Diagnosing issues within a car’s air conditioning (AC) system often begins with understanding gauge pressure readings. Pressure gauges are indispensable tools for automotive technicians, providing crucial insights into the health and operation of the AC system. This article serves as a comprehensive Car Ac Diagnosis Chart, detailing common gauge readouts and their corresponding interpretations, enabling effective AC troubleshooting.
Understanding Car AC Gauge Readings for Diagnosis
To effectively use a car AC diagnosis chart, it’s essential to first understand how to interpret gauge readings. These readings, typically taken from the high and low pressure sides of the AC system, provide valuable data about refrigerant levels, system pressure, and component functionality. This guide will walk you through various scenarios you might encounter while using your gauges for AC diagnosis, helping you pinpoint problems accurately.
Normal Conditions
Air in Cooling System (Insufficient Suction)
Defective Expansion Valve
Excessive Refrigerant
Moisture Intrusion
Refrigerant Does Not Circulate
Summary Table of Gauge Diagnostics
Normal Conditions
Under normal operating conditions, when the AC system is functioning correctly, the gauge readings should fall within a specific range.
Conditions:
- Ambient Temperature: 86 – 96°F (30-38°C)
- Engine Speed: Approximately 1500 rpm (or about 75% of rated engine speed)
Expected Gauge Readings:
- Low Side Pressure: Typically ranges from 25 to 45 psi.
- High Side Pressure: Typically ranges from 200 to 300 psi.
These readings indicate a balanced and efficient AC system operation. If your readings deviate from these norms, it signals a potential issue that requires further investigation using the car AC diagnosis chart below.
Air in Cooling System (Insufficient Suction)
The presence of air within the AC system is a common problem that can significantly impair its performance. When air is trapped in the system, it leads to elevated pressure readings on both the high and low sides.
Pressure Range:
- Low Side: 36 to 50 psi (Higher than normal)
- High Side: 284 to 356 psi (Higher than normal)
Indications:
- Both pressures are TOO HIGH compared to normal operating pressures.
- The low-side piping is not as cold as expected, indicating reduced cooling efficiency.
Causes:
- Air in the system: Air contamination is the primary cause. This can occur due to leaks, improper evacuation during servicing, or system component failures allowing air ingress.
Remedy:
- Evacuate the system: Use a vacuum pump to thoroughly remove air and moisture from the AC system.
- Replenish the refrigerant: Recharge the system with the correct type and amount of refrigerant as specified by the vehicle manufacturer.
- Check the gauge readings: After recharging, monitor the gauge readings to ensure they return to normal operating ranges.
- Inspect for leaks: Identify and repair any leaks in the system that may have allowed air to enter.
- Replace liquid tank (receiver dryer): If the system has been operating with air contamination for an extended period, the receiver dryer may become saturated and require replacement to ensure proper system operation.
Defective Expansion Valve
The expansion valve plays a crucial role in regulating refrigerant flow and pressure within the AC system. A malfunctioning expansion valve can lead to abnormal pressure readings, often indicating higher than normal pressures on both sides.
Pressure Range:
- Low Side: 30 to 36 psi (Slightly higher than normal)
- High Side: 313 to 327 psi (Higher than normal)
Indications:
- Both pressures are TOO HIGH compared to normal, although sometimes less dramatically high than with air in the system.
Causes:
- Improper refrigerant charge: Overcharging the system can cause elevated pressures.
- Defective expansion valve: A valve that is stuck or malfunctioning can restrict refrigerant flow or cause pressure imbalances.
- Improper temperature sensor installation: If the temperature sensor for the expansion valve is incorrectly installed or poorly insulated, it can send incorrect signals, leading to valve malfunction.
Remedy:
- Verify refrigerant charge: Ensure the refrigerant level is correct and not overcharged. Recover and recharge to the specified amount if necessary.
- Check temperature sensor installation and insulation: Inspect the temperature sensor for proper placement and insulation. Correct any installation issues.
