I’ve been in the logistics game for over a decade, and I know that sinking feeling when you look at your reefer display or datalogger and see a jagged line instead of a smooth one. You are hauling sensitive cargo—maybe pharmaceuticals or fresh produce—and stability is everything. You find yourself asking, why is my reefer temperature fluctuating?
Is the unit dying? Is the insulation failing? Or did the driver just load it wrong?
The truth is, temperature swings aren't always a sign of a broken compressor. Often, it's a combination of airflow management, settings, and wear and tear on the box itself. In this guide, I’m going to break down the technical reasons behind these fluctuations, backed by industry data, so you can stop guessing and start fixing.
It Might Be Your Loading Pattern (The #1 Culprit)
Before you call a technician to tear apart the refrigeration unit, take a look inside the trailer. Based on fluid dynamics simulations, the way you stack your cargo has a massive impact on temperature uniformity.
• The "Short-Cycling" Problem: If you stack pallets too high or block the return air bulkhead, the cold air shoots out from the unit and gets sucked right back in without circulating through the trailer. This tricks the sensors into thinking the load is cold, causing the unit to shut off prematurely, only to restart minutes later when the hot spots in the rear trigger a warm reading.
• Close vs. Intermediated Stacking: Research shows that intermediated stacking (leaving gaps between cargo) significantly improves airflow uniformity compared to close stacking . If the air can't move between the boxes, the heat stays trapped, causing local temperature spikes.
• The "Red Line" Rule: Never load cargo above the red load line or block the evaporator outlets. If the discharge air is blocked, the unit cannot push the cold air to the back doors ,.
Pro Tip: Always ensure the floor channels (T-floors) are clear of debris. Air needs to flow under the cargo to remove heat effectively .
The "Silent Killer": Insulation Failure & Air Leaks
Here is a perspective you won't find in a basic manual: your reefer unit might be working perfectly, but your trailer box might be failing it.
As trailers age, their insulation performance drops. The core materials, like XPS (Extruded Polystyrene) or PU (Polyurethane), can degrade over time.
• Moisture Intake: If the outer skin of your trailer is damaged, water gets into the insulation. Studies on XPS boards show that after 200 freeze-thaw cycles, the volume water absorption can hit 12.5%, which drastically kills the insulation's ability to hold temperature .
• VIP Aging: Even advanced Vacuum Insulation Panels (VIP) have a lifespan. Over time, air permeates the barrier film, increasing the internal pressure and reducing thermal resistance .
• The Speed Factor: Did you know driving faster makes your unit work harder? Research indicates that when a vehicle travels at 90 km/h, the air pressure difference on the box can reach 250 Pa, causing massive air leakage if the seals aren't perfect . This forces warm air in and cold air out, causing the temperature to fluctuate wildly as the unit struggles to catch up.
Check your door seals and the seal around the refrigeration unit. If you see daylight, you're losing money and temperature control .
Mechanical Glitches & Sensor Drifts
If your loading is perfect and your box is tight, then we need to look at the hardware. Modern units like the Thermo King SLXi or Carrier Transicold systems rely on complex sensors.
1. Sensor Disagreement
The controller constantly compares the Supply Air Temperature (STS) and Return Air Temperature (RTS). If these sensors drift out of calibration, the logic gets confused. For example, if the return air sensor reads artificially low, the unit will reduce cooling capacity, allowing the actual cargo temp to rise .
2. The Electronic Expansion Valve (EEV)
The EEV controls the flow of refrigerant into the evaporator. If this valve gets stuck or malfunctions, it can lead to "hunting"—where the system alternately floods and starves the evaporator. This causes the suction pressure and discharge temperature to swing, resulting in unstable air temperatures .
3. The Defrost Cycle
Sometimes, a spike is just a spike. During a defrost cycle, the unit uses hot gas or electric heaters to melt ice off the coil. It is normal for the air temperature to rise briefly during this process . However, if your unit is defrosting too frequently (e.g., every hour), you likely have an air leak or a humidity issue .
Are You Using the Wrong Mode?
Finally, check your controller settings. Are you running in Continuous mode or Start-Stop (Cycle-Sentry)?
• Start-Stop Mode: In this mode, the engine shuts off once the setpoint is reached and restarts when the temperature drifts a few degrees away. By definition, this creates temperature fluctuations. It is great for fuel saving on frozen loads, but terrible for sensitive fresh produce that needs constant airflow .
• Economy Modes: Features like "FuelWise" or "Economy Mode" allow for a wider temperature tolerance to save diesel . If your shipper requires tight temperature control (e.g., ±0.5°C), you must disable these economy settings.
Next Steps
If you are asking "Why is my reefer temperature fluctuating?", start with the basics before calling for a tow.
1. Check the airflow: Is the chute blocked? Is the return air bulkhead clear?
2. Inspect the seals: Are the door gaskets tight?
3. Verify the mode: Switch to Continuous Run if you are hauling fresh produce.
If these don't solve it, you may be dealing with a degraded box or a failing sensor. Your next step should be a full Pre-Trip Inspection (PTI) to run diagnostics on the sensors and valves . Don't risk the load—get it checked.