A/C Warm at Idle, Cold at Speed
A 2019 compact SUV with 74,000 miles uses R-1234yf. On a 95°F day at 60% humidity, vent temperature is 68°F at idle and 48°F at 2,000 rpm. At idle, low-side pressure is 55 psi and high-side is 190 psi. At 2,000 rpm, low side is 38 psi and high side is 230 psi. The condenser electric fan runs, but scan-tool command is 90% while measured fan current is 6 A; the manufacturer’s spec is 12 A at 100%. Condenser fins are clean, the cabin filter is new, and the blend door moves through its full range. The system has not been serviced recently and the refrigerant charge is unknown. No A/C DTCs are stored.
1. Use the pressure and vent-temperature data to decide whether the system is undercharged, overcharged, or airflow-limited. Show the comparison at idle versus 2,000 rpm.
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The system is not behaving like a simple undercharge. At idle the low side is high at 55 psi and the vent is warm at 68°F; at 2,000 rpm the low side drops to 38 psi and the vent improves to 48°F. An undercharged system usually shows low low-side and low high-side pressures at both speeds, and the vent does not improve this much with rpm. An overcharge usually shows high high-side pressure at both speeds; 190 psi at idle and 230 psi at 2,000 rpm are not extreme for 95°F. The pattern is airflow-limited at idle: the condenser cannot reject heat until ram air or higher fan speed helps. The fan command is 90% but current is only 6 A against a 12 A spec, so the fan circuit is the first suspect.
2. How would you test the condenser fan circuit without throwing parts? Include command, current, and voltage checks and expected results.
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Command the fan to 100% with the scan tool and measure command, supply voltage, ground voltage drop, and current. If command is 100% and current is still about 6 A, the motor or its internal control is weak; a direct battery feed to the fan should produce full speed and near 12 A. If direct feed produces full speed but the scan-tool command does not, check the PWM signal, relay, and ground. A voltage drop of more than 0.5V on either side is excessive. Also verify that the fan blade is not loose and that the condenser is not blocked. I would not recharge the system until the fan is proved good and the system is retested at idle with A/C on.
3. The customer wants a quick recharge. What is the risk, and what evidence would justify opening the refrigerant circuit?
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A quick recharge is risky because the pressures do not show a clear undercharge and the system has an identified airflow/fan defect. Adding refrigerant could overcharge the system, raise high-side pressure, and reduce cooling further, while masking the real cause. It also releases no diagnostic information because the charge is unknown. I would open the refrigerant circuit only after the fan repair and a repeat performance test still show low vent temperature with low suction pressure, or if a leak test and charge-weight check show a loss. At that point, recover, evacuate, weigh in the specified charge, and verify pressures and vent temperature at idle and 2,000 rpm.