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The Technician’s Log: Complex Diagnostics Protocols Decoded, Part II

Welcome to Part II of our latest in The Technician’s Log, our long-running series examining issues that impact commercial motor vehicle technicians. In Part I of our series, we looked at the top diagnostic issues of our day, how to address diagnostics complexity, and troubleshooting electric components. Now, to start Part II of our series, we will have a look at transmission and transmission failures.

Why do we focus on manual transmissions? After all, advanced transmission technologies are everywhere nowadays. Well, ATM models are more prevalent, but many truck drivers all over the world still operate their CMVs through a manual transmission. And although manual transmission noises are fairly straightforward to isolate, sounds are used to determine what causes transmission failure.

Gear and bearing sounds may vary depending on gear selection and vehicle speed. Yet, using sound, technicians can have a look at transmission power flow charts to determine which gears and bearings are in use when the sound occurs. Yet, the real work begins when a transmission is removed. A cardinal rule should be to never just tear the transmission apart and begin putting in parts orders.

Create a Transmission Troubleshooting Checklist

Any technician worth their salt will tell you that when you are troubleshooting and repairing a big-ticket item like a transmission, you’ve got to first put a checklist together. The value of checklists in ensuring you don’t miss anything simply cannot be overstated.

Try this basic transmission tear-down checklist:

  • Check transmission fluid level and condition.
  • Ensure the fluid is the proper type for the transmission and application.
  • Inspect bell housing mounting surfaces, mounting bolt tightness, and mounting wear and tear.
  • Examine flat gasket, input and output surfaces and shift levers.
  • When draining the transmission fluid, take note of any material that collects in the drain.
  • Don’t hesitate to refer to your manual or other service information guide.
  • Check for product updates before beginning disassembly.

Incorrect fluid types are often responsible for premature gear and bearing failures. And no one wants a premature failure. Shifting difficulties are also sometimes attributed to fluid and fluid-related issues.

You also want to pay close attention to measurements to help identify shift-related failures. It is important to make note of excessive or insufficient play points. Finding end play prior to disassembly provides technicians with the opportunity to order end play spacer washers, as needed.

These few simple steps can go a long way in helping technicians more thoroughly and efficiently identify transmission failure. They can also use the checklist to identify impending transmission failure. In the end, technicians must take a comprehensive approach to transmission troubleshooting to prevent failure.

Transmission Leak Detection Can Be Tricky

Transmissions are some of the most vexing parts of any vehicle. Just ask ASE-certified technicians and diesel technicians across the country. They will tell you: Transmissions are not their favorite. Still, it is easy to blame a transmission for something that is not its fault. Could transmissions be getting a bad rap?

Transmissions are, as a rule, placed behind the engine in the powertrain order of things. This means they are also usually underneath the cab. Why is this important? Because by the nature of their location, transmissions catch all the leaks from everything in front of and on top of them. This often leads to novice technicians’ assumptions that the transmission is the source of the leak.

To make matters worse, some leaks may be difficult to pinpoint because it might be harder to pinpoint the fluid you examine. Take engine coolant as one example. In diesel big rigs, engine coolant is generally orange. But there are plenty of examples of technicians mistaking a dark orange instead for the red of transmission fluid, especially if the orange fluid has collected dirt and grime as it has leaked to the bottom of the vehicle.

Still, it takes a keen technical eye to notice that rusty orange fluid is neither transmission, nor coolant fluid. You could also be looking at rusty freeze plugs behind the flywheel oozing fluid. You must be very careful to make sure you properly diagnose your fluid leaks.

Follow the Noise for Proper Troubleshooting

While we are talking about using sounds to diagnose problems, it would also be a good time to talk about noises and vibrations. CMV noises and vibrations pose unique challenges. Unknown noises and vibrations can often lead technicians in the wrong direction if they are not careful.

Even relying on truck drivers to help with the troubleshooting is not fool proof. Truck drivers are very tuned in to the sounds their trucks make. Technicians, on the other hand deal with different vehicles all throughout the day. Many of these vehicles may have their own configurations and repair patterns. Fortunately, there are several factors which can help technicians better isolate and troubleshoot anomalous vehicle vibrations and sounds.

Here is a sound and vibration troubleshooting checklist:

  • Check with the truck driver for more information on the sound/vibration.
  • Get familiar with the “feel” of the truck, it’s make, and model.
  • Have a good working knowledge of vibration diagnostic equipment.
  • Identify the vibration or sound to uncover its source.

The key to troubleshooting is in identifying the root cause of the problem. Technicians cannot succeed by fixing a problem on a truck without also fixing the root cause of that problem.

Addressing the “Why” Behind a Failure on Your CMV

It is only natural for technicians who work on similar vehicles all the time to notice patterns in failure points. And this is especially true for diagnostic trouble codes. For each new truck that comes into the bay, technicians need to closely follow diagnostic steps. At least this is what is supposed to happen in a perfect world. Is it what really happens, however?

Going after known faults can save time, but what if the problem is actually related to something different or unexpected? It can be easy for shop technicians to get tunnel vision when it comes to finding common failure. Good diagnostic techniques must take a holistic look at how a system works to properly pinpoint a failure.

Why did said component fail? The answer is usually fairly simple: There are numerous things that could be happening in and around a system that only a technician will be able to see. When it comes to effective diagnosis, the most important element is still the human element. You are the critical factor in making a proper diagnosis and creating a plan of action.

Never Assume You Know Everything

When it comes to proper engine and powertrain diagnostics and repair, you want to make sure you do not forget to ask questions. When a truck rolls into your bay with a trouble code light on, do not be afraid to ask the right questions. Talk to the truck driver and utilize all the diagnostic tools available to you before jumping to an erroneous conclusion.

Consider the following as vital questions to get you started down the right road of diagnosis:

  • What operating conditions was the vehicle in prior to the trouble code appearing?
  • Were there any other strange noises, lights, or vibrations noted prior to the trouble code?
  • Can you duplicate the problem?
  • Have you checked the vehicle’s service history?

It does not matter what system you work on, virtually every single one has initial diagnostic inspection steps that are unique to that system.  The simple step of asking the right questions, checking vehicle history, and conducting proper research before you begin a repair will ensure the job gets done right the first time and with minimal downtime.

Adopt a Staged Approach

Never tackle something all in one fell swoop. It is important to develop your plan in stages. Knowing that inspections can take a lot of time, having a staged approach allows you to tackle your job in manageable pieces.

We have provided you with a list of checklists in this two-part series, but do not hesitate to come up with your own checklists and benchmarks. Get creative and survey your truck drivers to see what is most important to them when it comes to diagnosis and repair of their vehicles.

Thanks for joining us in this series looking at how you can tackle complex diagnostics on modern big rigs. We know it isn’t easy, but like with everything else, change will become normal in no time. Stay tuned for our next Technician’s Log blogs coming up!

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