Pinpoint Test L : The A/C Is Inoperative: Notes

WARNING: This page is about a different car, the 2015 Lincoln MKT. However, it is still accessible from the selected car via links, so may be relevant.

Diagnostic Overview

Diagnostics in this article assume a certain skill level and knowledge of Ford-specific diagnostic practices. Refer to Diagnostic Methods in Service Information for information about these practices.

Refer to Automatic Climate Control System , for schematic and connector information.

Normal Operation and Fault Conditions

When the A/C is requested, the FCIM sends the button press message over the I-CAN to the IPC. The IPC sends the button press message to the BCM over the HS-CAN. The BCM sends the button press message to the HVAC module over the MS-CAN. The HVAC module sends an A/C request message through the MS-CAN to the BCM and the BCM sends the A/C request through the HS-CAN to the PCM.

Voltage is provided to the A/C clutch relay coil and switch contacts. When A/C is requested and A/C line pressures allow, a ground is provided to the A/C clutch relay coil from the PCM, energizing the A/C clutch relay. When the PCM energizes the relay, voltage is supplied to the A/C compressor clutch field coil from the relay. Ground is supplied for the A/C compressor clutch field coil.

The evaporator temperature sensor is used for A/C compressor cycling. An accurate evaporator temperature is critical for compressor engagement. The PCM uses the temperature measurement to turn the A/C compressor off before the evaporator temperatures are cold enough to freeze the condensation on the evaporator core.

When an A/C request is received by the PCM, the A/C compressor clutch only engages through the A/C clutch relay if all of the following conditions are met:

The HVAC control module does not detect an evaporator temperature below approximately 1 °C (33.8 °F).
The PCM does not detect excessively high or low refrigerant pressure from the A/C pressure transducer.
The PCM does not detect excessively high engine coolant temperature.
The PCM does not detect an ambient air temperature below approximately 0 °C (32 °F).
The PCM does not detect a temperature sensor reading that does not correlate with the other temperature sensor readings.
The PCM has not detected a WOT condition.

Possible Sources

Fuse
Wiring, terminals or connectors
A/C system discharged or low refrigerant charge
Network concerns
Evaporator temperature sensor
A/C pressure transducer
FCIM
A/C compressor clutch field coil
A/C compressor clutch air gap
A/C clutch relay

Visual Inspection and Diagnostic Pre-checks

Make sure the AAT AAT sensor reading and the other temperature sensor readings are similar when the engine is cold and the vehicle has not been in direct sun light.