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Electrical systems can confuse techs

Understanding the basics of electricity is key for electrical technicians

By Robert Buchwalter
Service Training Manager

A friend of mine, a WWII veteran, was trained as a radio operator on USAAF airplanes. That he wound up as a tailgunner is another story. He recently gave my wife and me his training notes and books from his radio school at Scott Field in Illinois. Reviewing that archival material, which covers everything from simple lighting circuits to super-heterodyne radios, I see a lot of the same topics Robert Hitt and I discuss in our Prevost training classes: the basics!




On those old yellowed pages is information on series and parallel circuits, resistors, connectors, voltage, amperage. For a geek like me, it is interesting reading. But how do 70 year old texts and notes relate to today’s motorcoaches that are controlled by multiplex electrical systems?

The answer can be found in the basics. Despite the sophisticated control systems found on today’s coaches, we still rely on wires, connectors, and a few relays to achieve an output. Our world is also comprised of vehicles with conventional electrical systems, controlled not by modules but by relays and diodes. Regardless of the control system, we need to ensure our shop employees understand the basics. If they do not have this foundation, then we cannot expect them to understand and master the seemingly more complex electrical systems and networks of today’s environment.

The biggest electrical issue is safety. Today’s 24 volt systems can, if improperly handled, deliver a significant electrical shock and most (emphasis on most) of us also understand the inherent danger in batteries. A good shop electrical training program must ensure everyone understands these hazards, wears the proper safety equipment and works safely. From that safe beginning, we can proceed.

Do your techs understand Ohm’s Law? The basic relation between voltage, resistance and current must be clear to everyone who deals with electrical systems. There are different methods built into multiplex systems for monitoring current flow and these can display codes related to electrical problems. But the MIDs, PIDs, SIDs, PPIDs, PSIDs, and FMIs mean little to someone who is uncertain regarding electrical systems. Our people who interpret these codes need to know what the system is telling them and how it relates to their diagnosis. A laptop cannot think, nor develop a diagnostic path, for itself. Our techs need to relate the codes to the coach.

A test question we use in training is, how much electrical energy does a Prevost consume in 2500 miles of driving? The answer can be determined to a high level of precision: none. No electrical system consumes energy; they convert the current flow into useful work. For coaches, this useful work is primarily torque (electric motors); light; heat (heated windshields or immersion heaters); and electro magnetism (relay coils, compressor clutches). Only when current is converted into one of these forms of useful work should we drop voltage. So, if you have 27.2 volts on one end of a wire and only 24.5 on the other end, there is something going on and we need to investigate.

Do your electrical technicians understand how a volt meter can mislead them? About a year ago, we had a problem with a Prevost in service with New York City Transit. It would not crank. Hooking up our laptop and using its software, we immediately saw we were missing a critical voltage supply and without it, cranking could not occur. Just as quickly we reviewed the schematic to determine where the problem was. Our voltmeters told us we had the required voltage in all the right places, but you can run a voltmeter on just a few strands of copper wire. A circuit load tester, however, showed us that while the voltage was present, when we loaded the circuit and needed current to flow, the load was more than the wiring could carry. A few more minutes of swimming upstream led us to the actual fault, a corroded wire, due to a small slice in the insulation.

The voltage was there but it could not do the work. Just a few minutes of diagnosis and the fault was found and repaired. (Also, the fact that the bus was “on the roof” at a parking garage and this event took place in February provided added incentive to find the problem quickly!)

Prevost, MCI, and Van Hool all have different electrical systems. But there is a common theme found in all: there are fewer and fewer electrical terminals. In older coaches, the electrical systems usually had ring terminal boards in the front and rear electrical boxes. On today’s coaches, the wiring is more “point to point” with fewer places that you can probe the circuit to check the voltage. Unfortunately, this results in some techs sharpening the points on their test lights in order to pierce a wire to check for voltage. This is a practice that only leads to more problems.

Despite what people think, water can penetrate the smallest hole and this will lead to corrosion of the copper wiring strands. If the actual cause is a poor connection due to corrosion, the movement of the harness sometimes improves the connection for a few days or weeks, masking the actual problem but it will eventually return. If we cannot pierce the wiring, the alternative is often building your own breakout harness, using connectors mated to the original harness, in order to determine voltage levels while moving the harness. It takes some time and effort, but it is the best practice to employ.

Electrical problems are usually handled by a senior technician and less experienced individuals are left out of the diagnostic procedure, but I think just about anyone can understand electrical systems. The challenge for us is to invest the time and instruction to try to help our techs. Start with the small problems; develop training methods and examples that they can relate to, in order to help them along the way.

Posted by on Oct 1 2014. Filed under Features. You can follow any responses to this entry through the RSS 2.0. You can leave a response or trackback to this entry

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