No Cold Air After Evacuation and Recharge - Moisture and Leak Judgment
A 2016 medium-duty truck with automatic climate control had its compressor replaced at another shop three weeks ago. The customer returns with poor cooling and a musty smell. You find the low side pulling down to 20 psig and cycling rapidly, high side around 175 psig at 85°F ambient, and the accumulator is cold and sweating. The system was reportedly evacuated for 20 minutes and charged from a 30-lb cylinder without a scale. The A/C control head shows the setpoint at 60°F but the blend door actuator was recently replaced. A recovery machine was used that has not had its filter-drier changed in over a year. The shop is busy and the customer needs the truck tomorrow.
1. Which findings point to moisture/air in the system versus a blend-door or control problem, and what tests confirm each?
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Pointing to moisture/air/charge issues:
- Low side at 20 psig with rapid cycling and high side only 175 psig at 85°F ambient is low on charge or has a restriction; a properly charged R-134a system at 85°F should sit closer to 35–45 psig low and 200–230 psig high at idle.
- The accumulator being cold and sweating can be normal, but combined with low pressures it suggests a starved evaporator.
- Charging from a cylinder without a scale means the charge is unknown - a prime suspect.
- A recovery machine with a spent filter-drier can pull moisture and acid back into the system, so contamination is plausible.
Pointing to blend door/control:
- Setpoint at 60°F with a recently replaced actuator - verify the actuator is calibrated and the door actually moves to full cold. A scan tool can command the door and watch actuator position/feedback.
Confirming tests:
- Weigh in the exact charge after recovery - if pressures normalize, it was charge.
- Check for a restriction: temperature drop across the accumulator, orifice tube, or expansion valve; a large temperature split indicates a restriction.
- Command the blend door full cold and full hot and watch vent temperature change - if vent temp does not follow, it is a door/control issue.
- If moisture is suspected, check for acid with a test strip and inspect the orifice tube/expansion valve screen for debris or sludge.
Do not condemn the compressor until charge and blend door are verified.
2. You recover the charge and find only 14 oz in a system that specifies 32 oz, and the recovered refrigerant tests positive for acid. What is the correct repair sequence and what would you tell the customer about the recovery machine?
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Findings: The system is roughly half-charged (14 oz vs. 32 oz spec) and contaminated with acid. That explains the low pressures/rapid cycling; the acid explains the musty smell concern and raises a durability question for the new compressor.
Repair sequence:
- Do not reuse the recovered refrigerant. Recover into a dedicated contaminated/recovery cylinder and label it; acid-contaminated refrigerant must not go back into the system or a bulk cylinder.
- Flush or replace components as needed - at minimum replace the accumulator (it holds desiccant and moisture) and the orifice tube/expansion valve screen; inspect for debris.
- Verify the source of the leak that caused the low charge (leak test with electronic detector or dye) and repair it before recharging.
- Evacuate to a deep vacuum (typically below 500 microns, held for a stability check) to remove moisture.
- Recharge by weight to the 32 oz spec using a scale.
- Verify vent temperature, pressures, and clutch operation under load.
Customer communication: The recovery machine with a spent filter-drier may have introduced moisture/acid. Have the machine serviced (new filter-drier) before further use, and note that contaminated refrigerant from the prior repair is the likely source. Document the acid test and charge weights on the repair order.