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.
Rule of thumb: 2.5 × ambient °F. Hot refrigerant leaving compressor. Pressure increases with ambient temperature. Excessive high pressure indicates condenser problems or overcharge.
10-35 psi at 70-110°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.
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).
| Ambient Temp | High-Side Pressure (2.5 × ambient °F) |
|---|---|
| 80°F | 200 psi |
| 90°F | 225 psi |
| 95°F | 238 psi |
| 100°F | 250 psi |
| 105°F | 263 psi |
| 110°F | 275 psi |
Before starting the machine, at typical ambient temperatures, the high and low sides of the AC system will be stabilized at pressures that are close to the ambient temperature. Higher than normal pressures at this point mean there is contamination in the system. Also, normal pressures at this point do not guarantee the system is properly charged — it only takes about 10% of a full system charge to reflect normal pressures when the system is off and stable.
Connect high side (red) to discharge port, low side (blue) to suction port. If using a manifold gauge, keep it clean and purged.
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 and fan operation.
| Low Side | High Side | Cycling | Suction & Discharge Lines | Duct Temperature |
|---|---|---|---|---|
| 10–35 PSI Pressure will be higher at higher cab temperatures. |
2.5x ambient High side pressure is typically 2.5 times the ambient temperature. |
Varies - controlled by MMC measuring evaporator temperature. More load = less cycling. | Suction line is cold and frosted. Discharge line is hot (too hot to hold on to). | 45°–53° |
| Low Side | High Side | Symptoms | Diagnosis | Solutions |
|---|---|---|---|---|
| Low | Low | Poor or no cooling. Foamy bubbles in the sight glass. Compressor cycles rapidly. Warm evaporator outlet line. | Low or improper refrigerant charge. | Check and repair any leaks in the system. Recharge system as needed. |
| Low or vacuum. | Low to normal. | No cooling. Sight glass is clear. Warm evaporator outlet line. | High side restriction. Bad expansion valve. | Look for icing on the high-side lines as an indication of restriction. Clear the restriction or replace necessary components. Inspect and replace the expansion valve and dryer. Evacuate and recharge the system. |
| Normal to low. | Normal | Unit works fine for a while, then begins to blow warm air. Evaporator pipes frozen. Compressor doesn't cycle. | Evaporator freeze-up. Water drains plugged. Bad MCC or bulb sensor is in the wrong place. | Clean water drains and evaporator. Correct the temp sensor or replace the MCC. |
| High or equal to high side gauge. | Low or equal to low side gauge. | No cooling. Warm evaporator outlet pipe. Compressor won't cycle. | Expansion valve stuck open. Bad compressor. | Repair or replace compressor. Replace expansion valve. Evacuate and recharge the system. |
| Normal to high. | High | Fair to poor cooling. Sight glass clear to foamy. Evaporator outlet cool to warm. | System overcharged. | Recover excess refrigerant until system operation returns to normal. |
| Normal to high. | High | Fair to poor cooling. Sight glass clear. Evaporator outlet cool to warm. | Engine overheating. Restricted airflow past condenser. | Check cooling system operation. Check cooling fan operation. Clear radiator or condenser restriction. |
Indicates a TXV that is stuck closed or a drastic restriction on the high side. Look for massive temperature drops on the high side — if none, remove and test the TXV.
Fluctuating 5-7 psi indicates a malfunctioning TXV or icing from water contamination.
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 (high ambient temperature, sitting in sun, heater valve open): 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, operating in a shop): The compressor may run for a short time and
then remain off for several minutes.
Notes:
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.