Remote reading of DTC faults on lorries

SAE J1939 is the standard protocol for exchanging data between control units in heavy machinery. Besides ordinary parameters such as engine revolutions and consumption, it also describes diagnostic messages. The most important of them for maintenance is DM1, active faults in real time.

What is inside DM1. The first part is the state of the lamps on the instrument panel. From it you can see whether the warning indicators are lit and how serious the fault is, that is, whether the vehicle has to be stopped at once or can reach base.

The second part is the DTC codes themselves. Each code consists of several fields. The SPN (Suspect Parameter Number) says which assembly or system is at fault. The FMI (Failure Mode Identifier) describes the type and degree of the failure. The OC (Occurrence Count) shows how many times the fault has recurred. The conversion method says whether this is a standard SPN or a manufacturer's proprietary code.

To put it by way of comparison, a lorry's control units are constantly broadcasting their condition, and the tracker is a receiver that listens to that transmission and passes it to the office. Without it the operator has nothing but a lamp on the dash, which tells them neither the cause nor the severity.

Besides DM1 the tracker reads DM2, previously active faults. These reveal chronic problems: a fault that appears and disappears once a week may turn out to be the first sign of a serious breakdown.

How the data is sent. The FMC650 sends the diagnostic messages as they are, as hexadecimal strings, without interpreting them itself. This is the same data a service scanner shows. An integrator can build their own decoding logic, and an operator with a diagnostic department of their own can receive the codes in the form they are used to.

Two CAN lines. The FMC650 connects to two buses at once and takes data from different sources. From the tachograph, for instance, the driver's remaining driving time, so as to choose a workshop they can reach before the compulsory rest. From the trailer's EBS module, tyre pressures. Together with the fault codes that gives the mechanic a noticeably fuller picture of the vehicle's condition than a phone call from the driver.

How the work changes. Let us go back to the tractor unit with the worn injector. As soon as the engine unit records a deviation, the code arrives at the server. The mechanic in the office sees which assembly and which type of failure, and decides: the vehicle will finish its run, and at base the parts and a free ramp will already be waiting for it. The chances of managing without a recovery truck and a repair at a random price on the road improve, although a code will not predict a sudden mechanical failure.

Which data you will get on a particular vehicle. Not every tractor unit transmits DM1 and DM2: it depends on the make, the model, the year and which units are connected to the available bus. Before fitting out a fleet you take one vehicle of each model, connect an FMC650 and see whether DM1 arrives, whether the dash lamps are in the message and which SPNs appear. The result is written into a fleet compatibility table.

How a code is matched to a service event. When a code has arrived at the server, the mechanic compares the SPN and the FMI with the service scanner readings when the vehicle comes in. If the codes agree, the arrangement can be trusted. From then on the maintenance log records the link: the date of the code, the assembly, the work carried out and the date it was cleared. After a few months the log shows which codes really did precede a repair and which turned out to be noise, and the notifications are re-tuned to the important ones only.

The limit of the scenario. The tracker sends codes without decoding them. Turning a particular manufacturer's proprietary SPNs into readable text is a job for the workshop or the platform, and for some makes the manufacturer's documentation is needed.