Short answer
Ignition-on time is how long a tracker saw its ignition input active. Engine hours is how long the engine control unit says the engine ran. The first includes accessory position, radio and cab air with the engine off; the second does not. Where a service interval is set in hours, the difference compounds.
Key takeaways
- SAE J1939 SPN 247, Engine Total Hours of Operation, is the ECU's own counter at 0.05 hours per bit in PGN 65253.
- A separate parameter, SPN 246 Total Vehicle Hours, tracks vehicle operating time as distinct from the engine specifically.
- A tracker's ignition-on figure measures a switch or a voltage, not combustion, accessory time is inside it.
- Teltonika devices let you pick the ignition source, and the choice changes the number the report shows.
- Manufacturers schedule service in operating hours, so scheduling on ignition-on time services vehicles early or late by a drift nobody sees.
01Two counters, two definitions
Both numbers arrive in a fleet report labelled in hours, and that shared unit hides the fact that they are answers to different questions. Engine hours, as SAE J1939-71 defines it in SPN 247, is Engine Total Hours of Operation: a four-byte counter maintained by the engine control unit itself, with a gain of 0.05 hours per bit, transmitted in PGN 65253. It advances when the engine runs and only then. Ignition-on time is a tracker measurement, and what it measures is the state of an input, a wire that is energised when the key is in a particular position, or a supply voltage that rises when the vehicle wakes up. Neither definition is wrong. But one is a statement from the engine about combustion, and the other is a statement from a sensor about a switch, and no configuration makes them the same quantity.
02Where the accessory position hides
The gap between the two counters is filled by everything a vehicle does with the key turned but the engine not running. A driver waiting in a loading queue with the radio on and the windows up. A technician running a diagnostic session on the bus. A refrigerated body on shore power while the cab electronics stay live. In each case an ignition-driven counter accumulates time and the ECU counter does not move at all, because there is no combustion to record. The direction of the error is always the same: ignition-on time is equal to or greater than engine hours, never less. That one-sidedness is what makes it dangerous. A symmetrical error averages out across a fleet and a month; a one-directional one accumulates, so the vehicles with the most waiting time carry the largest overstatement.
03The standard already separates engine from vehicle
It is worth noticing that J1939 does not treat "hours" as a single idea either. Alongside SPN 247 for engine operating hours, the standard defines SPN 246, Total Vehicle Hours, which tracks accumulated operating time of the vehicle as something distinct from the engine specifically. That the same standard bothers to define both is evidence that the distinction is real at the level of the vehicle designer, not an artefact of telematics. The FMS-Standard interface signed by Daimler, Scania, Iveco, MAN, Volvo Trucks, DAF and Renault Trucks made SPN 247 mandatory in all factory-fitted gateways from 1 October 2009, which tells you which of the two the truck manufacturers consider indispensable. If the people who build the engine keep these counters apart, a fleet report that collapses them into one row is losing information the vehicle was willing to give.
04How the tracker decides the engine is on
A tracker does not have one ignition definition; it has a configurable one. On the Teltonika FMC920 the selectable ignition sources are a digital input, power voltage, engine RPM read over OBD-II or CAN, and the accelerometer. Those four options are not interchangeable. A digital input follows the key. Power voltage follows the electrical system and can be fooled by a charger or a large accessory load. The accelerometer follows vibration, so it reports engine-on for a vehicle being towed or shaken. Only the RPM option is actually watching combustion, and it is the one that requires the CAN or OBD connection to exist in the first place. This is the practical crux: the phrase "engine on" in a report is defined by an installer's configuration choice, and two vehicles in the same fleet can be measuring different things.
05What the difference costs a maintenance schedule
Manufacturers write hour-based service intervals against running time. Cummins publishes an Onan generator-set schedule at 8, 50, 100, 250 and 500 hours on a whichever-comes-first rule against calendar time, and shortens the oil change to every 50 operating hours in dusty or dirty conditions. Caterpillar states that the hours-run meter on a generator set assists with maintenance and sets service at 12 months or 500 hours. Every one of those numbers means engine running hours. Feed an ignition-on figure into that rule and the counter reaches 500 before the engine has run 500, so the asset is serviced early, wasted cost, or, if the fleet compensates by raising the threshold, some vehicles are serviced late. Neither outcome is visible from the report, because the report shows hours and the label does not say which hours.
06When ignition-on is the right number
None of this makes ignition-on time a bad measurement. It is the right answer to a different set of questions, and often the only one available. Shift adherence, unauthorised use outside working hours, how long a vehicle sat with a driver in it, whether a key was turned at a site at all, these are questions about the vehicle being in service, not about the engine. Ignition state is also what a tracker can measure with a single wire and no adapter, so it exists on every installation, including light vehicles where no CAN adapter was fitted. The defensible position is to keep both, name each honestly in the interface, and route each rule to the counter that matches it: duty and utilisation to ignition, wear and service intervals to engine hours.