- Replace the expansion valve: If the refrigerant charge is correct and the temperature sensor is properly installed, a defective expansion valve is likely. Replace the expansion valve to restore proper system function.
Excessive Refrigerant
Overcharging the AC system with refrigerant can lead to excessively high pressures, particularly on the high side. This condition strains the system and can reduce cooling efficiency.
Pressure Range:
- Low Side: 36 to 43 psi (Slightly higher than normal)
- High Side: 327+ psi (Significantly higher than normal)
Indications:
- Both pressures are TOO HIGH, with the high side being exceptionally elevated.
Causes:
- Pressure increase due to excessive refrigerant: The system is simply overfilled with refrigerant.
- Insufficient condenser cooling: If the condenser is not effectively dissipating heat, it can contribute to high-side pressure increases, exacerbating the issue of excessive refrigerant.
Remedy:
- Verify the refrigerant level: Use gauges to confirm the high pressure reading and suspect overcharge.
- Clean the condenser: Ensure the condenser fins are clean and free of debris that could impede airflow and heat exchange.
- Check and adjust the fan belt and / or the condenser fan motors: Verify that the condenser fan is operating correctly and at the appropriate speed to provide adequate cooling. Repair or replace fan components as needed.
- Recover excess refrigerant: Use refrigerant recovery equipment to carefully remove excess refrigerant until the system pressures return to normal operating ranges. Charge to the correct refrigerant levels as per manufacturer specifications.
Moisture Intrusion
Moisture in the AC system is highly detrimental. It can lead to the formation of ice crystals, which can block refrigerant flow, particularly at the expansion valve. This often results in fluctuating and abnormal pressure readings.
Pressure Range:
- Low Side: 18 to 28 psi (Lower than normal, and fluctuating)
- High Side: 85 to 256 psi (Significantly lower and more unstable than normal high side readings)
Indications:
- Low pressure side alternates between a vacuum and normal pressure: This fluctuation is a key indicator of moisture freezing and thawing within the system.
Causes:
- Moisture has frozen in the air conditioner system, clogging the expansion valve: Water entering the system can freeze at the expansion valve, restricting refrigerant flow and causing pressure variations.
Remedy:
- Evacuate the system: Thoroughly evacuate the system to remove moisture. A deep vacuum is necessary to boil off and remove all traces of water.
- Replace the receiver dryer: The receiver dryer (or accumulator in some systems) is designed to absorb moisture. If moisture intrusion has occurred, the dryer is likely saturated and must be replaced.
- Recharge the system: Recharge the system with the correct refrigerant type and amount, ensuring the system is properly sealed and leak-free to prevent future moisture entry.
Refrigerant Does Not Circulate
When refrigerant circulation is impeded, it can lead to a range of pressure abnormalities, often characterized by a vacuum on the low side and a lower than expected high-side pressure. This scenario indicates a significant blockage or restriction within the AC system.
Pressure Range:
- Low Side: 0 to -29.99 psi (Vacuum – significantly lower than normal)
- High Side: 71 to 85 psi (Much lower than normal)
Indications:
- Low pressure side becomes a vacuum.
- High pressure is significantly lower than normal.
- Frost or condensation on the front and rear pipe connections of the receiver dryer or expansion valve: This indicates a temperature drop at the point of restriction due to rapid expansion of refrigerant trying to pass the blockage.
Causes:
- AC system is blocked by contamination or ice: Debris, sealant, or frozen moisture can block refrigerant flow.
- AC system is shut off by defective expansion valve or temperature sensor: A completely blocked expansion valve or a sensor erroneously shutting off refrigerant flow.
- AC line from evaporator to compressor has a kink in the line: A physical obstruction in the refrigerant lines.
Remedy:
- STOP OPERATION IMMEDIATELY: Continued operation can damage the compressor.
- Check for contamination or ice: Inspect the system for visible blockages or signs of ice.
