The 411 on 1939

If the numbers 1939, 1587, 1850 and 1700 make you think you're about to take a history test, you're partially right. Over the past several years, the Society of Automotive Engineers (SAE) has been setting data communication protocols, or basically telling everyone what language to speak. And SAE standards like 1939 and 1700 are examples of that.

It has been a good thing, because it has kept us all from learning as many different languages as there are manufacturers. Just think about how difficult it would be if each engine manufacturer used a different communication protocol, and what that would mean when trying to communicate with transmission computers and Anti-Lock Braking systems.

We recently provided you information on retrieving active codes from the new speedometer head (Maintenance Matters - April 2008) and an overview of different messages that can be gathered for your diagnostic routine. But just exactly what are some of these abbreviations, and how can you use them?

Let's start with a brief history of communication standards.

As electronics began to flourish in the transportation industry, engineers soon realized that technicians would become overwhelmed with this new technology and its many languages. As the predominant presence in the transportation technology, the SAE set about determining specific protocols, or rules, that everyone should follow. This also included the way data was transferred and retrieved as well as designations for data being read through a diagnostic scan tool, and even the abbreviations that would be used.

As time progressed, communications got faster and more data was being sent along the lines. And as the programs became more complex, the common fault codes began to become less and less helpful. So SAE standards progressed, and protocols evolved to include what would be classified as "identifiers," or smaller definitions that describe the many different factors that could cause a single fault to occur. Under the old system, a simple TPS code could relate to a voltage problem on the signal wire--but it was difficult to determine whether it was high- or low-voltage problem, and whether it was intermittent or a constant.

Today, the technician now has access to many identifiers to aid his diagnostic routine. Since many systems may share a common sensor and data, knowing where the Diagnostic Trouble Code (DTC) is coming from is just as important as what has caused it. This can all be determined by knowing the different messages and their abbreviation.

Some of these identifiers are:

MID - Message Identification

Identifies the Module or ECU that transmits the fault code

PID- Parameter Identifier

Identifies the data from the faulty electrical component

SID – Subsystem or Status Identification

Identifies the status or condition of the electrical component

FMI – Failure Mode Identifier

Identifies the TYPE of failure in the signal or sub-system (listed below)

As an example, received diagnostic information may look like:


128  –    084   -   002

128 = ENGINE (source of the diagnostic code)
084 = VSS Vehicle Speed Sensor (the faulty data pertains to)
002 = Data Erratic, Intermittent or Incorrect (signal quality)

So, in this case, we know that the engine is seeing an intermittent or faulty signal on the Vehicle Speed Sensor — useful information for making a repair. If all we had was a flash code, this is all we might be able to read:

DETROIT   54    
  CAT   84  
    CUMMINS   115

Keeping up with these advances in technology isn't always easy or cheap. The handheld scanner has largely given way to the laptop computer. And it takes a technician that knows how to use it. The fact of the matter is that as vehicles get more advanced, we have to keep stride with the knowledge base.

Fortunately, your MCI Fleet Support Manager can assist you with the proper software and diagnostic aids for your coach.

List of FMI messages:




Data valid but above normal range


Data valid but below normal range


Data erratic, intermittent or incorrect


Voltage above normal or shorted to high source


Voltage below normal or shorted to low source


Current above normal or open circuit


Current below normal or grounded circuit


Mechanical system not responding or out of adjustment


Abnormal frequency or pulse width or period


Abnormal update rate


Abnormal rate of change


Failure code not identifiable


Bad intelligent device identifiable


Out of Calibration


Special Instructions

15 – 30




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