause of the RBWL being on is low brake fluid caused by worn-out brake pads or a leaking brake line, wheel cylinder, or caliper. Therefore, the first step is to determine the cause of the lamp being on, then to repair the problem. STEP 1 Check the Level of the Brake Fluid. If low, carefully inspect the entire hydraulic brake system for leaks and repair, as necessary. STEP 2 Disconnect the Wire from the Pressure-Differential Switch. If the lamp is still “on,” the problem is due to the parking brake lever switch being “on” or grounded, or the wire going to the switch is shorted to ground. If the red brake warning lamp is “off” after being disconnected from the pressure-differential switch, then the problem is due to a hydraulic failure.

8. Proportioning Valve Purpose and Function A proportioning valve improves brake balance during hard stops by limiting hydraulic pressure to the rear brakes. A proportioning valve is necessary because inertia creates weight shift toward the front of the vehicle during braking. The weight shift unloads the rear axle, which reduces traction between the tires and the road, and limits the amount of stopping power that can be delivered. Vehicles with front disc and rear drum brakes require a proportioning valve for two reasons. Disc brakes require higher hydraulic pressure for a given stop than do drum brakes. In a disc/drum system, the front brakes always need more pressure than the rear brakes. Once braking has begun, drum brakes require less pressure to maintain a fixed level of stopping power than they did to establish that level. In a disc/drum system, the rear brakes always need less pressure than the front brakes.

9. Proportioning Valve Proportioning Valve Operation The proportioning valve does not work at all times. During light or moderate braking, there is insufficient weight transfer to make rear wheel locking a problem. Before proportioning action begins, brake system hydraulic pressure must reach a minimum level, called the split point. Above the split point, the proportioning valve allows only a portion of the pressure through to the rear brakes. Once system hydraulic pressure exceeds the split point, the rear brakes receive a fixed percentage of any further increase in pressure. Brake engineers refer to the ratio of front-to-rear brake pressure proportioning as the slope.

10. Proportioning Valve Proportioning Valve Parts A simple proportioning valve consists of a spring-loaded piston that slides in a stepped bore. The piston is exposed to pressure on both sides. The smaller end of the piston is acted on by pressure from the master cylinder, while the larger end reacts to pressure in the rear brake circuit. When the brakes are first applied, hydraulic pressure passes through the proportioning valve to the rear brakes. At pressures below the split point, the proportioning valve is open, and pressure to both the front and rear brakes is the same. This seals off the brake line and prevents any additional pressure from reaching the rear brakes. As system pressure (the pressure to the front brakes) increases, enough force is developed on the master cylinder side of the piston to overcome the pressure trapped in the rear brake circuit.

11. Proportioning Valve Proportioning Valve Parts The difference in surface area between the two ends of the piston determines the slope of the valve and, thus, the percentage of system pressure allowed to reach the rear brakes. Height-Sensing Proportioning Valves Many vehicles use a proportioning valve that varies the amount of pressure that can be sent to the rear brakes, depending on the height of the rear suspension. This type of valve is called a height-sensing proportioning valve. If the vehicle is lightly loaded, the rear suspension is high, especially during braking. In this case, the amount of pressure allowed to the rear brakes is reduced. When the vehicle is loaded, the rear suspension is forced downward. The lever on the proportioning valve moves and allows a greater pressure to be sent to the rear brakes.

12. Proportioning Valve Height-Sensing Proportioning Valves FIGURE 7–13 A height-sensing proportioning valve provides the vehicle with variable brake balance. The valve allows higher pressure to be applied to the rear brakes when the vehicle is heavily loaded and less pressure when the vehicle is lightly loaded.

13. Proportioning Valve Proportioning Valve Adjustment Height-sensing proportioning valves should be adjusted when replaced. The proper adjustment ensures that the proper pressure is applied to the rear brakes in relation to the loading of the vehicle. Proportioning Valve Diagnosis and Testing A defective proportioning valve usually allows rear brake pressure to increase too rapidly, causing the rear wheels to lock up during hard braking. When the rear brakes become locked, the traction with the road surface decreases and the vehicle often skids. Whenever rear brakes tend to lock during braking, the proportioning valve should be checked for proper operation. Pressure gauges can also be used to check for proper operation. Install one gauge into the brake line from the master cylinder and the second gauge to the rear brake outlet of the proportioning valve.

14. Electronic Brake Proportioning Purpose and Function Most new vehicles with antilock brake systems have eliminated the need for a conventional brake proportioning valve. Instead, this becomes a function of the antilock brake system controller. Often referred to as dynamic rear proportioning (DRP), this system removes the need for a mechanical proportioning valve. Parts and Operation Most proportioning valves are calibrated to reduce pressure to the rear brakes by a fixed amount, which may increase the risk of rear-wheel lockup if the vehicle is loaded differently or is braking on a wet or slick surface. Dynamic rear proportioning uses the ABS controller software and the rear wheel hydraulic control circuits to maintain optimum front-to-rear balance under all conditions.

15. Electronic Brake Proportioning Parts and Operation Electronic brake proportioning (EBP) in the ABS is accomplished by monitoring brake switch input and front- and rear-wheel speeds, and reducing pressure to the rear brakes as needed using the ABS solenoids when there is a difference in wheel deceleration rates. The controller energizes the inlet valve solenoids for both rear brakes to hold pressure in the lines and then energizes both rear outlet valve solenoids to release pressure, as needed.

