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11 Appendix C: J1939 Old DTC Conversion Methods
This appendix was created to assist the user in decoding the information contained in the
modbus DTC (DM1 and DM2) registers if the generator ECM does not support the
newest DTC conversion method. The diagnostic trouble codes on J1939 are specified in
a specific format. Older J1939 specifications had three conversion methods for the SPN
and it was impossible to tell them apart from the DTC data alone. One had to consult the
engine manufacturer.
Newer J1939 specifications follow one SPN method (Version 4) and the user can
determine if this method applies by looking at the CM bit. It will be set to 1 if Version 4
applies, and set to 0 if Versions 1, 2, or 3 applies. If the CM bit is 0, see below for the
three conversion methods. Byte 1 to Byte 4 refers to the individual bytes in the GSC400
DTC Modbus registers. See registers 40115 and 40116 on page 101, above.
DTC Conversion Method (Version) 1:
Byte 1: 8 most significant bits of 16 most significant bits of SPN
Byte 2: 8 least significant bits of 16 most significant bits of SPN
Byte 3: 3 most significant bits of byte contain the 3 least significant bits of SPN
5 least significant bits of byte contain the FMI
Byte 4: most significant bit of byte contains CM
7 least significant bits of byte contains OC
DTC Conversion Method (Version) 2:
Byte 1: 8 least significant bits of 16 most significant bits of SPN
Byte 2: 8 most significant bits of 16 most significant bits of SPN
Byte 3: 3 most significant bits of byte contain the 3 least significant bits of SPN
5 least significant bits of byte contain the FMI
Byte 4: most significant bit of byte contains CM
7 least significant bits of byte contains OC
DTC Conversion Method (Version) 3:
Byte 1: 8 least significant bits of SPN
Byte 2: 8 second byte of SPN
Byte 3: 3 most significant bits of byte contain the 3 most significant bits of SPN
5 least significant bits of byte contain the FMI
Byte 4: most significant bit of byte contains CM
7 least significant bits of byte contains OC
DTC conversion method 3 is the same as DTC conversion method 4 (the new J1939
standard) except that the CM bit is 1 so it is impossible to tell it apart from versions 1 and
2. Version 4 has the bit set to 0 which allows the user to know the conversion format
without consulting the engine manufacturer.
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