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Q: AC system

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On this 1998 Ford F150 the engine has been rebuilt so the miles that it says on here are the frame miles. On the truck there was a weird sound as if something was rattling, so I looked in the engine compartment and the AC clutch was bad. I replaced the AC clutch and when I turned the truck on and turned the AC on, the clutch continually moves. But once I turn off the air-conditioner (with the truck still running) the AC clutch disengages. I hooked an AC manifold gage to the high side and low side. The low side said 40 and the high side said 30. The truck blows cold but the gage reading does not make sense to me unless it's the expansion valve. Should I just evacuate the system and put new freon in? The truck does not have r24 the truck has r134. Ultimately, what should I do?

My car has 386 miles.
My car has an automatic transmission.

If the low side said 40 and the compressor was running, then the high side should be above 175 or more. I think your gauge is not working correctly. Try an check the pressures with the engine off and cool them. The pressure should be about 10 to 15 above your ambient temperature if the system is full. This means it is 90 degrees outside, then you will read around 100 to 115 psi on the gauge with the engine off. When you start the truck, the low pressure should go down as the inside cools down. It should cycle around 24 psi. If you want to have a mechanic take a look at this in person, a certified technician from YourMechanic can come to your home or office to diagnose your AC system.

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