A modern refrigerated trailer is a rolling computer. Box temperature, return air, supply air, ambient, GPS, door state, fuel level, engine hours, alarm state, gen-set load, gen-set hours, battery voltage, defrost cycle count, plus another 25 or so signals depending on the reefer unit you are running. Most fleet dashboards drown the dispatcher in noise.
The dispatchers we trust most look at six numbers. The rest are diagnostic, useful when something has already gone wrong, less useful when the load is moving.
Number one: return air against setpoint
Return air is the temperature of the air coming back into the evaporator from the box. It is the truest read of what the cargo is sitting in. If return air is sitting on setpoint, the cargo is in spec. If return air is drifting up while the unit is calling for cool, you have a real problem developing.
Watch the delta between setpoint and return air. Inside one degree is healthy. Beyond two is an alert. Beyond three for more than 15 minutes is an incident.
Number two: door state plus geofence
A door open event on a trailer that is geofenced inside a customer DC is normal. A door open event on a trailer geofenced on the Ahmedabad to Palanpur Highway is not. The signal on its own is noise. The signal in context is gold.
We tie door events to the dispatch plan in the TMS, so a planned stop logs without an alert and an unplanned door open pages the dispatcher in 30 seconds.
Number three: gen-set fuel and hours
Genset fuel is the slow leak. If a drayage box is sitting on a chassis with a gen-set burning fuel for 36 hours, you are accumulating gen-set hours, accumulating refuel cost, and accumulating risk that the box dies cold at 03:00 with nobody on the lot. Track gen-set fuel level per box, with an alert at 30 percent remaining, and gen-set hours per maintenance window.
Number four: discharge air after defrost
After every defrost cycle the unit has to pull discharge air back to setpoint. If the post-defrost recovery is taking longer this month than it did last month, the evaporator coil is fouling. That coil is going to fail on a hot day in May. Watch the discharge air recovery time. Schedule the maintenance.
Number five: alarm code rollup, weekly
Not every alarm code is an incident, and chasing every one of them clutters the dispatcher screen. But a weekly rollup of alarm codes by trailer surfaces the trailers that are starting to misbehave. The pattern matters more than the individual events. We pull this report every Monday morning, and it has saved us four mid-route breakdowns in the past 18 months.
Number six: pre-cool verification at dispatch
This one is operational, not telemetry, but it sits on the same dashboard. Every outbound trailer should be pre-cooled to a target colder than the highest customer setpoint before the first pallet rolls in. Pre-cool verification is a single number stamped on the BOL at the seal. If pre-cool is not verified at dispatch, the rest of the telemetry does not matter.
What we ignore
We do not actively monitor box temperature on its own, because return air is the more honest read. We do not monitor battery voltage day to day because it is a maintenance signal, not an operations signal. We do not monitor refrigerant pressure because the field crew can read it during PM. The point is not that the data is useless. It is that operations and maintenance are different jobs, and showing maintenance signals to the dispatcher slows down operations decisions.
A practical setup
Run two dashboards. Operations sees six numbers per trailer in real time. Maintenance sees 40 numbers per trailer on a weekly review. Tie alarms to dispatch plans so geofence and door state alerts are context-aware. Pull the alarm-code rollup every Monday.
That is it. Telematics is not a feature competition. It is a tool. Use the parts that move the cold-chain, and shelve the rest until you actually need them.