chap80 pps Automotive technology at University of Cambridge

start Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 OBJECTIVES: After studying Chapter 80, the reader should be able to: • Prepare for ASE Brakes (A5) certification test content area “D” (Power Assist Units Diagnosis and Repair) • • • • List the parts of a vacuum brake booster Describe how a vacuum brake booster operates Explain how to test a vacuum brake booster Describe how a hydraulic or electrohydraulic brake booster operates Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 KEY TERMS: atmospheric pressure • brake assist system (BAS) dual-diaphragm vacuum booster inches of mercury (in Hg) millimeters of mercury (mm Hg) power chamber • pressure differential supplemental brake assist (SBA) tandem-diaphragm vacuum booster • vacuum Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 THE NEED FOR POWER BRAKE ASSIST To double the stopping power of a disc brake, the driver must double the force on the brake pedal This is the reason that most vehicles equipped with disc brakes are power assisted The most commonly used power-assisted units are vacuum operated When a power booster is fitted, brake pedal ratio is decreased and the master cylinder bore size is increased The effect of these changes is to reduce pedal effort, greatly increasing pedal reserve Power boosters not alter the hydraulic system and allow braking even if the booster fails or its power supply is cut off All boosters have a power reserve that provides assist for at least one hard stop, and sometimes several light brake applications, even after power is lost See Figure 80–1 Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 80–1 Typical vacuum brake booster assembly The vacuum hose attaches to the intake manifold of the engine The brake pedal travel sensor is an input sensor for the antilock braking system Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Because power brake systems are designed with the added force of the booster taken into account, the amount of brake pedal pressure required to slow or stop a vehicle is much higher than in a nonboosted system once the reserve is used up Figure 80–2 A wide brake pedal allows two foot braking if power assist is lost For this reason, some vehicles with power brakes have a brake pedal that is wide enough to allow two-foot braking should the booster fail Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 PRINCIPLES OF VACUUM Most vacuum-powered brake boosters get their vacuum supply from the engine intake manifold An engine is essentially a big air pump because the pistons move up and down in the cylinders to pump in air and fuel, and pump out exhaust As a piston moves down on intake with the intake valve open, it creates a larger area inside the cylinder for air to fill Although it may seem as though the low pressure is pulling air into the engine, it is really the higher pressure outside that forces air in The difference in pressure between two areas is called a pressure differential Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Gasoline-powered internalcombustion engines normally operate with a low-pressure area, or partial vacuum, in the intake manifold Figure 80–3 Atmospheric pressure varies with altitude The term vacuum is used to refer to any pressure lower than atmospheric pressure Atmospheric pressure varies with altitude, but is about 14.7 pounds per square inch (psi) at sea level Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Measuring Vacuum Vacuum is measured in inches of mercury (in Hg) or in millimeters of mercury (mm Hg), a figure that indicates how far a column of mercury will rise when a vacuum is applied at one end, and atmospheric pressure at the other Vacuum is a measurement of the pressure differential between the lower pressure inside the tube, and the higher pressure outside it Manifold vacuum varies with throttle position The lowest vacuum (highest pressure) occurs when the throttle is wide open with the engine under load The highest manifold vacuum (lowest pressure) may be as much as 24 in Hg (610 mm Hg) Vacuum at idle typically falls between 15 and 20 in Hg (381 and 508 mm Hg) Most vacuum brake boosters operate with vacuum levels in this range Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Booster Vacuum Supply Vacuum boosters get their vacuum supply from the engine intake manifold Diesel engines, however, run unthrottled (engine speed is controlled strictly by the amount of fuel injected) and have little or no intake manifold vacuum If a vehicle with a diesel engine is equipped with a vacuum-powered brake booster, it must also be fitted with an auxiliary vacuum pump Figure 80–4 A belt-driven auxiliary vacuum pump Some small gasoline-powered and diesel engines use a beltdriven add-on pump Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 When the brake pedal is depressed, the lever and primary valve are moved The valve closes off the return port, causing pressure to build in the boost pressure chamber The hydraulic pressure pushes on the power piston, which applies force to the output rod that