chap82 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 82, the reader should be able to: • • • • Prepare for ASE Brakes (A5) certification test content area “F” (Antilock Brake System Diagnosis and Repair) Describe normal ABS dash lamp operation Discuss visual inspection of the various types and brands of ABS Explain how to retrieve trouble codes 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 OBJECTIVES: After studying Chapter 82, the reader should be able to: • • • List the methods used to clear trouble codes Explain the various methods for bleeding ABS Discuss methods and tools needed to diagnose an ABSequipped vehicle 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: amber ABS warning lamp breakout box (BOB) red brake warning lamp (RBWL) 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 ABS DIAGNOSTIC PROCEDURE Customer concerns about a fault with the antilock braking system can be best handled by performing the following: Step #1 Verify the customer concern This is very important because often a problem with the base brakes is thought by the customer to be a fault with the antilock braking system It is helpful for the owner to drive the vehicle with a service tech if the problem is not readily apparent 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 Step #2 Perform a visual inspection If both the red and the amber brake warning lights are on, look for a fault in the hydraulic system, including leaks at the following locations: • • • • • Master cylinder Electrohydraulic control unit Flexible brake hoses Brake lines and fittings Calipers and/or wheel cylinders Step #3 Check for stored diagnostic trouble codes (DTCs) Use a scan tool or other necessary methods to retrieve diagnostic trouble codes If found, follow the specified factory procedures to isolate and determine the cause 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 Step #4 Complete the repair This step may involve replacing a hydraulic component If so, then the hydraulic system should be bled using the factory specified procedure Clear all diagnostic trouble codes Step #5 Verify the repair Always test-drive the vehicle under the same conditions that were needed to verify the problem to be sure that the cause has been corrected 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 BRAKE WARNING LAMP OPERATION The first step in the visual diagnosis of an antilock braking system problem is to check the status of the brake warnin lamps Red Brake Warning Lamp A red brake warning lamp (RBWL) warns of a possible dangerous failure in the base brakes, such as low brake fluid level or low pressure in half of the hydraulic system The red brake warning lamp will also light if the parking brake is applied and may light due to an ABS failure, such as low brake pressure on an integral system See Figure 82–1 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 82–1 Typical brake warning lamp operation chart Not all vehicles use the same light sequence The top of the chart indicates normal warning lamp operation if the system is okay See the chart on Page 1004 of your textbook 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 Amber ABS Warning Lamp The amber ABS warning lamp usually comes on after a start during the initialization or startup self-test sequence The exact time the lamp remains on after the ignition is turned on varies with the vehicle and the ABS design Figure 82–2 On most vehicles equipped with ABS, the ABS and the BRAKE warning lamp should come on as a bulb check when the ignition in first switched on 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 Scope Testing Attach the scope leads to the sensor terminals or to the input connector on the breakout box With all four wheels off the ground, rotate the wheel by hand or by engine power A good WSS should produce an alternating current (AC) sine wave that increases in frequency and amplitude with increasing wheel speed Figure 82–13 A scope can be used to check for proper operation of a wheel speed sensor 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 Damaged or missing teeth on the tone ring will cause flat spots or gaps in the sine wave pattern.A bent axle or hub will produce a wavelike pattern that fluctuates as the strength of the sensor signal changes with each revolution Figure 82–14 A broken tooth on a wheel speed sensor tone ring shows on the scope trace as a missing wave 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 Scan Tool Testing A scan tool can be used to check for the proper operation of the WSS As an assistant drives the vehicle, connect the scan tool and monitor the speed of all of the sensors All of the sensors should indicate the same speed If a sensor shows a slower or faster speed than the others, carefully check the tone ring for damage such as a crack 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 WHEEL SPEED SENSOR ADJUSTMENT Some ABS applications use adjustable WSSs Most sensors adjust by first loosening a set screw, then inserting a nonmagnetic brass or plastic feeler gauge between the tip of the sensor and a high point on the tone ring Figure 82–15 Use a nonmagnetic brass or plastic feeler gauge to check wheel speed sensor gap A steel gauge would be attracted by the magnet in the sensor and would produce a drag on the gauge as it is moved between the sensor and the tone ring This drag could be interpreted as a correct clearance reading Adjust the sensor position so there is slight drag on the feeler gauge, and tighten the setscrew to lock the sensor in place 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 82–16 (a) Always use a nonferrous (brass or plastic) feeler (thickness) gauge when measuring the gap between the toothed ring and the wheel speed