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Bus and Motor Coach Library

Wiring Harnesses – a Source of False Signals & Warnings

Author – Brian Niddery (2005)

Most large buses, being rear-engined, carry their own unique and sometimes interesting characteristics.  Firstly you have engines that are often mounted front-to-back, and these engines must share their engine bay with rear drive axles, and in the case of motorcoaches an additional tag axle.

            There is usually more heat build-up in rear-engined models than front-engined vehicles.  All systems and components housed in a rear engine bay are also subject to an extraordinary amount of wheel splash.  This is particularly true of motorcoaches, where the operating regimen is usually at highway speeds, and where both the drive and tag wheels are churning up debris, water, grime, salt and other corrosive matter.  At high speeds you also get considerable "blow-back" - wind vortices at the rear of the unit that re-enter the engine bay carrying with it still more oil, road grime, and exhaust gases.  This operating environment represents cruel and unusual punishment to any and all components that happen to share that engine bay. This alien environment contrasts with that of trucks, which at high speeds enjoy a relatively clean and unobstructed cool air flow, and much of the spray thrown up by the single front steer axle tends to travel aft of the engine block.  And of course there is little or no blow-back of rear air.             Add to this the fact that today's highly sophisticated vehicles employ state-of-the-art multiplexing, on-board computers, and sensors that generate vast amounts of data that continually monitor and control vehicle functions and performance. These digitized systems require the need for additional wiring to enable complex data and information to stream between the various components.  Data streams require extremely low voltage signals, and consequently can be very susceptible to even minor interferences.

            So now what we have are very complex, sensitive electronic systems that in the case of motorcoaches are wired into a harsh operating environment known as the engine bay.  Overlay this with the fact that in rear-engined vehicles there is a need to combine these massive amounts of wiring into inordinately long harnesses running almost the entire length of the bus.  Wiring harnesses run from the driver's console to fuse box, through the length of the bus to the control module, and to the various components and systems.  For the most part buses tend to use truck parts, parts that have been developed and refined over many years to function well in a certain environment.  Change this environment however, and there is a very good chance that these parts may not function quite as well or quite as efficiently.             In recent years there seems to be an abnormally high incidence of wiring problems associated with motorcoaches.  At least so far - it hasn't shown in any pronounced way to affect transit buses or trucks.  This may possibly be due to the higher speeds and more severe wheel splash and blow-back encountered by motorcoaches, which is not present to nearly the same extent in city transit buses. It also doesn't appear to be related to any one manufacturer or any specific model.  However the incidence of wiring issues seems to be most prevalent in eastern Canada and the United States, and on motorcoaches built in the early nineties.

            Given the relatively long winter season in north eastern part of the continent, and the often more than generous use of road salt, it is thought by some industry people that this may have a critically adverse effect on wiring systems.  If this is the case, it may serve as an early warning sign for what may eventually, or perhaps is already occurring in some other regions of North America.              It bears mentioning that there seems to be less in the way of wiring problems associated with coaches built in the eighties.  Given this fact some believe a contributing problem may be sourced back to the wiring manufacturers.  It is plausible that at that time they began to use a less expensive material or different grade of loom or wire covering, or changed a manufacturing process.  We're receiving information that wire manufacturers are now taking some corrective action on new vehicles. 

            The problem is a gradual and abnormal wear of certain wiring systems that can lead to electrical shorts and crossed circuits.  A wiring system is subject to considerable forces.  There is corrosion from salt and spray, and a multitude of other chemical compounds.  Additionally there is vibration which causes adjoining wires to rub against each other, and even to the extent that the metal wire itself will sometimes rub inside its own coating.  Fine sand and grime blowing around the engine bay at high speeds tends to have a sand blasting effect that over time can act as an abrasive wearing down the exterior loom.  There are bending forces and metal fatigue if a wire is strung tightly or if there is a kink, or if there is too much flexing motion all of which can result in breakage over time.  Chafing and cutting can occur where wiring is clamped tightly to a bulkhead.  There are hundreds of wires subjected to varying degrees of wear and tear.             Here then is the difficulty.  Minor shorting out, and faults in the wiring can cause false signals to be transmitted to the control module, phantom signals, as they are sometimes known.  These are often intermittent, and sometimes not easily noticed.  However the on-board computer certainly reads these signals very clearly and instantly tries to take corrective action by attempting to readjust certain functions.  These false readjustments can often affect performance and reduce the efficiency of any number of components, including the engine, transmission, and even the air conditioning.  In extreme cases these can possibly result in unnecessary wear on these components, or albeit rare, even a complete shutdown of a component.  If the wiring faults increase in frequency or duration, it may produce symptoms that may falsely emulate what would seem to be an engine or transmission problem.  False adjustments can affect engine timing, fuel mixture, a rough idle, loss of power, or occasionally result in random transmission downshifting occurrences, something that can look and feel like a major event. 

            Although a careful diagnosis will usually isolate and identify the problem, patience and care is sometimes a virtue that is not always present in a service garage.  Often the symptoms can appear to imitate an engine or transmission related problem, and therefore many valuable hours can be wasted looking for a problem where a problem doesn't exist.  There have been instances where an engine or transmission has been opened up at major cost only to find that a minor wire or connector somewhere along the wiring harness is the culprit.  Oftentimes the fault is intermittent, and in the comfortable surroundings of a heated and dry garage the problem doesn't always show itself.  But as we all know it will easily resurface somewhere out on a cold and wet highway, usually with a load of passengers. 

What can be done to solve this problem?

            Teo Teodoro is the bus and motorcoach account manager with Harper Detroit Diesel/Allison in Toronto.  He points out, "With today's state-of-the-art technology, the symptoms of a problem can often not be what they appear.  Our recommendation is that no matter how obvious a problem may seem it is vitally important to follow proper procedures, particularly in taking that extra time to carefully check diagnostic readouts.  Otherwise a great deal of shop time and garage resources can be needlessly wasted."   A spokesperson for Coach Canada, out of Peterborough, Ontario, has indicated that their service logs have shown a marked and perhaps a predictable pattern of wiring problems over a period of years.  As a result they have instituted a PM policy to replace certain wiring components at scheduled service intervals.

To Summarize:

1.   Your first line of defense is ensuring that your service people take greater care in checking diagnostics.  Even if they are dead sure that they know what the problem is, they need to spend that extra time to carefully check the diagnostics for a wiring problem. 

2.    Whenever a vehicle is scheduled in for routine maintenance, a policy should be in place to more closely inspect those areas where wiring problems can most frequently occur.  Many of these problems occur in the engine bay.  It is a "best practice" PM policy to maintain a pre-determined schedule to carry out a thorough diagnostics check at recommended intervals. 

3.    If your operation is located in a "high incidence" region such as that found in the north eastern part of the continent, it may be beneficial to develop a PM replacement policy of "at most risk" wiring components at specified intervals, for example whenever an engine or transmission undergoes a major service or overhaul.

            Today's commercial vehicles are sophisticated and complex, and that complexity puts a greater responsibility and more weight on preventive maintenance policies and practices.  As a coach operator it might well be an idea to keep this on your radar screen in case similar problems should begin appearing with your coach units in future.