Auto Service Professional

DEC 2018

Magazine for the auto service professional

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D e c e m b e r 2 018 A SP 37 a module that will receive inputs from various parts of the braking system and with the inputs from the vehicle's driver. It then analyses the inputs that it receives and makes a decision on hydraulic pressures that should be applied so the vehicle will stay in a safe position based on the current conditions it has received. When the module receives the inputs neces- sary it then processes the information and gen- erates the proper output. When the inputs to the control module are not sending the correct signals or no signals at all, that's when servicing the ABS will present our greatest challenges as technicians. Some of the inputs that the EBCM will need to make the correct decision are from the wheel speed sensors (WSS), see Figure 2, vehicle speed sensors, brake pedal position (BPP) sensor, steering angle sensor (SAS), yaw rate (YAW) sensor, and many other sensors plus the added electrical circuits, so- lenoids, control valves and connections. It also looks at the hydraulic circuits as well. If the conventional brake hydraulic pressure isn't correct the system will not operate properly. e EBCM not only monitors the compo- nents in the braking system, but it also com- municates with several other electrical systems on the vehicle. e ECM, TCM, SRS, BCM and other modules communicate on a bus network and share information with each other. If the EBCM is generating incorrect informa- tion, the other modules on the network could possibly set DTCs of their own, indicating a problem with the ABS. When working on an ABS, it is extremely important to have access to a capable scan tool (see Figure 3). Capable means it has to be able to access ABS data, be able to have the capabilities of bidirectional control, miscellaneous func- tion tests, and more importantly, be updated to the current model year you are working on. ABS diagnostics using a scan tool is mission crit- ical when looking at the data pids and without a doubt your scan tool should have graphing capa- bilities (see Figure 4). e wheel speed sensors are probably the most important sensors from which the EBCM receives its information. When testing an ABS, set up your scan tool in graphing mode and road test the vehicle. For safety reasons, set up your scan tool to re- cord a movie. When you return back to the shop you can playback your recorded movie and look at all the WSS to see if they are reporting the cor- rect data. ere you can see if the sensors are tell- ing the EBCM the truth. If for some reason you find a glitch or a dropout in your scan data waveform you can look at that sensor in a little more detail for a problem. In the screen shot shown in Figure 5, you will see a drop- out with this particular sensor. A dropout could be caused by an erratic signal generated by the sensor. Some causes could involve a toner ring that has a problem with a missing tooth or it possibly got bent or knocked out of place and the sensor can't pick it up. ere could also be a problem with the internal part of the sensor or the wiring and or connection going to that sensor. Wheel speed sensors generate an AC signal which is converted back to a DC signal. If there is any type of interference in the signal from the sensor the EBCM will receive that as a problem and thus flag that particular circuit in the form of a DTC. Most WSS are a permanent mount type but there are some that are mounted in a sleeve and can be adjusted to the correct position. When testing an individual wheel speed sen- sor, a capable DVOM (digital volt ohm meter) or a lab scope is needed. A common test on a WSS is a resistance test and an AC voltage test. When Figure 1: The ECBM module receives inputs from various signals from the braking system.

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