2.5.6
CAN (SAE J1939) DTC (Diagnostic Trouble Codes) messages
It will be required to know the CAN source address (SA) of the controller (ECU), which has sent a DTC message, so that the
problem can be located.
The SAE J1939 specifies 13 different diagnostic modes (DM1 to DM13). But for CANcockpit only DM1 is relevant.
In DM1 active DTCs are broadcast on the CAN-Bus. Active means, that these faults are existing at the moment, compared to
faults, which existed sometime in the past.
2.5.6.1CAN SAE J1939 Elements
DTCs consist of 4 elements defined in SAE J1939:
1. Suspect Parameter Number (SPN) 19 bits
In the DTC, the SPN identifies the faulty element, component, sensor or parameter of a device (ECU).
2. Failure Mode Indicator (FMI) 5bits
The FMI defines the type of failure detected. It provides additional information about the nature of the fault. There are 32 pre-
defined FMIs.
Examples are:
"Out of calibration", "Data Valid but above normal operational range", "Mechanical System not responding or out of adjust-
ment"….
3. Occurrence Count (OC) 7bits
Shows the number of times a fault was active
4. SPN Conversion Method (CM) 1bit
The February 1996 version of SAE J1939-73 contained inadequate definitions to assure consistent implementations.
Consequently the specification of the DTC format was changed. To indicate the new consistent format this bit must be set to 0.
Also the old versions will be supported by CANcockpit.
The number of possible DTCs is very large, therefore efficient filtering is important for this function.
There are two main applications for DTCs (Diagnostic Trouble Codes):
•
To inform the operator, that something is not as expected.
Depending on the DTC it can have the nature of an Alarm (most severe), Warning or only Information
•
To provide diagnostic information for the workshop, so that system faults can be analyzed and repaired.
2. Functional Specification
Product Manual
DTC
Data Byte 3
8 least significant bits of SPN
Data Byte 4
8 medium significant bits of
SPN
Data Byte 5
3 most significant bits of SPN
plus 5 bit FMI
Data Byte 6
Conversion Method bit plus 7
bit Occurrence Count.
Technische Änderungen vorbehalten Technical details subject to change
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