DTC P1639 5 Volt Reference 2 Circuit. Buick GL7 (Export) 2000

For 1990-2009 cars

### Circuit Description

The PCM uses the 5 volt reference B circuit as a sensor feed to the A/C pressure sensor and the fuel tank 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 condition is present for more 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

Inspect for the following conditions:

• | A poor connection at the PCM. Inspect harness connectors for the following conditions:

- | Backed-out terminals

- | Improper mating

- | Broken locks

- | Improperly formed or damaged terminals

- | Poor terminal-to-wire connections

• | A damaged harness. Inspect the wiring harness for damage. If the harness appears to be OK, observe the A/C pressure display on the scan tool while moving the connectors and the wiring harnesses related to the A/C pressure sensor.

• | Ensure the PCM grounds and the engine grounds are secure and clean.

Reviewing the Failure 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

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

1. This vehicle is equipped with a PCM which utilizes an electrically erasable programmable read-only memory (EEPROM). If the PCM is replaced, the new PCM must be programmed.

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 digital multimeter (DMM) between the 5 volt reference circuit and ground.

Is the voltage 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 the voltage.

Is the voltage more than the specified value?

0 V

Go to Step 5

Go to Step 7

4 | Is the voltage less than the specified value? | 4.9 V | Go to Step 6 | Go to Step 8
5 | Repair a 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 a 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 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