Unit Protection Devices
Coolant Level Switch: The coolant level switch closes if the coolant level drops below an acceptable level. If it stays closed for a specified time, the microprocessor records Alarm Code 37.
Engine Coolant Temperature Sensor: The microprocessor uses the engine coolant temperature sensor to monitor the engine coolant temperature. If the engine coolant temperature rises above an acceptable level, the microprocessor records Alarm Code 41 and possibly 18. The microprocessor might also shut the unit down.
Fuses: Various fuses protect circuits and components.
| Fuse | Size | Function |
| F1 | 5A | 2A Power for REB |
| F2 | 15A | On/Off Switch Circuit |
| F3 | 40A | Fuel Solenoid/Starter Circuit |
| F4 | None 2A | No Fuse - All Bosch and Thermo King Alternators 2A Fuse - All Prestolite Alternators |
| F5 | 60A | Preheat Circuit (See Note) |
| F6 | 15A | High Speed Solenoid Circuit |
| F7 | 2A | 8X Power for CAN bus |
| F8 | 5A | 2A Power for CAN bus J12 |
| F10 | 15A | On/Off Relay Circuit |
| F12 | 5A | 2A Power for CAN bus J13 |
| F13 | 2A | Status Light Circuit |
| F15 | 2A | SR-4 Power Supply Circuit |
| F20 | 2A | Alternator Sense Circuit |
| F22 | 10A | Fresh Air Door Circuit |
| F25 | 7.5A | High Pressure Cutout Circuit |
Smart FETs: Smart FETs in the base controller protect circuits and components.
High Pressure Cutout Switch: The high pressure cutout switch is located on the compressor discharge manifold. If the compressor discharge pressure becomes excessive, the switch opens the circuit to the run relay to stop the unit. The microprocessor will record Alarm Code 10.
High Pressure Relief Valve: This valve is designed to relieve excessive pressure in the refrigeration system. It is located on the receiver tank. If the high pressure relief valve opens, much of the refrigerant will be lost. Take the unit to a Thermo King dealer if this occurs.
Low Oil Level Switch: The low oil level switch closes if the oil drops below an acceptable level. If it stays closed for a specified time, the microprocessor shuts the unit down and records Alarm Code 66.
Low Oil Pressure Switch: The low oil pressure switch closes if the oil pressure drops below an acceptable level. If it stays closed for a specified time, the microprocessor shuts the unit down and records Alarm Code 19.
Preheat Buzzer: The preheat buzzer sounds when the controller energizes the preheat relay. This warns anyone near the unit that the controller is about to start the engine.
Overload Relay-Automatic Reset (SmartPower Units): An overload relay protects the standby electric motor. The overload relay opens the circuit to the electric motor if the motor overloads for any reason (e.g., low line voltage or improper power supply) while the unit is on electric standby operation. The microprocessor will record Alarm Code 90.