sensor (b) Sometimes a sensor is equipped with a paper spacer that is the exact thickness of the spacing required between the toothed ring and the sensor If equipped, the sensor is simply installed with the paper touching the toothed wheel A typical gap ranges from 0.020 to 0.050 in (0.5 to 1.3 mm) (a) (b) Adjust the sensor so the tip just touches the tone ring and you can slip the paper or plastic out without ripping it Tighten the setscrew and the air gap is properly set 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 Digital Wheel Speed Sensor Diagnosis Test a digital WSS by first checking that battery voltage is available at the sensor with the key on, engine off If the sensor does not have 12 volts, the problem is the ABS controller or wiring between controller and sensor If there are 12 volts at the sensor, measure the signal voltage The voltage should switch from about 0.8V to about 1.9V as the wheel is rotated by hand 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 HYDRAULIC ABS SERVICE Before doing any brake work on a vehicle equipped with antilock brakes, always consult the appropriate service information for the exact vehicle being serviced Some manufacturers recommend discharging the hydraulic accumulator by depressing the brake pedal many times before opening bleeder valves Many service checks require that a pressure gauge be installed in the system Air can easily get trapped in the ABS electronic-hydraulic (E-H) assembly whenever the hydraulic system is opened Some E-H units can be bled through use of a scan tool where the valves are pulsed in sequence by the electronic brake controller (computer) 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 Some units are equipped with bleeder valves while others must be bled by loosening the brake lines Bleeding the E-H unit also purges out the older brake fluid, which can cause rust and corrosion damage Only DOT brake fluid is specified for use in an antilock braking system Always check the label on the brake fluid reservoir and/or service manual or owner’s manual CAUTION: Some ABS units require that the brake pedal be depressed as many as 40 times to discharge brake fluid fully from the accumulator Failure to discharge the accumulator fully can show that the brake fluid level is too low If additional brake fluid is added, the fluid could overflow the reservoir during an ABS stop when the accumulator discharges brake fluid back into the reservoir 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 Bleeding ABS During routine brake service, attempt to keep the air from entering the hydraulic system by doing the following: Do not allow the brake system to run dry Use a brake pedal depressor or plug any open brake line to keep brake fluid from flowing out of the brake master cylinder reservoir Do not allow the master cylinder to run dry during the bleeding operation Check the master cylinder reservoir and keep it filled with fresh brake fluid from a sealed container Always bench bleed a replacement master cylinder to help prevent against introducing air into the hydraulic system NOTE: To avoid bleeding the hydraulic unit, use a brake pedal depressor during service to avoid losing brake fluid This precaution keeps air from getting into the hard-to-bleed passages of the hydraulic unit 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 depressing the unit as per manufacturer’s recommended procedures, the brakes can be bled using the same procedure as for a vehicle without ABS Air trapped in the ABS hydraulic unit may require that a scan tool be used to cycle the valves Figure 82–17 Special bleed valve tools are often required when bleeding some ABS units such as the Kelsey-Hayes 4WAL 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 Figure 82–18 Two bleed valve tools are needed to bleed the Kelsey-Hayes 4WAL system, which attaches to the bleed valves on the accumulator The bleeding procedure for vehicles equipped with antilock brakes is often different than vehicles without ABS Consult service information for the specified bleeding procedure and sequence 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 ABS SAFETY PRECAUTIONS Avoid mounting the antenna for the transmitting device near the ABS control unit Transmitting devices include cellular (cell) telephones, citizen-band radios, and so on Avoid mounting tires of different diameter than the original tires Different size tires generate different wheel speed sensor frequencies which may not be usable by the ABS controller Never open a bleeder valve or loosen a hydraulic line while the ABS is pressurized The accumulator must be depressurized according to the manufacturer’s recommended procedures If arc welding on a vehicle, disconnect computers (electronic control modules) to avoid damage due to voltage spikes Do not pry against or hit the wheel speed sensor ring 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 ABS diagnosis starts with checking the status of both the red brake warning lamp and the amber ABS warning lamp The second step in diagnosis of an ABS problem is to perform a thorough visual inspection The third step in diagnosis of an ABS problem is to test-drive the vehicle and verify the fault Always consult the factory service information for the specific vehicle being serviced for the proper procedure to use to retrieve and clear diagnostic trouble codes (DTCs) 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.) breakout box is used with a digital multimeter to diagnose electrical ABS components Hydraulic service on most integral ABS units requires that the brake pedal be depressed as many as 40 times with the ignition key “off” to depressurize the hydraulic 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