AC warms at higher speeds

[Pete3323]

Hello and thanks in advance for any input. I have a 1998 Mustang Cobra. The AC works great if I’m just putting around town. Once I get on the freeway I lose the cold air very quickly. Starts blowing warm. Any idea what would cause this? I don’t think it needs recharge as it works fine and plenty cold at low speeds. I’m stumped!

[tbirdtbird]

Sudden or gradual onset?
Still could be low on charge.
Be certain that the heater function is disabled when you are on AC, there are water valves and flap doors to control this.
The refrigerant pipe exiting the evaporator should be cold, as should be the input pipe.

You may need to put a set of gauges on it and get some readings. If you get readings post the hi and lo here at 1500 RPM
Do you have a set of manifold gauges? You can also borrow them from Autozone, etc

[bohica2xo]

Check all of the vacuum hoses and check valves underhood.

Loss of vacuum signal to the control head on older Ford products lets the system vacuum motors return to default locations. Usually a defrost setting.

My old F150 had a bad check valve. It would blow out the defrost vents at WOT on hills.

[Cusser]

Check all of the vacuum hoses and check valves underhood.
Loss of vacuum signal to the control head on older Ford products lets the system vacuum motors return to default locations. Usually a defrost setting.

I had a 1984 Jeep Cherokee and the 3 vacuum "motors" themselves that controlled air flow system went bad at various times.

[Dougflas]

I agree it may be low on charge. The most common problem is proper charging and it is the easiest to check. Once the charge is verified, then on to other causes.