Introduction: Identifying the A/C Cooling Issue
A 2018 Honda Fit equipped with a 1.5L engine and CVT transmission was brought in for inspection due to poor A/C performance. The customer noticed that the air conditioning failed to provide sufficient cooling shortly after acquiring the vehicle. An independent repair shop could not pinpoint the issue despite topping off the refrigerant. The unresolved problem led the customer to visit an authorized 4S dealership.
Initial Observations: Cooling Problem Symptoms
Technicians tested the vehicle by starting the engine and setting the A/C to its coldest setting with maximum fan speed. A thermometer measured the vent output temperature at 15°C, confirming suboptimal cooling.
During inspection, the A/C compressor and condenser fan were observed cycling. Each operation lasted 7 seconds before shutting off for 14 seconds. Despite these irregularities, diagnostic tests using Honda’s HDS scan tool revealed no error codes.
Understanding the Manual A/C System
This vehicle’s manual air conditioning system comprises:
- A/C compressor
- Condenser
- Powertrain Control Module (PCM)
- Evaporator
- A/C pressure switch
- Evaporator temperature sensor
- Heater control panel
The PCM plays a central role, receiving data from the pressure switch and evaporator temperature sensor to regulate the A/C compressor and fans. If system pressure surpasses 2850 kPa (413.4 psi) or drops below 196 kPa (28.4 psi), the PCM shuts off the compressor to prevent damage. Additionally, the PCM temporarily disables the compressor when the evaporator temperature nears freezing.
Testing and Analysis
Using A/C pressure gauges, technicians measured system pressures during compressor operation:
- High pressure: 1517 kPa (220 psi)
- Low pressure: 276 kPa (40 psi)
Upon compressor disengagement, high pressure dropped to 1448 kPa (210 psi), and low pressure rose to 414 kPa (59 psi). These findings ruled out refrigerant shortage. To ensure the refrigerant’s quality, it was evacuated, and new refrigerant was added, yet the problem persisted.
Data from the HDS scan tool showed abnormal evaporator temperature readings. When the A/C clutch disengaged, the evaporator temperature rapidly dropped from 3°C to 1°C within 3 seconds—indicating a fault.
Comparing Normal vs Faulty Systems
In a properly functioning system, it takes at least 17 seconds for the evaporator temperature to drop from 3°C to 2°C, ensuring a stable cooling cycle. However, the faulty sensor caused the temperature to plummet too quickly, leading the PCM to disengage the compressor to avoid evaporator freezing.
Solution: Replacing the Faulty Sensor
The faulty evaporator temperature sensor was replaced, ensuring it was installed in the designated evaporator area. Following standard refrigerant recharging procedures, the system was retested. Results showed the vent temperature dropped to 4.3°C, and the cycling issue was resolved.
Lessons Learned: Accurate Diagnosis Matters
This case highlights the importance of monitoring sensor data using advanced tools like the HDS scan tool. By comparing pressure and temperature patterns, technicians confirmed the evaporator temperature sensor fault. For vehicles with automatic A/C systems, entering diagnostic modes via control panel buttons or accessing HVAC unit data with an HDS can yield valuable insights.
Diagnosing no-error-code issues like this one becomes far more efficient when leveraging sensor data analysis, saving both time and effort.
