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

### Circuit Description

The PCM uses the 5 Volt Reference B circuit as a sensor feed for the A/C Refrigerant 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.

• | Above condition for longer than 10 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

Important: : Inspect the PCM and engine grounds for being secure and clean.

Check for the following conditions:

• | Poor connection at the PCM.

| Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.

• | Damaged harness.

| Inspect the wiring harness for damage. If the harness appears to be OK, observe the A/C High Side Pressure display on the scan tool while moving connectors and wiring harnesses related to the A/C refrigerant pressure sensor. A change in the A/C High Side Pressure display will indicate the location of the malfunction.

Reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.

### Test Description

Number(s) below refer to the step number(s) on the Diagnostic Table.

1. This step is checking for a short to voltage on the 5 volt ref line.

1. This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is being replaced, the new PCM must be programmed. Refer to PCM Replacement/Programming

#### DTC P1639 - 5 Volt Reference B Circuit

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | Go to Step 2 | Go to Powertrain On Board Diagnostic (OBD) System Check
2 | Visually/physically check the PCM and engine grounds. Refer to Ground Distribution in Wiring Repairs Was a problem found? | -- | Go to Step 12 | Go to Step 3
3 |

1. Disconnect the A/C refrigerant pressure sensor.
2. Using a DMM, measure voltage between the 5 volt reference B circuit and the sensor ground circuit.

Is voltage greater than the specified value?

5.5 V

Go to Step 4

Go to Step 6

4 |

1. Turn OFF the ignition switch.
2. Disconnect the PCM.
3. Turn ON the ignition switch.
4. Measure voltage between the 5 volt reference B circuit and the PCM ground circuit at the PCM harness connector.

Is voltage greater than the specified value indicated?

0 V

Go to Step 5

Go to Step 10

5 | Locate and repair short to voltage in the 5 volt reference B circuit. Is action complete? | -- | Go to Step 12 | --
6 | Is voltage less than the specified value indicated? | 4.5 V | Go to Step 8 | Go to Step 7
7 |

1. Check for a short to voltage in the A/C refrigerant pressure sensor signal circuit wiring.
2. If a problem is found, repair as necessary. Refer to Wiring Repairs .

Was a problem found?

--

Go to Step 12

Go to Step 9

8 |

1. Turn OFF the ignition switch.
2. Disconnect the PCM.
3. Check the 5 volt reference B circuit for a short to ground or to the sensor ground circuit.
4. If a problem is found, repair as necessary. Refer to Wiring Repairs .

Was a problem found?

--

Go to Step 12

Go to Step 10

9 |

1. Turn ON the ignition switch with the A/C refrigerant pressure sensor disconnected.
2. After at least 10 seconds, check for DTC P1635.

Is DTC P1635 set?

--

Go to Step 10

Go to Step 11

10 | Important:: The replacement PCM must be programmed. Refer to Powertrain Control Module Replacement/Programming . Replace the PCM. Is action complete? | -- | Go to Step 12 | --
11 | Replace the A/C refrigerant pressure sensor. Refer to A/C Refrigerant Pressure Sensor Replacement . Is action complete? | -- | Go to Step 12 | --
12 |

1. Review and record scan tool Fail Records data.
2. Clear DTCs.
3. Operate the vehicle within Fail Records conditions.
4. Monitor Specific DTC info for DTC P1639 until the DTC P1639 test runs, using a scan tool.

Does the scan tool indicate DTC P1639 failed this ignition?

--

Go to Step 2

System OK