Requesting Specific PGNs from Heavy Equipment
Not all Telemetry data is published by default to the Diagnostic Port. The standard allows you to request specific information from the equipment. And, it is different for different types of equipment. For example, the following Table shows you information that is typically not available, but can be requested using the associated SPN and PGN combination:
You need specialized programming knowledge to send these requests to the equipment, read and interpret the responses.
Requesting Diagnostic Trouble Codes (Fault Codes)
Fault Codes are not published by default on the CAN bus. Youʼd have to request the fault codes using specific PGNs. The PGNs to get the fault codes are the DM1 and DM2 messages. The formal name for Fault Codes in the j1939 specification is Diagnostic Trouble Codes (DTC)
DTCs are standardized codes that indicate specific problems or malfunctions in a vehicleʼs or equipmentʼs systems. When the onboard diagnostics system detects an issue, it generates a DTC to pinpoint the problem. This code helps technicians and operators identify and address issues promptly.
Structure of a DTC in J1939 In the J1939 protocol, a DTC typically consists of several parts that provide detailed information about the fault:
SPN (Suspect Parameter Number): The SPN identifies the specific parameter or component that is experiencing an issue. For example, an SPN might indicate a problem with engine speed, oil pressure, or temperature.
FMI (Failure Mode Indicator): The FMI describes the type of failure detected in the parameter identified by the SPN. There are several standard FMIs, such as “data valid but above normal operational range,” “data erratic or intermittent,” or “mechanical system not responding or out of adjustment.
.” All the FMI values are defined in the Appendix A of the j1939 spec (-73 is the latest version) – you can purchase this spec from the SAE website (https://sae.org)
OC (Occurrence Count): The OC shows how many times the particular fault has been detected. This helps determine if an issue is intermittent or persistent.
Source Address: This part of the DTC indicates which electronic control unit (ECU) reported the fault. This is useful in systems with multiple ECUs, as it helps locate the source of the problem.
Two examples of a Diagnostic Trouble Codes
Example 1:
Letʼs say you have a DTC with the following details:
SPN 190: Engine Speed
FMI 3: Voltage Above Normal
OC 2: The fault has occurred twice
Source Address: ECU 1 (Engine Control Unit)
This DTC tells you that the engine speed sensor has reported a voltage that is higher than normal, and this issue has been detected twice by the engine control unit.
Example 2:
SPN 157: Injector Metering Rail 1 Pressure
FMI 1: Data valid but below normal operational range – Most severe level
OC 3: The fault has been detected three times
Source Address: ECU 1 (Engine Control Unit)
This DTC indicates that the pressure in Injector Metering Rail 1 is below the normal range and that this issue has been detected three times by the engine control unit.
The DM1 message
The DM1 message contains several key pieces of information to help you understand and address the issues in your vehicle or equipment. Hereʼs what youʼll find in a typical DM1 message:
Message ID (PGN 65226): The DM1 message is identified by the Parameter Group Number (PGN) 65226. This PGN tells the receiving device that the message is a DM1 diagnostic report.
Lamp Status:
- Malfunction Indicator Lamp (MIL) Status: Indicates whether the MIL is on or off, signaling a serious issue that needs attention.
- Red Stop Lamp Status: Indicates a severe problem that requires the vehicle to be stopped.
- Amber Warning Lamp Status: Indicates a warning that something is wrong, but the issue is not critical enough to stop the vehicle.
- Protect Lamp Status: Indicates that protective measures are in place to prevent further damage.
SPN (Suspect Parameter Number): Each active DTC includes an SPN, which identifies the specific parameter or component that has the issue. This helps pinpoint what part of the vehicle or equipment is affected. FMI (Failure Mode Indicator): The FMI provides details about the type of problem detected with the parameter identified by the SPN.
Common FMIs include indicators for values that are above or below normal, data being erratic, or mechanical failures.
Occurrence Count (OC): The OC indicates how many times the specific fault has been detected. This can help determine if the issue is a one-time event or a recurring problem.
SPN Conversion Method: This field indicates how the SPN should be interpreted. It tells whether the SPN follows a standard conversion method or a manufacturer-specific one.
The DM2 Message
The DM2 message, known as Diagnostic Message 2, contains historical data about DTCs that were once active but have since been resolved or are not currently detected. This historical data is valuable for understanding past issues and trends that may indicate intermittent problems. The structure is the same as the DM1 message. However, the PGN ID is 65227.
