For 1990-2009 cars

### Circuit Description

The powertrain control module (PCM) uses the 5-volt reference 2 circuit as a sensor feed for the A/C refrigerant pressure sensor. The PCM monitors the voltage on the 5-volt reference 2 circuit. If the voltage is out of tolerance, the PCM will set DTC P1639.

### Conditions for Running the DTC

The engine must be running.

### Conditions for Setting the DTC

• | The PCM detects a voltage out of tolerance condition on the 5-volt reference 2 circuit.

• | Above condition is present for longer than 10 seconds.

### Action Taken When the DTC Sets

• | The PCM will illuminate the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test has been run and failed.

• | The PCM will store conditions which were present when the DTC set as Freeze Frame/Failure Records data.

### Conditions for Clearing the MIL/DTC

• | The PCM will turn OFF the malfunction indicator lamp (MIL) during the third consecutive trip in which the diagnostic has run and passed.

• | 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.

### Test Description

The number below refers to the step number on the diagnostic table.

1. The ignition must be turned OFF and the PCM powered down or the test may become inaccurate.

Step | Action | Values | Yes | No
--- | --- | --- | --- | ---
Schematic Reference: Engine Controls Schematics
1 | Did you complete the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | Go to Diagnostic System Check - Engine Controls
2 |

1. Observe the Freeze Frame/Failure Records data for this DTC.
2. Turn OFF the ignition for 30 seconds.
3. Start the engine.
4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text or as close to the Freeze Frame/Failure Records data that you observed.

Does the DTC fail this ignition?

--

Go to Step 3

Go to Intermittent Conditions

3 |

1. Visually and physically inspect the PCM and engine grounds. Ensure that the grounds are clean and secure.
2. If a problem is found, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 4

4 |

1. Turn OFF the ignition.
2. Disconnect the A/C pressure sensor connector.
3. Connect a DMM between the 5-volt reference and a good ground.
4. Turn ON the ignition, with the engine OFF.

Does the voltage measure near the specified value?

5 V

Go to Step 10

Go to Step 5

5 | Does the voltage measure more than the specified value? | 5 V | Go to Step 6 | Go to Step 7
6 |

1. Turn OFF the ignition.
2. Disconnect the PCM and check for a short to voltage on the 5-volt reference circuit and repair as necessary. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and repair a problem?

--

Go to Step 12

Go to Step 11

7 | Important: The ignition must me turned OFF for this test.

1. Turn OFF the ignition.
2. Using a DMM check for continuity between the 5-volt reference circuit and the low reference circuit at the A/C pressure sensor connector. Refer to Circuit Testing in Wiring Systems.

Does the resistance measure at the specified value?

∞ or OL

Go to Step 8

Go to Step 9

8 |

1. Leave the ignition OFF.
2. Using a DMM check for continuity between the 5-volt reference circuit and a good chassis ground. Refer to Circuit Testing in Wiring Systems.

Does the resistance measure at the specified value?

∞ or OL

Go to Step 11

Go to Step 9

9 | Repair the circuit for a short to chassis ground or a short to A/C pressure sensor low reference. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 12 | --
10 | Replace the A/C pressure sensor. Refer to Air Conditioning (A/C) Refrigerant Pressure Sensor Replacement in Heating, Ventilation, and Air Conditioning. Did you complete the replacement? | -- | Go to Step 12 | --
11 | Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? | -- | Go to Step 12 | --
12 |

1. Use the scan tool in order to clear the DTCs.
2. Turn OFF the ignition for 30 seconds.
3. Start the engine.
4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC run and pass?

--

Go to Step 13

Go to Step 2

13 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | Go to Diagnostic Trouble Code (DTC) List | System OK