Troubleshooting a Thermo King APU Not Cooling: Yellow Triangle Warning

Thermo King Auxiliary Power Units (APUs) are essential components in trucks that provide heating, cooling, and electrical power when the main engine is turned off. However, encountering issues with the APU’s cooling system can be frustrating, especially when accompanied by a yellow triangle warning. In this article, we will explore the possible causes of a Thermo King APU not cooling and triggering the yellow triangle warning, as well as potential solutions to resolve the issue.

Identifying the Problem:

  1. Checking the Alarms: To begin troubleshooting, connect a laptop to the Thermo King APU’s motherboard and access the software. Navigate to the “Alarms” section to identify the specific fault code triggering the yellow triangle warning. One common code is “High Engine Coolant Temperature.”
  2. Inspecting the Cooling System: After identifying the fault code, it’s time to inspect the APU’s cooling system. Start by removing the cover and checking the control panel motherboard for any blown fuses. Ensure that all fuses are intact and functioning correctly.

Possible Causes and Solutions:

  1. Low Coolant Level: If the coolant level is low, it can lead to overheating and trigger the yellow triangle warning. Check the coolant reservoir and top it up if necessary. Ensure that there are no leaks in the coolant system.
  2. Faulty Temperature Sensor: A malfunctioning temperature sensor can provide inaccurate readings, leading to false alarms. Consider replacing the temperature sensor to ensure accurate temperature monitoring.
  3. Defective Fan: A non-functioning fan can result in inadequate cooling. Check if the fan is operating correctly when the APU is turned on. If the fan is not working, it may need to be replaced.
  4. Seized Fan: In some cases, the fan may be physically blocked or seized, preventing it from spinning and cooling the APU. Inspect the fan for any obstructions or signs of damage. If necessary, clean or replace the fan.
  5. Blown Fuse: A blown fuse can disrupt the APU’s cooling system, causing the yellow triangle warning. Double-check all fuses related to the cooling system and replace any blown fuses.
  6. Refrigerant Leak: If the APU is not cooling and there is a refrigerant leak, it can lead to insufficient cooling capacity. Inspect the high-pressure bulb and low-pressure bulb for any signs of leakage. If a leak is detected, replace the bulbs and recharge the system with the appropriate amount of refrigerant.

When troubleshooting a Thermo King APU not cooling and triggering a yellow triangle warning, it is crucial to check the coolant level, temperature sensor, fan functionality, fuses, and potential refrigerant leaks. By systematically addressing these possible causes, you can resolve the issue and ensure proper cooling functionality in your Thermo King APU. If the problem persists, it is recommended to consult a professional technician or contact Thermo King customer support for further assistance.

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