Your compressor might be going out, suggest to get it checked, your local a/c shop can take care of it, or your condensor might have a leak and when it is running on high it is putting enough for the "low" cool setting. We dont do much HVAC but thats my guesstimation.
Are you refering to the fan only comes on when the selector is put on High? If so, there is a resistor block which controls fan speed and it is probably bad.
This information came from a Ford Service DVD...
Blower Motor Switch Resistor
The heater blower motor switch resistor has the following features:
The assembly is located on the passenger side of the plenum assembly behind the glove compartment (06010).
Three resistance elements are mounted on the resistor board to provide four A/C blower motor speeds.
Depending on the heater blower motor switch position, series resistance is added or bypassed in the A/C blower motor circuit to decrease or increase A/C blower motor speed.
An overheating protective device (thermal limiter) will open the resistor coil circuit when the temperature reaches 105°-110°C (221°-230°F), interrupting the blower motor operation in all speeds except HI.
The thermal limiter cannot be reset and is not serviceable.
Looks like it is time to replace the resistor block. :bigthumbsup
I'm not sure what you mean here? Does the compressor only turn on when the fan is set to high, or does the fan only blow when its on high? The first is a pretty rare problem. If the fan only comes on when set to high, the switch is bad or the blower motor is, or wiring, most likely the switch/resistor block.
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