P1540

DTC P1540

Circuit Description

The engine control module (ECM) or powertrain control module (PCM) monitors the high side refrigerant pressure through the A/C refrigerant pressure sensor. When the pressure is high, the signal voltage is high. When the pressure is low, the signal voltage is low. When pressure is high, the ECM/PCM commands the cooling fans ON. When pressure is too high or too low, the ECM/PCM will not allow the A/C compressor clutch to engage.

DTC Descriptors

This diagnostic procedure supports the following DTC:

DTC P1540 Air Conditioning (A/C) Refrigerant Overpressure - Air Conditioning (A/C) Disabled

Conditions for Running the DTC

* Engine running
* The A/C refrigerant pressure is greater than 2 951 kPa (428 psi).
* The ECM/PCM must be powered.

Conditions for Setting the DTC

Any of the conditions for running the DTC are met for more than 500 milliseconds.

Action Taken When the DTC Sets

The ECM/PCM disables the A/C clutch.

Conditions for Clearing the DTC

* When the A/C refrigerant pressure drops to less than 2,068 kPa (300 psi) the ECM/PCM will clear the current DTC and go to a history DTC. If the A/C is still requested, the ECM/PCM will then enable the A/C clutch.
* The DTC will become history if the ECM/PCM no longer detects a failure.
* The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
* The DTC can be cleared by using a scan tool.

Diagnostic Aids

* If DTC P1540 is stored as a history code it indicates that the ECM/PCM did receive a high A/C refrigerant pressure message. This code could also be caused by an intermittent short to voltage on the sensor signal circuit, which would cause the ECM/PCM to set DTC P0533. The code could be the result of an actual A/C refrigerant high-pressure condition.
* It is possible for the refrigerant system pressure to exceed 2,951 kPa (428 psi) during extended engine idle speed with A/C on when engine coolant temperatures are greater than 123°C (253°F). This could be caused by:

- An overcharged A/C system
- A failed A/C pressure sensor
- Blockage in a refrigerant line
- Debris in front of the A/C condenser
- Radiator or A/C condenser air shields missing or out of location
- Inoperative engine cooling fans
- Extended periods at idle in city traffic followed by rapid acceleration
- Aftermarket products affecting cooling system performance

* Refer to Testing for Intermittent Conditions and Poor Connections in Diagnostic Aids. Component Tests and General Diagnostics

Test Description

Step 1 - Step 8:

Step 9 - Step 17:

The numbers below refer to the step numbers on the diagnostic table.
4. Tests for the proper operation of the circuit in the low voltage range.
5. Tests for the proper operation of the circuit in the high voltage range. If the fuse in the jumper opens when you perform this test, the signal circuit is shorted to ground.
6. Tests for a short to voltage in the 5-volt reference circuit.
7. Tests for a high resistance or an open in the low reference circuit.