connects to the master cylinder piston In the event of a power steering pump failure, power assist is still available for several brake applications See Figure 80–24 During operation, hydraulic fluid under pressure from the power steering pump pressurizes an accumulator See Figure 80–25 Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 80–24 A hydro-boost hydraulic booster as the brakes are applied Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 80–25 A hydro-boost hydraulic booster in the holding position Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Diagnosis Power for hydro-boost units comes from the power steering pump The first step of troubleshooting is to perform a thorough visual inspection, including the following: Checking for proper power steering fluid level Checking for leaks from the unit or power steering pump Checking the condition and tightness of the power steering drive belt Checking for proper operation of the base brake system Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 After checking all of the visual components, check for proper pressure and volume from the power steering pump using a power steering pump tester The pump should be capable of producing a minimum of gallons (7.5 liters) with a maximum pressure of 150 psi (1000 kPa) with the steering in the straight-ahead position With the engine “off,” the accumulator should be able to supply a minimum of two power-assisted brake applications See Figures 80–26 and 80–27 Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 80–26 A typical hydro-boost hydraulic line arrangement showing the pump, steering gear, and brake booster assembly Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 80–27 Pressure and flow analyzer installation to check the power steering pump output Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 HYDRO-BOOST FUNCTION TEST With the engine off, apply the brake pedal several times until the accumulator is depleted completely Depress the service brake pedal and start the engine The pedal should fall and then push back against the driver’s foot Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 The Hydro-Boost Accumulator Test The accumulator stores hydraulic fluid under pressure to provide a reserve in the event of a failure of the power steering system The accumulator is designed to provide three or more power-assisted stops with the engine off If the accumulator fails, it does not hold pressure To easily check whether the accumulator has lost its charge, grasp the accumulator with your hand and try to twist or move it The accumulator should have so much pressure that it should not move or wiggle If the accumulator moves, it has lost its ability to hold pressure and the hydro-boost unit should be replaced Figure 80–28 The accumulator should be able to hold pressure and feel tight when hand force is used to try to move it Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 HYDRO-BOOST SYMPTOM-BASED GUIDE Excessive Brake Pedal Effort Loose or broken power steering pump belt No fluid in the power steering reservoir Leaks in the power steering, booster, or accumulator hoses Leaks at tube fittings, power steering, booster, or accumulator connections External leakage at the accumulator Faulty booster piston seal, causing leakage at the booster flange vent Faulty booster cover seal with leakage between the housing and cover Faulty booster spool plug seal Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Slow Brake Pedal Return Excessive seal friction in the booster Faulty spool action Broken piston return spring Restriction in the return line from the booster to the pump reservoir Broken spool return spring Grabby Brakes Broken spool return spring Faulty spool action caused by contamination in the system Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Booster Chatters—Pedal Vibrates Power steering pump belt slipping Low fluid level in the power steering pump reservoir Faulty spool operation caused by contamination in the system Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 SUMMARY Vacuum brake boosters use air pressure acting on a diaphragm to assist the driver’s force on the brake master cylinder At rest, there is vacuum on both sides of the vacuum booster diaphragm When the brake pedal is depressed, atmospheric air pressure is exerted on the back side of the diaphragm The use of two diaphragms in tandem allows a smallerdiameter booster with the same area The larger the area of the booster diaphragm, the more air pressure force can be applied to the master cylinder Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 SUMMARY (cont.) Hydraulic-operated brake boosters use either an electric motor driven pump or the engine-driven power steering pump When replacing a vacuum brake booster, always check for proper pushrod clearance To be assured of power-assisted brake application in the event of failure, hydraulic power-assisted brake systems use an accumulator to provide pressure to the system Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 end Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458