Analysis and Solutions to Bus AC Pressure Problems

The stable operation of the bus air conditioning system is directly related to passenger comfort and driving safety. Abnormal pressure is a comprehensive fault signal, often involving multiple subsystems.

This article draws on the research framework of the article “Fault Modal Analysis of Thermal Management System for Commercial Vehicles” published in the 6th issue of the *China Transportation Equipment Technology Journal* in 2025, and combines authoritative industry insights to break down the topic of “air conditioning pressure failure” into six sub-problems: mechanical, hydraulic, electrical, refrigerant, usage scenarios, and system maintenance, and analyzes each in depth.

Content Block 1: Bus AC Pressure Problems—Core Compressor Faults

Problem Status: As the “heart” of Bus AC , mechanical wear, bearing jamming, or internal leakage of the compressor are common causes of insufficient high-pressure side pressure and abnormal low-pressure side pressure. Statistics from the aforementioned article in 2025 show that approximately 30% of poor air conditioning cooling problems are directly related to the mechanical condition of the compressor.

Problem Analysis: Long-term high-load operation, poor lubrication, or refrigerant contamination will accelerate the wear of internal compressor components. Wear and tear leads to decreased volumetric efficiency, manifesting as an inability to effectively establish discharge pressure and a narrowing of the suction-discharge pressure difference. Li Ming, a senior engineer at the National Bus Engineering Technology Research Center, points out: “Abnormal compressor noise accompanied by violent fluctuations in the pressure gauge pointer is often a clear indication of severe wear of mechanical components, requiring immediate shutdown and repair.”

Conclusion: Compressor mechanical failure is one of the sources of systemic pressure disturbances. Regularly listening for abnormal noises and monitoring the relationship between compressor operating current and pressure are key to preventative diagnosis.

Content Block 2: Bus AC pressure problems—Abnormal hydraulic circulation resistance

Current situation: The pipes connecting various components, the condenser, and the evaporator constitute the refrigerant’s “blood circulation network.” Pipe blockage (such as a faulty dryer causing a blockage), dirty condenser fins, or fan malfunction can cause abnormally high pressure spikes; evaporator frosting or insufficient fan airflow can lead to excessively low low pressure.

Analysis: Blockage or poor heat dissipation in the high-pressure pipes prevents the refrigerant from effectively condensing and liquefying, causing pressure buildup on the high-pressure side of the system. Conversely, if heat exchange at the evaporator end is obstructed, the low-pressure side pressure will continue to decline. This is not merely a problem with a single component; it reflects the overall cleanliness and heat dissipation environment of the system.

Conclusion: Abnormal pressure values require prioritizing checks on piping connectivity and heat exchanger efficiency. Maintaining clean condenser and evaporator surfaces and regularly replacing the dryer filter are fundamental to maintaining system pressure balance.

Content Block 3: Bus AC Pressure Problems—Sensor and Actuator Malfunctions

Current Situation: Modern bus air conditioning systems commonly employ electronic control. Distorted pressure sensor signals, malfunctioning expansion valves (or throttle bodies), and fan relay failures can disrupt the system’s control logic, leading to pressure regulation failure.

Analysis: Electrical faults are often insidious. For example, pressure sensor drift can send incorrect pressure signals to the ECU, causing misadjustments in the expansion valve opening and resulting in abnormal pressure. Zhang Hua, a specially appointed expert from the China Society of Automotive Engineers, emphasizes: “After failing to troubleshoot mechanical and piping issues, the focus should be on using a diagnostic tool to read the pressure sensor data stream and ECU control commands to distinguish between a ‘real pressure fault’ and a ‘signal illusion.'”

Conclusion: Electrical control faults are the core challenge in modern air conditioning pressure problems. Accurate diagnosis relies on specialized electronic diagnostic tools to systematically verify sensors, actuators, and control logic.

Bus AC pressure problems

Content Block 4: Bus AC Pressure Problems – Refrigerant Medium

Current Problem: Inaccurate refrigerant charge (too much or too little) or the introduction of non-condensable gases such as air and moisture are typical non-hardware causes of abnormal pressure.

Problem Analysis: Excessive refrigerant leads to excessively high condensing pressure; insufficient refrigerant leads to excessively low evaporating pressure and reduced cooling capacity. The introduction of non-condensable gases such as air causes abnormally high pressure and a decreased pressure differential, resulting in a sharp decline in compressor efficiency. This is usually related to improper maintenance operations.

Problem Conclusion: Strictly adhering to the manufacturer’s specified refrigerant type, charge quantity, and vacuuming operation procedures is the fundamental prerequisite for ensuring normal system pressure. Combining pressure gauge readings with sight glass observation is an effective means of diagnosing medium problems.

Content Block 5: Bus AC Pressure Problems – Dynamic Load and Environmental Stress

Current Problem: The bus operating environment is complex and variable. Prolonged idling cooling, extreme high temperatures, and operation at high altitudes all place extreme demands on system pressure.

Problem Analysis: At idle, low engine speed results in insufficient compressor and condenser fan speed, easily triggering high-pressure protection. Low air pressure at high altitudes affects condensation efficiency and pressure switch activation points. These dynamic factors necessitate a wider safety margin and more adaptable system design.

Problem Conclusion: Pressure fault diagnosis must be combined with the vehicle’s actual operating conditions. Pressure protection occurring under specific harsh scenarios may require targeted system enhancements or operational procedure adjustments, rather than simply attributing it to system malfunction.

Content Block 6: Bus AC Pressure Problems – Prevention and System Deficiencies

Current Situation: The lack of regular, professional, and systematic maintenance is the underlying cause of the cumulative outbreak of most pressure problems. This covers the inspection and maintenance of all the sub-items mentioned above.

Problem Analysis: The air conditioning system is a closed-loop system; any lack of maintenance in any sub-item will ultimately be reflected in pressure indicators. Fragmented “add refrigerant when it’s low” repairs cannot solve the root problem and may even introduce new faults.

Conclusion: Establishing a regular preventative maintenance system with pressure testing as the core indicator is crucial. The China Academy of Transportation Sciences (CATS) recommends in its “2025 White Paper on Road Passenger Vehicle Operation and Maintenance” that “bus air conditioning systems should undergo comprehensive pressure performance testing and maintenance annually before seasonal changes, creating health records and shifting from emergency repairs to predictive maintenance.”

Summary: Bus air conditioning pressure issues are not isolated phenomena; they are the result of the combined effects of six dimensions: mechanical, hydraulic, electrical, media, environmental, and maintenance. The solution lies in adopting a systems perspective and following a complete diagnostic logic chain from media to components, from mechanical to electrical, and from static testing to dynamic scenarios. Only through comprehensive and meticulous management and preventative maintenance can we ensure that bus air conditioning systems continuously and stably output coolness and comfort under various complex environments.