Getting to the logic of it all

Your new MCI coach is equipped with an electronically controlled electrical system. Call it "drive by wire" if you want, but it really isn't all that different than other electrical systems already in use on your coach. The throttle cable was replaced years ago by a sensor that simply told the engine computer what the driver wanted to do — drive by wire.

Multiplex electrical systems only take that to a new level by adding computer programming to the formula. Diagnostics now involves knowing what the computer is thinking.

Fortunately, two things are in our favor when it comes to diagnostics for the Multiplex systems:

  1. The system has built-in test lights
  2. The programming is logical

Test lights are easy enough to read — they are either on or off. Computers, too, are guided by on and off signals. There are just a lot more of them to think about at one time. Knowing how to read the logic program associated with your electrical system will take the guesswork out of your diagnostics and keep your fleet rolling.

This article uses the VANSCO system as example, as it is the most popular. If your coach is a D Series with the DINEX system, don't fret, because the language is nearly the same.

Making sense of it all

We are used to thinking that when we flip a switch on, we turn on a light or a motor. To a computer, "on" and "off" are just specific conditions. If we think of a circuit as being "active" and "inactive," we are starting to think like the program that operates our coach. Reading the program logic files supplied with the coach is just a series of noting active and inactive conditions. These conditions can be due to a variety of conditions such as a switch position, a sensor reading or vehicle speed.

The logic is written with symbols arranged like rungs on a ladder, and it is often called ladder logic. Completing all of the required conditions on a rung completes the logic and turns an output circuit active.

In other words

Every task on the coach has a specific title, just like every job in the shop has a title. The tasks are completed by modules, which all have names, just like the guys in the shop. Each individual name is referred to as an "address." And all of the modules talk to each other on the data bus, which is a dedicated circuit, much like the PA system in a shop. All commands and communication are broadcast across that communication link, and they all hear what is going on. Much like if Ed the shop foreman might tell Jim to carry out the trash over the PA. Everyone hears the command, but only address Jim completes the task.

To extend the metaphor, this is a busy shop with lots of tasks and lots of commands. Every switch on the coach is a vital part of the command structure. Jim cannot carry out the trash until Bob has opened the door. If the door is open, Bob will tell everyone that the door is open. This is an input signal to Ed. Ed now knows that it is okay to tell Jim he can carry out the trash.

All of the correct active inputs allow tasks, or outputs, to become active.

Learning to read the new language

What's in a name? First, we need to learn everyone's name, or address. The address is the on/off electrical code that computers read to determine a name. Just like everyone in the shop has a unique name, so does each module. If there are two Jims or two Bobs in the shop, things can get confused. We must also be careful that if there is an electrical failure in the code, we do not duplicate someone's address. So we have to use a unique numbering system.

This table indicates the module number and location, and its ADDRESS number for each module's specific location. You will see that there was a change in address numbers vs. module location with the 2008 model year on E/J Coaches. Keep this in mind as you read the logic.

  E/ J Series – VANSCO D Series - VANSCO
Prior to 2008 Location Address Location Address
  #1, Top, FJB #2 #1 Top FJB #1
  #2, Bot, FJB #3 #2 Mid. FJB #2
  #3 LH P Rack #5 #3 Bot FJB #3
      #4 RJB #4
      #5 RH PRack #6
Eff. 2008 #1, Top FJB #1 #1 Top FJB #1
  #2, Bot, FJB #2 #2 Mid. FJB #2
  #3 LH P Rack #3 #3 Bot FJB #3
  #4, RH RJB #4 #4 Top RJB #4
      #5 Bot RJB #5
      #6 RH PRack #6

Now that we know everyone's name (address) and where their work station is at, let's look at the symbols that are used in our new language.

Below is the input symbol, or information received by Ed (aka the program) so he can determine what command to produce. Without the slash through the two vertical lines, the input is determined to be active. With the slash, the input is inactive.

You also see labels of information above and below the symbol. For a 2007 E/J coach, 'I3-1' is broken down as:

  • 'I' = INPUT
  • '3' = Module #2
  • '-1' = Input #1
To satisfy this CONDITION as true, we should see that input LED #1 on module #2 is ON, or the condition is MASTER SWITCH ACTIVE

The status of an Input is shown by the LED with the number c orresponding to the input #. For example, Input #1 = LED 1. Also,

  • 'I' = INPUT
  • '2' = Module #1
  • '-14' = Input #14
To satisfy this CONDITION as true, we should see that input LED#14 on module #1 is OUT, or the condition is PARK BRAKE SWITCH INACTIVE

This is a rung of logic for cranking the engine on an E/J series coach. We will use this common task as we learn to read the logic programming. As we work across the logic from left to right, we see an active master switch, two inactive Starter Lockout inputs, and an active crank or start-switch input. And there are a couple of new symbols that we haven't seen yet.

Can you sort through this symbol? The "O" is for "output."

  • Output #15
  • Module #2, (address #3)
  • 24 volt delivered to the starter relay

The output LED #15 will be on solid if all logic is satisfied, and the starter solenoid should engage and crank the engine

Here's a little test: Where would you check to verify that the step well light switch is on? (get answer)

You also see another rung in this ladder for alternative inputs. This is drawn as a link of code around or underneath the main line of code. This link can be thought of as an "or" condition.

The required conditions could be the first line of inputs or the second line of conditions, or inputs.

Either line would satisfy the logic for this series of conditions.

In this specific case they appear to be the same inputs just listed in a different order. In some instances, this is due to the programming combinations available. Having similar program lines is sometimes needed to break the sequence of a line of logic once the task is active. In this example, the engine is running.

The other new symbol of the logic in our example looks like and is referred to as a flag. A Flag represents a complete chain of events in a single symbol. The master neutral flag simply tells the logic that the transmission is in neutral, but with the different engine and transmission combinations, it is simpler to just draw a single symbol in the ladder.

We use a flag in the logic rung so that we do not have to write out every symbol of a repetitive process. To see what conditions are necessary to "set a flag," we refer to the program logic for a specific module, in this case module #3 (address 5).

In this example, there are three different conditions that can satisfy a neutral signal. First is actually another flag, which once again is just a series of conditions already set in place. "F5-3" represents that there is no physical neutral switch on a late model ZF transmission. Instead, the neutral condition is a transmitted signal across the data line to the modules. In this case there would be no input light to check.

"I5-6" is the neutral switch for an Allison transmission (module 3 input 6)
"I2-23"- is the neutral switch on an early ZF transmission (module 1 input 23)

This allows us to write a single program that will serve in multiple applications.

Just a couple more and we're done.


Timers can be for how long a circuit stays on or a rate of flash, like in turn signals.

Timer "C5-1" turns the baggage lights out after 30 minutes.

Timer 'C2-1' flashes the turn signals, on for ½ second, off ½ second.

There are other symbols and programming beyond the scope of this article. But for most of your diagnostics, the basic ladder logic and new E-Plan schematics will be the best roadmap for diagnostics. To go any deeper will require the VANSCO computer software, available through your Fleet Support Manager. For more information, see our Service Bulletins 2922 and 2933.

Answer: (Module #2, input LED light #13)

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