Short answer
A crane, an excavator or a site generator does its work standing still, so its odometer barely moves while its engine wears normally. Operating hours are the counter that tracks that wear, which is why SAE J1939 defines an engine-hours parameter and why equipment makers write service schedules in hours rather than distance.
Key takeaways
- Distance is a proxy for wear only on a machine that works by moving; stationary equipment breaks that assumption completely.
- SAE J1939 SPN 247 counts engine operating hours at 0.05 hours per bit in PGN 65253, a standardised counter, not a telematics invention.
- Cummins writes its Onan generator schedule at 8, 50, 100, 250 and 500 operating hours, always "whichever comes first" against calendar time.
- Caterpillar fits an hours-run meter specifically to assist with maintenance and sets service at every 12 months or 500 hours.
- A platform that only stores kilometres cannot schedule anything on a crane, a compressor or a generator, it needs an hours counter per asset.
01The counter that reads zero while the machine works
An odometer is a useful maintenance counter for one reason only: on a vehicle that does its work by travelling, distance travelled rises in step with engine wear. Break that link and the counter stops meaning anything. A tower crane lifts all day and travels nothing. A tracked excavator digs for eight hours and repositions itself by perhaps a hundred metres. A site generator runs continuously and has no wheels at all. In each case the engine turns, the oil ages, the filters load with dust and the coolant cycles, while the distance figure a fleet platform was built around stays flat. The reading is not inaccurate; it is measuring the wrong quantity. Any maintenance rule expressed in kilometres will simply never fire on these assets, which means in practice that the machine is serviced when somebody remembers rather than when it is due.
02What the standard itself counts
This is not a telematics workaround. SAE J1939-71 defines SPN 247, Engine Total Hours of Operation, as a four-byte parameter with a gain of 0.05 hours per bit, transmitted in PGN 65253. That resolution, three minutes per bit, is deliberate for a counter meant to drive service intervals. The same standard separately defines SPN 246, Total Vehicle Hours, which tracks accumulated operating time of the vehicle as something distinct from the engine specifically. The fact that one standard bothers to define two different hour counters tells you the distinction matters to the people who design machines, not only to the people who report on them. The FMS-Standard interface signed by Daimler, Scania, Iveco, MAN, Volvo Trucks, DAF and Renault Trucks made SPN 247 mandatory in every factory-fitted gateway from 1 October 2009, which is a strong statement about which counter manufacturers treat as essential.
03How manufacturers actually write the schedule
Read an equipment manual and the unit of maintenance is unmistakable. The Cummins operator manual for the Onan HDKAG generator set lays out its schedule at 8, 50, 100, 250 and 500 operating hours, each tied to a calendar equivalent with the phrase "whichever comes first". The same manual shortens the oil change to every 50 operating hours in dusty or dirty conditions, a clause that matters on a Saudi construction site far more than in a workshop. Caterpillar takes the same approach and goes one step further by explaining why the meter is fitted at all: its generator set manual states the hours-run meter "assists with maintenance", and sets service at every 12 months or 500 hours. Neither manufacturer offers a distance-based alternative, because for a machine that never moves there is no distance to offer one against.
04Three counters, not one
The word "whichever comes first" in a manufacturer schedule is the operative instruction, and it means a platform needs to hold more than one counter per asset and compare them. Wialon implements exactly this, tracking mileage, engine hours and calendar days as three independent counters calculated simultaneously, so whichever reaches its threshold first triggers the service. On a mixed construction fleet that structure is not optional. A haul truck will hit its distance threshold first. An excavator will hit its hours threshold first. A standby generator that runs an hour a month will hit its calendar threshold long before either, and a calendar rule is the only one that will ever fire on it. Storing a single counter forces the platform to choose one failure mode: either the stationary assets are never scheduled, or the moving ones are scheduled far too often.
05Where the hours figure comes from when there is no bus
Older construction plant frequently has no CAN bus to read SPN 247 from, and this is where installations go wrong. The fallback is to infer running time from an ignition or alternator signal wired to a digital input, and that inference is not the same quantity. Ignition-on time includes every period when the key is turned but the engine is not turning, and on equipment left keyed-on between lifts the gap can be large. The honest position is to record which source each asset uses and never mix them in one report. An asset whose hours come from the engine bus and an asset whose hours come from an ignition input are measuring two different things, and averaging them produces a figure that describes neither. Labelling the source per asset costs nothing at commissioning and is nearly impossible to reconstruct later.
06Utilisation, not just maintenance
Once hours are recorded properly they answer a second question that distance never could: whether the machine is earning. A rented excavator charged by the day and running four hours a day is half-idle capital, and the only evidence of that is the hours counter. The same figure exposes the opposite case, where a single machine carries the whole site and the fleet is one breakdown away from a stoppage. Neither conclusion is visible in a position trail, because a machine parked on the pad and a machine digging beside the pad occupy nearly the same coordinates. This is why hours belong in the asset record next to the maintenance thresholds rather than in a separate maintenance module: the same number that schedules the service also prices the rental decision.
07What to specify before the first machine is fitted
The practical requirement list is short. Each asset needs a declared primary counter, hours, distance or calendar, and the platform needs to hold all three so "whichever comes first" is computable. Each asset needs its hours source recorded, whether that is the engine bus or a wired input. Each asset needs its own thresholds rather than a fleet default, because the Cummins 250-hour step and the Caterpillar 500-hour step are different numbers from different manufacturers and flattening them into one interval discards the manufacturer's judgement. Finally, the dusty-conditions clause needs somewhere to live: if the schedule halves in dust, the site condition is part of the asset record and not a note in somebody's notebook. None of this is expensive to capture at installation, and all of it is painful to add afterwards.
| Machine | Distance accrued | Hours accrued | Counter that should schedule it |
|---|---|---|---|
| Haul truck on a site loop | High | High | Distance, with hours as a check |
| Tracked excavator | Almost none | High | Engine hours |
| Tower crane | None | High | Engine hours |
| Site generator running continuously | None | Very high | Engine hours |
| Standby generator, an hour a month | None | Very low | Calendar days |
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)
- 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)
- Teltonika FMC003 wired inputs available for an ignition or alternator running signal where no CAN bus exists: 2 digital inputs; 1 analog input (0–30 V) (Source: ENTEK.AI product catalogue, as of 2026-08-31)
Questions
Why can I not just use kilometres for an excavator?
Because an excavator does its work without travelling. It may dig for a full shift and reposition itself by a hundred metres, so a distance threshold will never be reached while the engine, oil and filters wear at a normal rate. The counter is not inaccurate, it is measuring a quantity the machine does not produce.
Where does the hours figure come from?
From the engine bus where one exists: SAE J1939 defines SPN 247, Engine Total Hours of Operation, at 0.05 hours per bit in PGN 65253. On older plant with no CAN bus, running time is inferred from an ignition or alternator signal wired to a digital input, a different quantity that should be labelled as such per asset.
Do I still need a calendar rule if I track hours?
Yes. Manufacturer schedules are written as "whichever comes first", Cummins pairs its 8, 50, 100, 250 and 500-hour steps with calendar equivalents, and Caterpillar sets service at every 12 months or 500 hours. A standby generator that runs an hour a month will only ever be scheduled by the calendar rule.
Does site dust change the interval?
It does in the manufacturer documentation. The Cummins Onan operator manual shortens the oil change to every 50 operating hours in dusty or dirty conditions. That means the site condition belongs in the asset record alongside the thresholds, because otherwise the halved interval exists only in an engineer's memory.