07A short audit for your own reports
The check takes an afternoon and it is worth doing before the next service cycle. First, for each vehicle group, find out which ignition source is configured, input, voltage, RPM or accelerometer, because that single setting defines what your hours column contains. Second, for any vehicle with a CAN or OBD connection, pull SPN 247 alongside the tracker figure over the same month and look at the gap; a large one identifies your highest-waiting assets. Third, read your maintenance rules and confirm each hour-based one is bound to the ECU counter rather than the tracker one. Fourth, where no CAN connection exists, do not pretend otherwise, label the column as ignition hours, and set the interval knowing it overstates. An honest label costs nothing and prevents a silent drift.
| Situation | Ignition-on time | Engine hours (SPN 247) | Which one the rule needs |
|---|---|---|---|
| Driving on a highway | Accumulates | Accumulates | Either |
| Idling in a loading queue | Accumulates | Accumulates | Either |
| Key in accessory, engine off | Accumulates | Does not move | Engine hours |
| Diagnostic session, engine off | Accumulates | Does not move | Engine hours |
| Engine running, tracker asleep | May miss it | Accumulates | Engine hours |
| Vehicle towed, engine off | Depends on source | Does not move | Engine hours |
| Shift adherence check | The right measure | Understates duty | Ignition-on |
Figures and sources
- SAE J1939 SPN 247, Engine Total Hours of Operation, resolution and PGN: 0.05 hr/bit, PGN 65253 (Source: SAE J1939-71 (Revised MAR2011), Vehicle Application Layer, as of 2026-08-31)
- SAE J1939 SPN 246, Total Vehicle Hours, a counter the standard keeps separate from engine hours: tracks accumulated vehicle operating time as distinct from the engine specifically (Source: SAE J1939-71 (Revised MAR2011), Vehicle Application Layer, as of 2026-08-31)
- FMS-Standard mandating SPN 247 in all factory-fitted gateways, and its issuing OEM group: mandatory from 01.10.2009; Daimler, Scania, Iveco, MAN, Volvo Trucks, DAF, Renault Trucks (Source: FMS-Standard Interface description Vers. 02.00 (HDEI Working Group), as of 2026-08-31)
- Ignition sources selectable on the Teltonika FMC920, the setting that defines the hours column: DIN1, Power Voltage, Engine RPM (OBD II/CAN), Accelerometer (Source: Teltonika Wiki, FMC920 System settings (Ignition Source), as of 2026-08-31)
- Cummins/Onan generator maintenance schedule, expressed in operating hours: 8, 50, 100, 250, 500 h, "whichever comes first" vs calendar time (Source: Cummins Operator Manual, Onan Generator Set HDKAG, as of 2026-08-31)
- Cummins shortened oil-change interval in dusty/dirty conditions: every 50 operating hours (Source: Cummins Operator Manual, Onan Generator Set HDKAG, as of 2026-08-31)
- Caterpillar generator set hours-run meter, stated purpose, and service interval: "assists with maintenance"; service every 12 months or 500 hours (Source: Caterpillar Generator Set Operator & Maintenance Instruction Manual, as of 2026-08-31)
- Wialon service-interval implementation, three independent counters tracked simultaneously: mileage, engine hours, and calendar days, each calculated independently (Source: Wialon Hosting User Guide, Service intervals, as of 2026-08-31)
Questions
Which counter is bigger, and by how much?
Ignition-on time is always equal to or greater than engine hours, never less, because it includes key-on-engine-off periods. The size of the gap depends entirely on duty pattern, a highway truck may differ by little, a vehicle that waits with the cab powered can differ substantially. Measure it on your own fleet rather than assuming a ratio.
I have no CAN adapter. Can I still schedule maintenance in hours?
Yes, but label the column honestly as ignition hours and set the interval knowing it overstates running time. The alternative is to select engine RPM over OBD-II as the ignition source where the port is available, which is the one option among the four that actually watches combustion.
Why does the standard define both SPN 247 and SPN 246?
Because vehicle operating time and engine operating time are genuinely different quantities to the people who design the vehicle. SPN 247 counts engine running; SPN 246 counts the vehicle as a whole. The FMS-Standard made SPN 247 mandatory in factory gateways from October 2009, which shows which one truck makers treat as essential.
Should I stop reporting ignition-on time altogether?
No. It answers a real class of questions, shift adherence, out-of-hours use, how long a vehicle was in service with a driver, and it exists on every installation, including those without an adapter. Keep both counters and route each rule to the one that matches the question it asks.