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Car Trouble's cause not always clear

As modern vehicles get more advanced with modern luxuries and safety equipment, so do the challenges of diagnostic and repair.

Most folks think you hook up the scanner and it tells you what is wrong. This is so incorrect, it doesn’t. It will sometimes give you an idea of where to start looking but very often digging in deep to consider all the remote possibilities is the only way we get to the answer.

Newer vehicles have an incredible amount of information that is gathered from sensors placed around the chassis, brakes, engine, interior, transmission, even sun sensor on the dash to determine the best setting on you climate control.

All of this information is feed into control modules or electronic control units (ECU) placed around the vehicle.

Newer vehicles have many sensors and ECUs for safety equipment to help avoid crashes or surviving a crash a higher probability. These include anti-lock brakes (ABS), traction control, stability control and the air bags, air curtains system. Each one of these systems makes vehicles much safer to drive and survive.

Most times when these systems are in need of repair, a sensor, wiring, or control unit is defective. These systems keep very close tabs on when the vehicle is doing, many times a second, and making logical decisions with all the sensor inputs.

Recently a customer brought a 2010 Volkswagen into our shop with an unusual symptom. He was driving on the freeway at about 70 mph when he started feeling his car “hiccup” every few miles. Out of his corner of his eye, he saw a light flash on this dashboard. As he continued to drive, the hiccupping got worse and the stability control light would flash more often, then came on the ABS light and air bag light. The bucking continued until he manually turned off the stability control. This happened the next day as he drove back to Eugene and turning off the stability control stopped this bucking.

When he brought the vehicle to us, we had many questions for him, one being what was the weather like when the problem occurred? Well, it was summer, so there was no rain or snow, and while driving on a dry freeway there should be no reason for the stability control system be engaged. This system is designed to keep you from sliding in wet conditions or going into a corner with too much speed.

To start troubleshooting we hooked up our enhanced VW and Audi vehicle scanner and found stored fault codes for the ABS, air bag system and stability control. We cleared these codes, then gave the vehicle a long test drive. The codes didn’t return and the vehicle was returned to the customer with the explanation that we couldn't duplicate the problem and therefore didn't know why the codes were set.

The customer understood, took his vehicle but the problem returned after driving for about 60 miles and only at speeds around 70 mph. In further discussing the problem we asked if there was kind of recent “event” that preceded it. It turns out he had gotten new tires and rims installed along with having the tires “siped” - a process in which small slots are cut into the surface of the tires to increase traction. We went for a test drive down I-5 as he was explaining the problem, hoping it would happen again. We could not duplicate this problem. But as we were driving I believe I knew what was wrong. We need to check the rolling diameter of the tires that is make sure all the tires were identical The modern vehicle counts very precisely every rotation of tires and wheel and feeds this information into control units.  If there is a differential, the vehicles will react by modifying inputs and setting to make sure your vehicle is in control.

Back at the shop, we marked each tire at the bottom with paint, rolled the vehicle forward and found the front and rear tires had a different rolling diameter. I theorized that this was the cause of the problem but the proof is always in the pudding.

So the customer went back to the tires store and told them of my theory.  They said they had never heard of siping having that effect, but they did replace his tires without siping this set.

Two weeks later and hundreds of miles later, this symptom has not returned so I feel somewhat confident that this was indeed the problem.

What I don’t know is if the tires were different or the siping caused the rolling diameter to be different.

Modern vehicles are very sensitive to all inputs and this is a case in point of that. The vehicle was doing its job, analyzing the information it got. The information was just on the cusp of showing what it “thought” was a problem and reacting.

The moral of this story is, modifications should be done carefully and due noted in case problems do happen later. Without the customer providing the information on tires, we would have spent much more time with this VW. When you have problems with your vehicles, please remember the events and take the time to pass this information on to your repair shop. It’s not as easy as just hooking up the machine to get the answer to what is wrong with your vehicle. In this case, nothing was wrong with this VW other than being sensitive.