16. Metering Valve (Hold-off) Function Purpose and Function A metering valve is used on many front-disc, rear-drum-brake-equipped vehicles. The metering valve prevents the full operation of (holds off) the disc brakes until between 75 to 125 PSI is sent to the rear drum brakes to overcome rear-brake return spring pressure. This allows the front and rear brakes to apply at the same time for even stopping. FIGURE 7–17 The metering valve may be a separate component or part of the combination valve.

17. Metering Valve (Hold-off) Function Parts and Operation A metering valve consists of a piston controlled by a strong spring and a valve stem controlled by a weak spring. When the brakes are not applied, the strong spring seats the piston and prevents fluid flow around it. At the same time, the weak spring holds the valve stem to the right and opens a passage through the center of the piston. When the brakes are applied and pressure in the front brake line reaches 3 to 30 PSI (20 to 200 kPa), the tension of the weak spring is overcome and the metering valve stem moves to the left, which closes the passage through the piston and prevents fluid flow to the front brakes. The small amount of pressure applied to the calipers before the metering valve closes is enough to take up any clearance, but not enough to generate braking force.

18. Metering Valve (Hold-off) Function Parts and Operation When the pressure at the metering valve reaches 75 to 300 PSI, the tension of the strong spring is overcome and the valve stem and piston move farther to the left. This opens a passage around the outside of the piston and allows fluid to flow through the valve to the front brake calipers. When the brakes are released, the strong spring seats the piston and prevents fluid flow around it. At the same time, the weak spring opens the fluid passage through the center of the piston.

19. Metering Valve (Hold-off) Function Systems Without Metering Valves Braking systems that are diagonal split, such as those found on most front-wheel drive vehicles, do not use a metering valve. A metering valve is only used on front/rear split braking systems, such as those found on most rear-wheel-drive vehicles. There are three reasons why front-wheel-drive vehicles do not use metering valves. Front-wheel-drive vehicles usually have a diagonally split dual braking system that would require a separate metering valve for each hydraulic circuit. This would make the brake system more costly and complicated. Front-wheel-drive vehicles have a forward weight bias that requires the front brakes to supply up to 80% of the total braking power. A metering valve would create a slight delay. Until all the clearance in the brake system is taken up, there will not be enough pressure in the brake hydraulic system for the front disc brakes to overcome the engine torque applied to the driven front wheels. Most rear-wheel-drive vehicles without metering valves are equipped with four-wheel disc brakes.

20. Metering Valve (Hold-off) Function Meter Valve Diagnosis and Testing A defective metering valve can leak brake fluid and/or cause the front brakes to apply before the rear brakes. If the front brakes lock up during these conditions, the front wheels cannot be steered. Inspect the metering valve for two possible faults: Look around the bottom of the metering valve for brake fluid leakage. (Ignore slight dampness.) Replace the metering valve assembly if it is leaking. As the pressure builds to the front brakes, the metering valve stem should move. If it does not, replace the valve. More accurate testing of the metering valve can be accomplished using pressure gauges.

21. Combination Valves Most vehicle manufacturers combine the function of a proportioning valve with one or more other valves into one unit, called a combination valve. On a rear-wheel-drive vehicle, a combination valve consists of the following components all in one replaceable unit: Metering valve Proportioning valve Pressure-differential switch

22. Combination Valves Some combination valves have only two functions and contain just the pressure differential and metering valve. Other units combine the pressure differential with the proportioning valve

23. Brake Pedal Position Sensor/Switch Purpose and Function On older vehicles, the brake switch was used to turn on the brake lights. Most new vehicles use the brake pedal position (BPP) sensor or switch as an input to the body computer for many functions, including the following: Brake lights ABS input signal Traction control is disabled when the brake pedal is depressed Electronic stability control (ESC) system input signal Vehicles with ABS and ESC may also use a brake pedal position sensor in addition to the brake pedal switch. This sensor is a three-wire potentiometer that returns a variable voltage signal to the ABS controller to indicate the position of the brake pedal. FIGURE 7–24 The brake pedal position (BPP) sensor (switch) and arm mounts to the brake pedal, under the dash.

24. Brake Pedal Position Sensor/Switch Brake Light Switch Testing The brake light switch or sensor is most easily checked with the use of a scan tool. Both the powertrain data list and the ABS data list show the brake switch as an “ON/OFF” status when the pedal is pressed. A DMM (digital multimeter) set to ohms can be used to check a brake switch on or off the vehicle. Access the two wires from the switch and back probe the connectors with the meter test leads.

25. Summary A pressure-differential switch is used on vehicles built after 1967 with front/rear master cylinders to warn the driver of a loss of pressure in one of the two separate systems by lighting the dashboard red brake warning indicator lamp. Since the late 1990s, master cylinders use a brake fluid level sensor or switch in the master cylinder reservoir. This sensor lights the red brake warning lamp. A proportioning valve improves brake balance during hard stops by limiting hydraulic pressure to the rear brakes. Most new vehicles with antilock brake systems have eliminated the need for a conventional brake proportioning valve. Instead, this becomes a function of the antilock brake system controller, often referred to as dynamic rear proportioning (DRP), and this system removes the need for a mechanical proportioning valve. A metering valve is used on many front-disc, rear- drumbrake - equipped vehicles. The metering valve prevents the full operation of (holds off) the disc brakes until between 75 and 125 PSI is sent to the rear drum brakes to overcome rear-brake return spring pressure. Most vehicle manufacturers combine the function of a proportioning valve with one or more other valves into one unit, called a combination valve. On older vehicles, the brake switch was used to turn on the brake lights. Most new vehicles use the brake pedal position (BPP) sensor or switch as an input to the body computer for many functions.