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Q: I have a 2000 Mercury Mountaineer and the AC only works if I'm going under 40-50mph

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The AC works great but only when I'm driving below 40ish mph, if I'm on the highway it blows really warm and the heater only blows cold air.

My car has 150000 miles.
My car has an automatic transmission.

Hi, thanks for writing in. I suggest checking the AC pressures to see if the refrigerant is full and check if the compressor cycles in and out at the proper pressures. The low pressure side should cycle out at about 24 psi. If the low side gets below that, then your evaporator may freeze causing no air flow through it and will blow warm or if the compressor cycles out and not back in at high speeds. The pressure cycling switch may be bad. If you would like help, consider having an expert automotive technician from YourMechanic come to your home or office to [https://www.yourmechanic.com/services/ac-is-not-working-inspection), and make or suggest any repairs as needed.

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