Commercial Transport Mechanic Insider Advice

imported_John Whelan - April 23, 2014

Commercial Transport Mechanic Notes

This post is for you young guys just getting started in the commercial transport trade who have not had a lot of “hands on” training. I've had a lot of training and experience in this field and mixed into that is a lot of failures as well. You will have to deal with me. If it's your fault you need to wear it and don't blame someone else ...take responsibility.

There is a favourite saying I have that has probably been said thousands of times and that saying is “always check the simple things first”. It sounds pretty basic but that's “basic training” if you're just getting into the trade. The thinking process is very important for each job you take on. If you jump into a task feet first you're going to end up with a frustrating experience on your hands.

It never hurts to ask questions when you are stuck with a problem. I still ask around and it really makes a difference. You will never know everything... I guarantee you that. Everyday is a journey maybe that's why they call a certified mechanic a “journeyman”? It makes sense to me and as in life it's a journey of living and learning every step of the way.

Always Check The Source

Let's apply the title above to a few common repairs that come along in this industry. I'm just going to touch lightly on these problems because of the varying symptoms and solutions but you'll get the picture. Go to the FORUM at and ask questions there. It's a great place to get answers.


What is the source...what makes diesel engines run? Fuel and air. Check the fuel tank, fuel lines and air filter...don't laugh it's happened :) If you suspect a fuel line run the fuel suction line into a clean bucket of fuel and run the engine to see what happens. You have now bypassed the fuel tank and associated fuel lines. If it makes a difference you have a fuel supply problem.


Hook up to the laptop and check it out. If you have a diesel engine 2007 and newer you NEED diagnostic software. You may have a battery supply problem to a module specifically between the engine ECM and Chassis module. This is still related to the “engine will not run” problem.

In a school bus application our Mercedes models communicate using signals and modules and if there is an open or resistance problem the on-board computer won't allow the engine to start if all the parameters are not met. This has happened before to us and it was a wiring problem along the chassis frame and another time the CPC (common powertrain controller) failed.


Is there an exhaust brake installed on the vehicle? There has been several instances where I have found a stuck exhaust brake. It has been stuck closed because of a seized air cylinder. This condition causes major back pressure and NO POWER. You can test back pressure by manually using a water manometer and for a Cummins diesel engine the last specification I heard on those engines for allowable back pressure was 2 psi.


What's the source? Batteries along with connections and cables. Load test the batteries after they have been recharged and checked out the charging system as well (if the batteries pass the load test). Perform a voltage drop test which will check the cables and connections. Using a multimeter you can check how much voltage you are losing at each cable length and connection. Set the multimeter to VOLTS and go from one end of the battery cable to the other and you should not lose more that .2 Volts. There are many tests you can use.

Go to Google and search for “voltage drop testing” or “how to voltage drop”. I've done so already and the information that comes up using those searches is right on. This was just a tidbit of what you could experience if you're in the commercial transport trade . Thank you for reading the post and please make a comment below.