Circuit Description
The powertrain control module (PCM) uses the 5-volt reference B circuit as a sensor feed for the A/C refrigerant pressure sensor and for 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 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 more than 10 seconds. |
Action Taken When the DTC Sets
| • | The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails. |
| • | The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records. |
Conditions for Clearing the MIL/DTC
| • | The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail. |
| • | A current DTC, Last Test Failed, clears when the diagnostic runs and passes. |
| • | A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic. |
| • | Clear the MIL and the DTC with a scan tool. |
Diagnostic Aids
Notice: Use the connector test adapter kit J 35616-A for any test that
requires probing the following items:
• The PCM harness connectors • The electrical center fuse/relay cavities • The component terminals • The component harness connector
Important: Verify that the PCM grounds and the engine grounds are secure and clean.
Check for the following conditions:
| • | A poor connection at the PCM--Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal-to-wire connections. |
| • | A damaged harness--Inspect the wiring harness for damage. If the harness appears OK, observe the A/C pressure display on the scan tool while moving the connectors and the wiring harnesses related to the A/C refrigerant pressure sensor. A change in the A/C pressure display will indicate the location of the malfunction. Also move the harness and the connector for the fuel tank pressure sensor. |
Reviewing the Failure Records vehicle mileage since the diagnostic test last failed may help to determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.
If the problem is intermittent, refer to Intermittent Conditions .
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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A component that does not affect the reference voltage when it is reconnected is considered passed. When the last component is reconnected, disconnect one of the passed components in order to obtain the voltage measurement.
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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 | ||||
|---|---|---|---|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics | ||||||||
1 | Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | |||||
2 |
Was a problem found and corrected? | -- | Go to Step 11 | Go to Step 3 | ||||
3 | Are any other DTCs set? | -- | Go to the applicable DTC table | Go to Step 4 | ||||
4 |
Is the voltage within the specified range? | 4.5-5.5 V | Go to Step 5 | Go to Step 8 | ||||
Reconnect one disconnected component at a time while measuring the voltage on the 5-volt reference circuit at any one of the disconnected harness connectors. Does any component being reconnected cause the voltage to vary outside the specified range? | 4.5-5.5 V | Go to Step 6 | Go to Diagnostic Aids | |||||
6 | For the affected component:
Was a problem found and corrected? | -- | Go to Step 11 | Go to Step 7 | ||||
7 | Replace the affected component. Is the action complete? | -- | Go to Step 11 | -- | ||||
8 |
Was a problem found? | -- | Go to Step 9 | Go to Step 10 | ||||
9 | Repair the 5-volt reference circuit as necessary. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | Go to Step 11 | -- | ||||
Replace the PCM. Refer to Powertrain Control Module Replacement . Is the action complete? | -- | Go to Step 11 | -- | |||||
11 |
Does the DTC run and pass? | -- | Go to Step 12 | Go to Step 2 | ||||
12 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK | |||||
