Trailer Refrigeration Units

Proper TRU maintenance

For conventional belt-driven TRUs, best practices recommend conducting comprehensive inspections at 750-hour intervals, and for both conventional and hybrid units, service intervals should be scheduled at 1,500-hour cycles or one year, whichever comes first.

For TRUs shipped with extended life oil filters, oil change intervals occur at 3,000 hours or two years, whichever comes first.

The recommended inspection and service cycle for conventional units is: inspection at 750 hours, inspection plus scheduled maintenance at 1,500 hours, inspection again at 2,250 hours and inspection, scheduled maintenance and oil change at 3,000 hours, and so on. Hybrid units don’t require the 750- and 2,250-hour inspections.

On average, a single-temperature refrigeration unit runs approximately 1,800 hours per year, so scheduled maintenance may occur every 10 months or so, with inspections half way between.

The control module keeps track of engine hours to help determine when it is time to do a unit service or inspection.


At an interim inspection on conventional units, the following should be checked while the engine is off:

  • Engine oil; add as necessary.
  • Air filter; reset the air filter indicator.
  • Battery cables and clamps.
  • Coolant level; add if needed.
  • Exhaust system and mounting hardware.
  • Belt condition; tighten as necessary.

The gearbox and compressor seal should also be checked for leaks. Be aware that with all open (wet) seals, such as the gearbox and compressor shaft seals, some seepage is normal.

Next, the engine should be started and checked for proper operation. The pre-trip routine should be run with all active and inactive alarms noted.

Fuel lines, oil lines, filters and the exhaust system should be observed for leaks. Technicians should listen for unusual bearing noise from components, such as belt idlers or fan shafts.

During interim inspections, water should be drained from the fuel tank and debris should be cleared from defrost drains.


When the 1,500-hour mark comes around for scheduled maintenance on hybrid and conventional units, the aforementioned inspection steps need to be performed. Note that some of the steps, such as checking the gearbox and compressor seal, only concern conventional units, because hybrid units don’t have these.

Some additional minor inspections and service are recommended at the 1,500-service interval. One is to clean the condenser and straighten the fins as necessary. Due to their location downwind of the truck’s exhaust, condenser coils can get coated with oily soot, and cleaning them helps maintain unit efficiency.

A pressure washer can be used, but care should be taken not to bend the fins. Fins that have been bent should be straightened using a fin comb.

The evaporator coil should also be inspected for restrictions and cleaned as necessary.

Other recommended additional measures:

  • Inspect the water pump, air cleaner housing and tubing.
  • Check the unit and engine mounting bolts for proper torque.
  • Check switches and electrical connections.

With hybrid units, the insulation resistance of high-voltage wiring must be checked with a megohmmeter.

At 3,000 hours, additional scheduled maintenance includes an oil change and replacing the oil, air and fuel filters. Synthetic oil can extend oil change intervals to 4,000 hours.

At 4,500-hour service intervals, the breather should be cleaned and valve lash gap should be adjusted.

The coolant should be replaced at 6,000 hours if standard coolant is used, or at 12,000 hours if extended life coolant is used. As a preventive measure, the water pump and thermostat may be proactively changed at the 12,000-hour mark.

Although the previous measures are common to both hybrid units and conventional units, at regular intervals, conventional systems also require belt changes and extra compressor attention.


The refrigeration unit – or at least its diesel engine – isn’t all that different from what’s under the hood of a truck, but with a truck, you always have a driver with a dashboard full of gauges to monitor its operation.

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