Pressure readings tell you what's happening inside the AC system. Learn to interpret high and low side pressures to diagnose problems quickly and accurately.
150-250 psi at 70-80°F ambient. Hot refrigerant leaving compressor. Pressure increases with ambient temperature. High pressure indicates condenser problems or overcharge.
25-45 psi at 70-80°F ambient. Cold refrigerant entering compressor. Pressure decreases with ambient temperature. Low pressure indicates undercharge or restrictions.
Normal operation at moderate temperatures. Both pressures should be stable with compressor cycling. Superheat 8-12°F, subcooling 10-15°F.
Higher pressures due to increased ambient heat. System works harder to reject heat. Expect slightly higher pressures with same cooling performance.
Maximum normal operating pressures. Higher ambient reduces cooling efficiency. Pressures may approach high-pressure switch limit (cuts out above 384 psi on high side).
Connect high side (red) to discharge port, low side (blue) to suction port. Purge hoses of air before connecting. Verify valve positions before opening.
Always note ambient temperature when reading pressures. Use pressure-temperature charts to determine if readings are normal for conditions.
Allow system to run 5-10 minutes before taking readings. Watch for pressure stabilization and compressor cycling patterns.
Take several readings over time. Note pressure changes with compressor cycling, fan operation, and engine RPM variations.
| Condition | Low Side | High Side | Sight Glass | Suction Line | Receiver/Drier | Liquid Line | Discharge Line | Discharge Air |
|---|---|---|---|---|---|---|---|---|
| Lack of Refrigerant | Very low | Very low | Clear | Slightly cool | Slightly warm | Slightly warm | Slightly warm | Warm |
| Loss of Refrigerant | Low | Low | Bubbles | Cool | Warm to hot | Warm | Warm to hot | Slightly cool |
| Compressor Failure | High | Low | Clear | Cool | Warm | Warm | Warm | Slightly cool |
| Condenser Malfunction | High | High | Clear to occasional bubbles | Slightly cool to warm | Hot | Hot | Hot | Warm |
| Expansion Valve Stuck Open | High | High or Normal | Clear | Cold — sweating or frosting heavily | Warm | Warm | Hot | Slightly cool |
| Expansion Valve Stuck Closed | Low | Low | Clear | Cold — sweating or frosting heavily at valve inlet | Warm | Warm | Hot | Slightly cool |
| Restriction Between Condenser and Expansion Valve | Low | Low | Clear | Cool | Cool or sweating or frosting | Cool or sweating or frosting | Hot to point of restriction | Slightly cool |
| Restriction Between Compressor and Condenser | High | High, normal, or low | Clear | Slightly cool to warm | Warm or hot | Warm or hot | Hot | Warm |
| Normal | Normal | Normal | Clear | Cool, possible sweat | Warm | Warm | Hot | Cool |
Bobcat systems have pressure switches that protect the compressor. Bobcat high-side pressure switch cuts out below 28 psi and above 384 psi. These switches will cause compressor cycling if pressures exceed limits.
Normal Weather (77–95°F): The AC compressor may cycle on and off every 5 to 45 seconds.
High Load (heavy traffic, sitting in sun): The compressor may run almost continuously, engaging for 60 seconds or longer with only very short off cycles of 10 to 20 seconds.
Low Load (cabin is cool, warmer temperature setting): The compressor may run for a minute or two and then remain off for several minutes.
These cycle times are general indicators and may vary based on the specific machine and its condition. If cycle times are significantly different from these averages, it may indicate a problem with the AC system that requires attention.
Protects compressor from running with insufficient lubrication. Cuts out when high-side pressure drops below 28 psi. Will cause rapid compressor cycling.
Protects system from overpressure damage. Cuts out when high-side pressure exceeds 384 psi. Usually indicates condenser airflow problem.
If compressor cycles more rapidly than normal (every 5–45 seconds in 77–95°F weather), check pressure switch operation. Low charge is most common cause of low-pressure switch cycling.
Don't confuse pressure switch operation with electrical problems. Verify pressures before replacing electrical components.
Connect gauge set to a running machine. Practice reading pressures while varying engine speed and fan operation. Record readings and compare to normal ranges for ambient conditions.