2014 Kenworth T680 - AC Blows Cold then Warm

Question

My AC is acting strange. When I first start the truck, it blows ice cold. After about 20-30 minutes of highway driving, it starts blowing humid, warm air. If I turn the AC off for 10 minutes and turn it back on, it works again for a little while.

I've checked the refrigerant levels with a manifold gauge set and they seem correct (30 low / 225 high at 85F ambient). The compressor clutch is still engaged even when it's blowing warm air. I'm stumped.

MisfireMatt · Accepted Answer

✅ Confirmed fix: It was the expansion valve (TXV) failing. It was sticking closed after it got to operating temperature, starving the evaporator of refrigerant.

On the T680, the TXV is located on the firewall. Replaced the valve, vacuumed the system for an hour, and recharged to factory spec (2.1 lbs). The vent temps now stay at a steady 42 degrees without icing up or cutting out.

ChevyChuck · Answer

Your evaporator is freezing up. This is usually caused by a faulty thermistor (evaporator temperature sensor) or low airflow. Since you checked the filter, check the sensor. If it doesn't tell the compressor to cycle off, the condensation turns to ice and blocks the airflow through the fins.

2014 Kenworth T680 - AC Blows Cold then Warm

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