High Engine Coolant Temperature Shutdown

Common Symptoms

• Unit alarms and shuts down mid-run, Smart Reefer 4 (SR-4) HMI touchscreen/hard-key control panel displays alarm code 18
• Alarm Menu on the SR-4 shows Code 18 as an active or stored alarm
• Engine stops and will not restart until fault is cleared and coolant temp drops
• Box temperature starts climbing because the refrigeration system is no longer running
• Coolant overflow reservoir is low, empty, or visibly boiling over
• Steam or coolant smell coming from the engine compartment of the reefer unit
• Unit may cycle on briefly after a cool-down period then shut down again repeatedly

Probable Causes (Ranked by Likelihood)

• Low coolant level due to a leak in the system (hoses, water pump, radiator, or head gasket) Very Likely
• Plugged or dirty radiator core restricting airflow and coolant heat transfer Very Likely
• Failed or stuck thermostat preventing coolant from circulating through the radiator Likely
• Water pump failure or worn impeller causing inadequate coolant flow Likely
• Cooling fan or fan clutch not spinning at proper speed, reducing airflow across the radiator Possible
• Defective coolant temperature sensor sending a false high reading to the controller Possible
• Combustion gas intrusion into the cooling system (head gasket failure), causing overpressure and boilover Less Likely

Step-by-Step Diagnostic Procedure

1. Before touching anything, let the engine cool completely, at least 20-30 minutes. Never open a hot pressurized radiator cap. Serious burns can result.

2. Check coolant level first. Remove the radiator cap only when cold and inspect the overflow reservoir. If the level is low, look for visible coolant on the ground, on hoses, or on the engine block. Top off with the correct pre-mixed coolant (50/50 ethylene glycol) and note whether the level drops again on the next run.

3. Inspect all coolant hoses end to end. Squeeze them. Look for soft spots, cracks, or loose clamps. Pay close attention to the lower radiator hose and any bypass hoses near the thermostat housing.

4. Check the radiator core from the front of the unit. If you see road grime, smashed bugs, or debris blocking the fins, clean it out with low-pressure water or compressed air from the engine side outward. A blocked radiator is one of the most common causes of Code 18 on units running in hot Central Valley summers.

5. With the engine cold, locate the thermostat housing (typically on the top of the engine near the water outlet). If you have a spare thermostat on the truck, swap it. A stuck-closed thermostat is a cheap and fast fix. Spec thermostat opening temp for the Yanmar TK486 is typically 180-185 degrees F.

6. Start the engine and watch the cooling fan. It should spin freely and at a consistent speed. If the fan belt is slipping, broken, or the fan itself wobbles, that will reduce cooling. Inspect belt tension and condition.

7. If coolant level checks out, the radiator is clean, the thermostat and fan look good, and the code keeps coming back, measure the coolant temperature sensor resistance at the sensor connector with a multimeter. Compare to the Thermo King spec chart. A sensor reading out of range will cause a false Code 18 even if the engine is not actually overheating. Replacing the sensor requires moderate skill and a basic socket set.

8. If you see white or gray exhaust smoke, find coolant in the oil (milky dipstick), or the overflow reservoir keeps pressurizing rapidly, suspect a head gasket. That is an advanced repair requiring the engine to come apart. Stop running the unit and call a Thermo King dealer or certified reefer tech immediately to avoid total engine damage.

Frequently Asked Questions