Mechanically Speaking: Diagnosing a problem doesn’t always come easy

"I was recently reminded by a customer how strange certain people’s impression of our industry can become."

In an ideal world I would fix cars for free. Unfortunately I have to make a living doing it. I was recently reminded by a customer how strange certain people’s impression of our industry can become. Some of these impressions are the result of our own industries marketing methods.

I was called by a customer in a relative panic. The windshield wipers on her late model European SUV were acting up. “Sometimes they work, sometimes they don’t” was the complaint. The weather had been relatively recalcitrant and we all know what driving without wipers in the rain can be like.

When a situation like this comes up even when our days our fully booked we will make some time to attack such an issue. I said bring in the vehicle and I would take a look at it.

Late model vehicles, particularly certain brands have a lot of built in diagnostics. They are accessible if you have the necessary diagnostic tools. In this case I knew this vehicle would have quite a bit of diagnostic information and I had the equipment to access it.

When the vehicle arrived the wipers were working so a solution was not going to be totally straightforward. It would still be worth using the computer system diagnostics.

The first diagnostic step is an attempt to reproduce the problem. Wiggling connections, putting extra load on the wipers and asking a few questions about what happens and what doesn’t happen ensued. The problem did not make itself apparent.

So a wiper system is an electromechanical system. Ultimately twelve volts of electrical power must be applied to an electric motor that starts to turn and through a mechanical linkage then the wipers move back and forth across the windshield. Simple. The failure mode can be either electrical or mechanical.

The wiper system in this vehicle uses switches, relays, motors, computer modules, and wiring. Typical. I was hopeful the computer system diagnostics were going to help me out by narrowing down the possibilities.

A scan of the computer network (twenty or so modules) produced a clue to the problem. The junction box electronics module  that supplies the 12 volts to the wiper system was intermittently recognizing that when it turned on the wipers they were not moving like they should.

That little tidbit of information helped to eliminate a lot of failure modes. I now knew that every part of the system up to that power module was working fine when the wipers were not. The problem was therefore not in the switch, not in the network wiring between the switch and the instrument cluster. There was definitely no problem with the communication between the instrument cluster module and the junction box electronics module.

At this point it was time to check for common issues. A search of technical service bulletins came up with some possibilities but not a definitive answer. A software error and a binding mechanical linkage could cause the problem but it looked like further investigation was required.

My time was used up for the day and further investigation was required. The power supply to and from the wiper motor was in question. There are relays, wiring a motor and mechanical bits still to look at but some vehicle disassembly would be required. I had other work to do.

I presented my results to the customer as well as a bill for my time. I felt I had made significant progress even though the wipers were currently working. I knew where I did not have to look. I gave the option of waiting until the wipers stopped working altogether or booking an appointment for further investigation.

My customers statement at this point caught me off guard. “I have never paid for diagnostics before.” “I guess I should have never come here unless my wipers were not working.”

Trail’s Ron Nutini is a licensed automotive technician and graduate of mechanical engineering from UBC. E-mail:

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