Troubleshooter: Common sense guide to engine diagnostics

Because of the sophistication of heavy-duty truck engines, when a customer comes into your shop complaining of an engine problem the tendency is to assume the problem is complex.

While sometimes that’s true, in addition to relying on electronic diagnostic tools and other high-tech devices, technicians should not forget common sense troubleshooting basics.

Proper diagnostics start with the service writer. The first thing a service writer needs to do is interview the driver, according to Mike Powers, product development manger, global on-highway, Caterpillar, Inc.

“Find out who, what, where and when the trouble started,” he says. “Has the truck recently been in for service, and if so, for what? Did the problem start after the service work was completed? Is this a performance complaint (low fuel mileage, won’t pull) or is the check engine light on?”

Tom Diefenbaker, director of technical support, Daimler Trucks North America, LLC, says, “It is important to understand if it is a symptom-based complaint, in which case you need a lot of information, or is it a complaint that is the result of a warning light?”

Powers explains that performance-based complaints don’t usually turn on the engine warning light.

Steve Scott, director of product development and technical support, IPD, encourages technicians to take time to make sure they completely understand the complaint. “Go over the details of the complaint with the customer so you both agree about what you’re expected to remedy.”

Diefenbaker also believes that an in-depth interview with the operator is an essential first step. It is important to know exactly under what operating conditions the problem occurs. Was the vehicle pulling a grade? Descending a grade? Was it making a right turn? A left turn? Did the problem occur at idle, in steady state or at cold start up? Is the problem intermittent? Does it happen at a specific RPM or temperature?

“Ask about the engine’s repair history and review the maintenance records,” Scott suggests. “Finding out if you are dealing with a repetitive failure or identifying a pattern of increased losses of fluids or fuel consumption can put you on the fast track to resolving the problem.”

Diefenbaker also suggests extracting a log file so you have a permanent record of any fault codes that are on the vehicle.

“Bottom line, what the service writer needs to do is make sure he gets enough information to paint a picture so the technician can understand exactly what is going on.”

Andrew Plant, manager, support systems development, Detroit Diesel, suggests going for a test drive with the customer, especially if you are dealing with a symptom-based problem.

“He might have a vibration complaint under certain operating conditions or a noise complaint where he indicates that the turbocharger in this vehicle does not sound like the turbocharger on his old truck.” By going for a test drive with the customer, the service writer will have a more complete understanding of the problem and can provide the technician with more information that can help him resolve the issue quickly.

Once the service writer turns the vehicle over to the technician, the technician needs to make sure he understands the complaint.

“The completion and cost of the repair often is directly related to the quality of the initial steps taken in diagnosing the problem,” Scott says. “Gathering and recording accurate descriptions, complete lists of facts and conditions, will aid in logically and systematically narrowing the problem down to the most likely root cause.”

Diefenbaker encourages technicians to perform their own quick inspection of the vehicle before beginning to troubleshoot. “A cursory inspection, both physical and electronic, can help ensure that there isn’t something obvious that has been missed,” he says.

While complete information is the place to start the troubleshooting process, the engine experts we spoke with agreed that using a systematic approach was also key to completing the repair properly and in a timely manner.

“Labor and parts are expensive and getting the repair done correctly the first time is the most cost effective,” Scott says. “Using a systematic approach to identify the root cause of the problem will save everyone involved time and money. Not checking the basics before you start the engine can make the problem worse, resulting in further damage or failure. This could leave you and the owner at odds as to who’s at fault.”

Powers believes that today’s engines must be approached in a holistic way during troubleshooting since many systems interact. “The symptoms may point to one system of the engine, but in reality the problem may be caused by another system. With that in mind, any troubleshooting must be performed in a systematic method using the engine manufacturer’s troubleshooting procedures.”

Plant says a systematic approach means technicians will not miss any steps. “It is easy to skip the first step of checking to see if you have a blown fuse when the headlights are not working. And obviously that is not an engine issue, but we get the same thing.”

Without a systematic approach, technicians may fail to check the simple things. “Did they simply unplug a sensor during a prior repair so that the check engine light is on and all they have to do is plug that sensor back in?”

Diefenbaker adds, “Not using a systematic approach generally means making assumptions, and with the complexity that we see in these engines, making assumptions is dangerous. Not following a systematic approach could result in misdiagnosis, extended repair time or increased downtime.”


Using Your Senses
In the past, a technician often could rely on his senses to help him troubleshoot an engine complaint. Today, however, that has become much more difficult, although they still can provide clues.

Steve Scott, director of product development and technical support, IPD, believes that the senses can help direct the technician. “Things like burnt odors, contaminated fluids, scored surfaces, high temperature and the color of smoke can help identify things that have gone wrong.”

Mike Powers, product development manger, global on-highway, Caterpillar, Inc. adds, “Senses – sight, smell and hearing – all help in troubleshooting, but the most important thing is that those inputs correspond to the symptoms the driver complained about and to some extent what the engine diagnostic codes are telling the technician.”

Andrew Plant, manager, support systems development, Detroit Diesel, says using your senses as a diagnostic tool is more difficult with newer engines. “Electronics can compensate for a lot of issues in an engine today, so a technician who relies on sight, smell and hearing is not getting as valuable information as he got in the past.”

He explained that the most important sense a technician can use is the sense of good judgment and avoid being intimidated by the new engines. “They are new and they are complex, but there are diagnostics and tools available to help.”


Tools of the Trade
While a systematic approach to engine problem diagnosis is critical, having the proper tools is also important. Here is a list of things the technician should have on hand before beginning the diagnostic process:

  • Up-to-date troubleshooting manuals and wiring diagrams
  • Volt/ohm meter
  • A sight glass
  • The manufacturer’s electronic service tool
  • Non-contact temperature gun
  • Multimeter
  • Test lights

Mistakes to avoid
Be aware of common mistakes that technicians make when diagnosing engine problems so you can avoid them.

According to Mike Powers, product development manger, global on-highway, Caterpillar, Inc., perhaps the biggest error technicians make is assuming they know what the problem is without asking all the appropriate questions. “They often will assume the problem is electronic or that it is something big instead of a simple thing like a loose nut or bolt or a bad connection.”

His advice: “Never forget that this is a mechanical engine and needs three basic things – fuel, air and compression.”

Steve Scott, director of product development and technical support, IPD, agrees that jumping to conclusions is a big mistake. “Take the time to eliminate the basics and be able to support your diagnosis before starting the repair. Too often blame is cast on the failed part(s) and a systematic, logical approach is not taken to find the real problem. When that happens, evidence of the root cause often is destroyed and the real problem is not adequately corrected, leaving the engine or part doomed to fail again.”

Replacing parts rather than diagnosing a problem is the thing Andrew Plant, manager, support systems development, Detroit Diesel, cited as the biggest error technicians make. He encourages technicians to rely on their experience and knowledge of diagnosing a problem rather than simply replacing the failed part.

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