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TECH TIPS

FORD A/C CLUTCH

If the engine is running hot, or if there is an engine misfire, the PCM will not allow the A/C clutch to come on. It monitors the throttle position sensor and shuts down the A/C clutch if it detects a wide open throttle. It shuts down the A/C clutch if it detects the idle is too low. These are all basic inputs that have been around for years.

Check Hood Weatherstrip

On some vehicles, a lack of power under heavy load (with no DTCs) might be caused by a loose or detached rubber weatherstrip on the underside of the hood, which could cause a restriction to the air intake. A perceived drivability problem could be fixed by this simple fix.

In recent years, Ford has added additional inputs that can also shut down the A/C clutch. On some models, they have replaced the high pressure switch with an air conditioning pressure (ACP) sensor that actually measures refrigerant pressure instead of a basic pressure switch that opens up at high refrigerant pressures.

Ford has also added an A/C evaporator temperature (ACET) sensor. This sensor monitors the evaporator core temperature. If the PCM detects the evaporator core is freezing up, it will shut down the A/C compressor. Both the ACP and the ACET sensors are highly overlooked, even though their data can be read on most scan tools. The ACET sensor may also be referred to as an evaporator air discharge temperature (EADT) sensor.

To test, monitor both sensors on scan data. If your scan tool does not show the parameter identification (PIDs), use a voltmeter and monitor the sensor voltage at the sensor or preferably at the PCM. If the ACET sensor voltage rises to 4.0 volts or higher, the PCM may interpret that the evaporator core is freezing up and will shut off the compressor.

The ACP sensor has a range of zero to 4.9 volts. Roughly speaking, 1 volt = 100 psi; 2 volts = 200 psi; 3 volts = 300 psi, etc. If the PCM sees high voltage from this sensor, again it will shut the A/C compressor down. If the actual A/C pressures are OK and the evaporator core temperature is not freezing, suspect that one of these two sensors is preventing the A/C compressor from coming on.