For 1990-2009 cars
Engine Controls Components
Engine Data Sensors
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice

### Circuit Description

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

### Conditions for Running the DTC

The ignition is ON.

### Conditions for Setting the DTC

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

• | The above condition is present for longer than 30 seconds.

### Action Taken When the DTC Sets

The PCM stores conditions which were present when the DTC set as Failure Records only. This information will not be stored as Freeze Frame Records.

### Conditions for Clearing the MIL/DTC

• | The DTC becomes history when the conditions for setting the DTC are no longer present.

• | The history DTC clears after 40 malfunction free warm-up cycles.

• | The PCM receives a clear code command from the scan tool.

### Diagnostic Aids

Inspect for the conditions that might cause intermittents.

Many situations may lead to an intermittent condition. Perform each inspection or test as directed.

Important: : Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.

• | Loose terminal connection

- | Use a corresponding mating terminal to test for proper tension. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair.

- | Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair.

• | Damaged harness--Inspect the wiring harness for damage. If the harness inspection does not reveal a problem, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the scan tool display may indicate the location of the fault. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair.

• | Inspect the powertrain control module (PCM) and the engine grounds for clean and secure connections. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair.

If the condition is determined to be intermittent, reviewing the Snapshot or Freeze Frame/Failure Records may be useful in determining when the DTC or condition was identified.

### Test Description

The numbers below refer to the step numbers on the diagnostic table:

1. This step ensures that the replacement PCM, which utilizes an electrically erasable programmable read only memory (EEPROM), is programmed when installed.

Step | Action | Values | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the Powertrain On Board (OBD) System Check? | -- | Go to Step 2 | Go to Powertrain On Board Diagnostic (OBD) System Check
2 |

1. Disconnect the A/C pressure sensor.
2. Turn ON the ignition.
3. Connect a DMM between the 5 volt reference circuit and a good ground.

Does the voltage measure more than the specified value?

5.0 V

Go to Step 3

Go to Step 4

3 |

1. Turn OFF the ignition.
2. Disconnect the PCM.
3. Turn ON the ignition.
4. With the DMM still connected, measure voltage.

Does the voltage measure more than the specified value?

0.0 V

Go to Step 5

Go to Step 7

4 | Is voltage less than the specified value? | 4.9 V | Go to Step 6 | Go to Step 8
5 | Repair short to voltage in the 5 volt reference circuit. Refer to Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 9 | --
6 | Repair short to ground in the 5 volt reference circuit. Refer to Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 9 | Go to Step 7
7 | Important: The replacement PCM must be programmed. Replace the PCM. Refer to Powertrain Control Module Replacement/Programming . Did you complete the replacement? | -- | Go to Step 9 | --
8 | Replace the A/C pressure sensor. Refer to Air Conditioning (A/C) Refrigerant Pressure Sensor Replacement in HVAC. Did you complete the replacement? | -- | Go to Step 9 | --
9 |

1. Use the scan tool in order to clear the DTCs.
2. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC reset?

--

Go to Step 2

System OK