Emergency vehicle technology has changed rapidly over the past decade, which has required fire departments to upgrade their skills to maintain their fleets. One of the biggest and most noticeable changes in vehicle electrical systems is the availability of networked data communication, called multiplexing.

Vehicle electrical systems have always been complex, overloaded and arguably the area most susceptible to problems on an apparatus. There are three factors contributing to this weakness: system complexity, lack of documentation and the proliferation of electro-mechanical devices. Multiplexed electrical systems can enhance a technician's ability to effectively deal with these factors.

Network know-how

Multiplexing can serve to reduce the physical complexity of the electrical system while still offering more complex operation of individual outputs. Safety and operational interlocking can become a simple matter of programming the system to “check off” the conditions that are desired before a load can become active, rather than adding relays and wiring. Multiple switches and remote operations do not require additional logic wiring and circuitry.

Changes to the function of the electrical system or additions of outputs may require only simple changes to the software database as opposed to extensive rewiring. Most multiplex systems offer solid-state output drivers that reduce or eliminate the need for older electro-mechanical relays, circuit breakers and fuses, which often were a major source of frustration because they were fragile and unreliable.

The intelligence of the multiplexed system includes well — thought-out programming that controls the inputs and outputs of the electrical system. That programming also can be used as an “as-built” wiring diagram. By using the proper tools, a technician can easily access this information for diagnostic purposes.

Simple systems checks

Multiplexing typically can provide diagnostic information through a linked computer, a personal data assistant or an information display. The use of these tools can make diagnostics simple and direct. The technician will need to have knowledge of the system layout and functions, plus the tools that are available to simplify the job. For example, you need the proper tools and documentation to successfully troubleshoot electronic engines. The same is true when working on multiplexed electrical systems.

The multiplexed electrical system consists of a number of modules linked together by a twisted pair of wires called a common data bus. These modules typically perform input, output, control and display functions. Special hybrid modules might perform several of these functions. All multiplex modules require at least power, ground and communications wiring to operate in a system.

Multiplex system inputs typically will be conventional toggle, rocker or push-button switches. An input channel reads the switch position as being either on or off, and that information is processed and sent along the data bus to any other component in the system. Using this data bus allows a substantial reduction in the amount of standard copper wire in the truck; the switch position is sent as data and not as another wire in the harness.

Because the switch condition is just another bit of data to the system, it can be acted on without any additional wiring. The parking brake circuit is a good example. All that is required to use the parking brake information anywhere else in the system is one switch and one wire to one input port. All scene-related loads can be tied to the parking brake data in the system without adding extra wires or relays.

Outputs in the system can be electro-mechanical, solid-state or a combination of both. Electro-mechanical relays have the advantages of handling more current and being familiar to most technicians, but they require a circuit breaker or fuse for circuit protection and are relatively slow. The advantages of solid-state outputs are their speed and flexibility. They can be driven using pulse-width modulation, thereby reducing their draw. Their outputs can be flashed at high rates of speed without worrying about arcing the contacts.

Most solid-state devices provide a current feedback circuit so that, within limitations, they can read how much current a particular circuit is drawing. That gives them the ability to self-limit their current output, negating the need for external protection devices. This function also can be used to test for non-functioning loads or monitor bulb outages in some cases.

Most multiplex systems include a display of some sort, either as an option or part of the basic system. The display can relate vital system information, faults or even act as a PDA, displaying address and telephone information. Some displays are used as the primary interface to the system for troubleshooting and displaying system program and input/output location.

A modem can be part of the system so that a truck can be dialed up to perform diagnostics from a remote computer. System fault information usually can be ascertained this way, as well as module or node communication status. Advanced diagnostics will probably require the vehicle maintenance technician and the electrical system expert doing the remote diagnostics. The remote site can force outputs and inputs on and off while the individual with the vehicle can verify circuit operation and perform voltage tests if necessary.

Getting started

You should have some basic information about the electrical system on hand before starting any troubleshooting, whether it's a conventional or multiplex system. This should start with a strong knowledge of the basic layout of the system, the number of modules, and their location and function. The next consideration would be the location of inputs and outputs and their relationship to the module. This could come from a wiring diagram, schematic or the system itself if available on the display module. Additionally, a description of each output's operation and its function is vital to the troubleshooting exercise.

Most troubleshooting begins with a user complaint about a load not working as expected. If the complaint is about a single output, diagnosis begins in a conventional manner: Turn on the problem load and check the wiring, the output device and the ground. If the root of the problem is not found with this process, then the next step is to check the multiplex system. Most multiplex systems are designed to perform a check of all installed modules and indicate whether they are on-line and communicating.

If all modules are communicating, the system will be operating and can probably assist the technician in further troubleshooting. If a module shows up as off-line, then a check of that module's power, ground and communications wiring is in order. This should be checked at the module connector first. Some modules have visual indicators that can simplify this check. If all appropriate wiring is correct, the problem would appear to be internal to the module itself. Further checking of the communications bus requires the use of a logic analyzer and may be beyond the capability of most original equipment manufacturers or their dealers.

If the suspect module is on-line and functioning, many systems have the ability to turn outputs and inputs on in the software. If this can be accomplished, it's a simple matter to turn an output load on in the software to see if it actually functions. If it does, then the problem is most likely in the input side of the system and that can be checked in a similar manner. If it doesn't, the module output port should be checked conventionally with a voltmeter.

If the system indicates that the output is on and a manual check fails, then the problem would be in the output driver, and the module would need to be replaced. Checking the input side is a matter of forcing the input on and checking operation of the load. If it functions normally, then the problem has been isolated to the switch circuit controlling that output.

Some multiplex systems perform electrical load management functions as part of their control scheme. This adds complexity to the troubleshooting process because system voltage, parking brake status and the master warning switch state become part of the configuration of each output.

A load may not come on because the vehicle electrical state, or voltage, has deteriorated to a point below that which will allow the output to turn on. Many loads will operate only in either the scene mode or the response mode — when the vehicle is either parked or driving. If the parking brake status is being registered incorrectly or not at all, this will affect the operation of any load dependent on that status.

The same holds true for the status of the master warning switch. This normally controls the operation of the apparatus warning light system. Whether you're looking at a multiplexed vehicle or a conventional one, determining if there is a common thread to the reported malfunctions is important. Many problems can be diagnosed without even looking at the vehicle if a few questions are asked and honest answers received.

A very powerful function available on some systems is the capability of “capturing” an event. The system can be set up to look for a change in the status of a particular output or input and collect information just prior to and just after the change so that an underlying cause can be found. This might involve the use of a “black box” or be part of the system itself. This can make the difficult task of locating an intermittent problem possible in a simpler and faster manner and in some cases, this capability will allow the technician to solve the situation that could not be isolated without this capability.

While the multiplex system is a very powerful tool for troubleshooting, field diagnostics will be limited unless the technician has had the appropriate training. However, even without extensive training, the technician should be able to determine whether a module is functioning properly and replace the module if it's not.

As the emergency vehicle industry moves toward manufacturing more and more electronic trucks, the multiplex system will become the cornerstone of the electrical and electronic components. More advanced troubleshooting capabilities will be included in these systems. Additional training and education will be necessary to keep technicians up to speed on advancing technology.

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Dick Smith is a product support specialist for Class 1 Inc. and has been with the firm for more than seven years. He has an extensive background in electronics and is heavily involved in OEM training on the multiplexing system, as well as on most other components.

Russ Walker works in the sales department at Class 1 Inc. and teaches technical training on the multiplexing system at EVT courses all over the country. His background includes sales and support for recreational and utility motor vehicles of all types.