Diagnosing Non-Engaging A/C Clutch
Not as Easy as It Used to Be

Posted 8/11/2011
By Bill Strehle

Every summer, we get a rush of hotline calls because the A/C clutch will not come on. It used to be simple: Check the pressures, check power at the low- and high-pressure switches, unplug and jump the suspect pressure switch to see if the A/C clutch comes on. It is not that easy anymore. There are many inputs that the powertrain control module (PCM) looks at to decide if it will allow the A/C clutch to run.

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 (TPS) and shuts down the A/C clutch if it detects a wide-open throttle (WOT). It shuts down the A/C clutch if it detects that the idle is too low. These are all basic inputs that have been around
for years.

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 that the evaporator core is freezing up, it will shut down the A/C compressor. Both the ACP sensor and the ACET sensor 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 temp is not freezing, suspect that one of these two sensors is preventing the A/C compressor from coming on.

Bill Strehle is an Identifix Ford specialist. He is an ASE master with L1 certification.

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