The Most Important Diagnostic ToolPosted 5/10/2004
By Brian Manley
a methodical process to troubleshooting.
As an automotive instructor, I teach my students system theory first. Next, I teach them how to apply that theory to a good system. Finally, they learn how to apply that knowledge when faced with a system that is not operating as designed. I also remind them often about which diagnostic tool is the most important one.
Case in point: a Ford Explorer with an antilock brake system (ABS) lamp that is illuminated (Figure 1). The customer who had this problem came to us through one of my second-year students, Gary, who has gained a reputation around school as a guy who knows how to fix Fords. He developed this reputation from being an apprentice at a local Ford dealer for two years, and from working on many students' vehicles on the side. He is also becoming a better diagnostic tech with each passing month. He is one of several outstanding students who are gaining an understanding of strategy-based diagnostics.
With the Explorer in our bay, Gary had already verified the customer concern: The ABS lamp was illuminated at all times. Gary's first instinct was to grab the scan tool and probe the ABS module. Fifteen minutes later, after running key-on engine off (KOEO) and key-on engine running (KOER) tests, Gary had a hand full of trouble codes (Figure 2):
I know where I would have started with this Ford - right or wrong. I would have looked to see why the ABS ECU was so unhappy with the front speed sensors. But, as I've become a better instructor, I've also become better at letting students discover their own diagnostic routine. When I am smart enough to step back and simply facilitate the learning process - instead of showing the entire procedure - students often learn more by meandering through testing and discovering their way to the correct diagnosis.
SERVICE CODE - 11
At this point, I strongly urged Gary to perform a thorough inspection of all brake system components, right down to the friction material. I stressed the fact it has been my experience that ECUs are usually not faulty, but they do fail occasionally, and we should keep this fact in mind. When faced with a "fatal error" type of code, I explained, always search for peripheral causes, which are cheaper to fix than taking the code charts for gospel and throwing ECUs at the problem. Further, we cover electrical signals in our first quarter each year, and if the ECU was claiming that there was an erratic wheel speed sensor, I fully expected Gary to back-probe each sensor with a lab scope and either confirm or deny that fact.
As an added benefit to our customers, we also check steering and suspension components, leaks, belts and hoses and fluid conditions. I try to instill a sense of ownership with my students - to treat each vehicle as if their mom were driving it out of town on a long trip.
Gary got the vehicle into the air, where we immediately began inspection of both wheel speed sensors for the front wheels, due to the multiple codes pointing in their direction. The picture of the ABS wheel speed wire against the wheel (Figure 5) shows what we initially discovered. Could this cause the ABS ECU to freak out? Absolutely! Upon closer inspection, we discovered that the edge of the wheel had rubbed right through the speed sensor's insulation, into the copper wire strands, creating an "erratic" connection to ground.
We inspected the rest of the brake system and found no other hose twists or worn-out friction lining, so Gary broke the news to his customer.
The customer was informed of our discovery. She then told us that a local brake specialty chain store had recently replaced the front brakes. The next day she told us that their response was, "We couldn't have twisted the brake hose." I gave her my card and asked her to tell them to call me so I could explain the situation; otherwise, they were responsible for repairing the damage to the speed sensor wires. I never received a call, and the Explorer was repaired by the chain store.
The Most Important Diagnostic Tool
Once again, using our most important diagnostic tool found the root cause of another customer concern. Properly functioning sensor inputs into this tool (sight, sound, smell and touch) are critical for the read-only memory (ROM), electronically erasable programmable read-only memory (EEPROM), and random access memory (RAM) circuits - grey matter - to function properly. Proper "feeding" (technical update training) and "watering" of our most important diagnostic tool will help keep it fully functional for years to come.
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