If a newer Trane roof top unit is not operating either on initial startup, or following maintenance where the main electrical supply lines were disconnected from the main contactor, the problem may be an incorrect phase condition.







Newer Trane roof top units have phase monitors that sense the voltage between the three conductors of the three phase power supply. The phase monitor is located inside the access panel with the other electrical control wiring and devices. The phase monitor has two LED lights (one red light and one green light) that provide indication. Phase monitors protect motors and compressors against problems caused by phase loss, phase imbalance, and phase reversal.

The green LED illuminates when the three phase supply conductors are properly connected to the main contactor (a three phase contactor). This is the normal arrangement for a unit.

The red LED illuminates when the three phase supply conductors are improperly connected to the main contactor (when there is a phase error). When the red LED is lit, the phase monitor prevents the unit from operating. There are two conditions that will result in the red LED being illuminated.

If any of the three phase conductors (L1, L2, and L3) are connected to the main contactor in an improper sequence (when L1 is not connected to T1, L2 is not connected to T2, or L3 is not connected to T3 on the contactor), then the red LED will be lit and the unit will be prevented from operating.

If line to line voltage is not between 180 volts and 633 volts.

The only time we have found the red LED illuminated on the Trane phase monitor is when initially connecting the line leads (L1, L2, and L3) to the contactor in the wrong phase sequence, or if the line leads are disconnected from the contactor for some maintenance reason and they are then subsequently reconnected incorrectly.








WARNING: These repair activities involve working on high voltage equipment that poses serious electrical shock risk. Follow all safe electrical work practices and ensure that only qualified, trained service technicians perform these repair activities.

When a Trane roof top unit will not operate and the red LED is illuminated on the phase monitor, then switch any two of the electrical line leads supplying the main contactor. Then check the LED status on the phase monitor. If the red LED goes out and the green LED illuminates, then you know that you have now established the correct phase sequence. A standard digital multimeter (DMM) does not have a feature to read voltage phase, so just swap any two of the leads and check for the green LED being lit on the phase monitor.



























It is a good practice to color code the individual line leads connecting to the main contactor before removing them from the contactor for any maintenance. Electrical tape in various colors is available and it is handy to wrap a strip of colored electrical tape around the electrical lead an inch or so from the end of the conductor. Have a different color for each lead. Then when the leads are reconnected to the contactor later, they can be reconnected in the same arrangement. Taking photos before and after is also a helpful check to ensure the phases are reconnected following maintenance in the correct order.

When performing maintenance on roof top units, it is good practice to ensure that the low voltage transformer high side is wired for the correct line voltage. It is common for the low voltage transformer that is shipped with the roof top unit to have options for either 230v or 208v primary voltage. Based on results of a voltage test using your voltage meter, ensure that the correct primary side terminals are connected.

To interpret the model number descriptions on Trane roof top units, refer to the Trane Roof Top Unit (RTU) Model Number Description blog entry on this site (still under additional development).

Here are a few situations that can prevent the roof top unit from operating normally that are easy to overlook during troubleshooting.

  • Check the air filters to make sure they are not dirty
  • Check the evaporator to ensure there is no dirt accumulation
  • Check the condenser to ensure there is no dirt accumulation
  • Ensure the flame sensor (in furnace mode) is clean and sending a proper microamp signal
  • Check for any error codes
  • Check that all electrical connections are tight (threaded fasteners should be checked with a screwdriver or nut driver for tightness and spade terminal connector fasteners can be tightened with a pair of needle nose pliers if loose)
  • Check for proper gas pressure on the inlet and outlet of the gas valve (in furnace mode)
  • Once in a while we will find moisture in the gas regulator supplying a roof top unit. At night the moisture freezes and prevents natural gas from being supplied to the unit but during the day, when the sun hits the regulator and melts the ice, the unit runs fine.