- Evacuate the system (if moisture is found): If moisture is suspected, evacuate the system and replace the receiver dryer.
- Replace the expansion valve (if defective): If the expansion valve is suspected to be the blockage, replace it.
- Replace receiver dryer: Always replace the receiver dryer after opening the system, especially if contamination or moisture is suspected.
- Recharge with refrigerant to proper level: Recharge the system after repairs, ensuring correct refrigerant type and amount.
- Repair any kinked hoses: Inspect and repair or replace any kinked refrigerant lines to ensure unrestricted flow.
Summary Table of Gauge Diagnostics
This car AC diagnosis chart summarizes common issues based on high and low side pressure readings, offering quick diagnostic guidance.
Low Pressure Side is TOO HIGH
Generally, a high low-side pressure often accompanies a high high-side pressure.
| Cause | Inspection | Remedy |
|---|---|---|
| Defective Thermoswitch | The magnetic clutch switch turns off before the outlet air temperature is sufficiently low. | Replace the thermoswitch |
| Poor Expansion Valve temperature sensor contact | The high and low pressure sides gauge pressures equalize when the magnetic clutch is turned off (within a short time duration). | Replace Compressor (This symptom may indicate internal compressor leakage or inefficiency) |
| Expansion Valve opens too far | Frost has adhered to the suction hose / pipe. | Install the temperature sensor against the low pressure pipe and cover with insulating tape. |
| Clogged compressor suction filter | Compressor fitting is cool but the low pressure hose is not. | Remove and clean the filter. If filter is excessively clogged, consider compressor replacement. |
Low Pressure Side is TOO LOW
| Cause | Inspection | Remedy |
|---|---|---|
| Insufficient Refrigerant | There is little temperature difference between the low and high pressure sides. | Repair any leaks and recharge the refrigerant to the correct levels. |
| Clogged liquid tank (receiver dryer, accumulator) | Considerable temperature difference between the inlet and outlet sides of the receiver dryer during operation. Also, tank may be frosted. | Replace liquid tank (receiver dryer). |
| Clogged expansion valve | Expansion valve inlet side is frosted. Generally, the low side pressure is near vacuum. | Replace expansion valve. |
| Expansion valve temperature sensor gas leak | Expansion valve outlet side is chilled and low pressure side is LOW. (Damaged capillary tube, etc.) | Clean or replace expansion valve. |
| Clogged or blocked piping | Low pressure readings decrease or a negative reading is shown. Indicates piping is clogged or blocked between the evaporator coil and the compressor. A frost spot may indicate the point of blockage. | Clean or replace piping. |
| Defective thermoswitch (cold control) | Evaporator is frozen showing ice along the face of the coil, not just frost on the manifolds of the coil. | Adjust temperature sensing tube to a cooler part of the coil or replace the thermoswitch. |
High Pressure Side is TOO HIGH
| Cause | Inspection | Remedy |
|---|---|---|
| Poor Condenser cooling | Dirty or clogged condenser fins, Cooling fans do not operate correctly. | Clean and / or repair the condenser core / fans. |
| Excessive refrigerant | Verify by gauge reading. High side significantly elevated. | Utilize your refrigerant recovery equipment to capture excess refrigerant. Charge to the correct refrigerant levels. |
| Air in the system | Pressure is high on both high and low sides. | Evacuate, vacuum, and recharge with refrigerant. |
High Pressure Side is TOO LOW
| Cause | Inspection | Remedy |
|---|---|---|
| Insufficient refrigerant | Little temperature difference between the low and high pressures. | Repair any leaks and recharge the refrigerant to the correct levels. |
This car AC diagnosis chart provides a starting point for troubleshooting. Always consult vehicle-specific service manuals for detailed procedures and specifications. Proper diagnosis using pressure gauges and this chart can significantly streamline AC system repairs, ensuring efficient and effective outcomes.
