A/C gauges tell you a lot once you know the patterns. The key is looking at the high side and low side together, then comparing them to the chart for the outside temperature.
Here are the patterns that come up over and over, on the job and on the A7.
Always check the ambient temperature
Normal pressures go up with outside temperature and humidity. You can't judge a reading without knowing the conditions, so compare to the chart at the temperature you measured.
Both sides low
Usually low charge. There isn't enough refrigerant, so the evaporator runs short. On an orifice tube system with a cycling clutch, the low side drops to the cut-out point fast, so the clutch cycles on and off a lot.
Both sides high
If condenser airflow checks out, think overcharge. Extra liquid backs up in the condenser and cuts its capacity, which pushes both sides up. Recover it, weigh it, and recharge to spec.
Also check the condenser fan. If the high side is high at idle but cooling gets better at highway speed, the fan probably isn't running. Ram air helps on the road, but at idle there's nothing moving air through the condenser.
Low side in a vacuum
Something's blocking flow. If the high side is above the chart and frost starts right at the orifice tube, the orifice tube is restricted.
If it cools fine for a while, then quits, and the low side drops into a vacuum, it may be moisture freezing at the orifice tube. Once the ice melts, it works again.
Low side high, high side low
If raising engine speed barely changes anything, the compressor isn't pumping. Worn valves or internal damage. Confirm the clutch is actually driving it first.
Restrictions show up as cold spots
Refrigerant cools fast when it drops pressure through a restriction. So a sudden temperature change, often with frost, marks the spot. A receiver-drier that's hot going in and frosty coming out is restricted.
Cheat sheet
- Always compare to the chart for the outside temperature
- Both low: low charge
- Both high: overcharge or a condenser airflow problem
- Low side in a vacuum: restriction or moisture freezing
- Low side high, high side low, no change with rpm: compressor not pumping
- Frost or a sudden cold spot marks a restriction
Common questions
What does low pressure on both sides mean?
Usually a low refrigerant charge.
Why is my AC high side pressure high at idle only?
The condenser isn't getting enough airflow at idle, often because the condenser fan isn't running.
What does frost on an AC line mean?
A sudden cold or frosty spot usually marks a restriction, where the refrigerant is dropping pressure.
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