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. |
| • | Above condition for longer than 10 seconds. |
Action Taken When the DTC Sets
| • | The PCM will not illuminate the malfunction indicator lamp (MIL). |
| • | The PCM will store conditions which were present when the DTC set as Failure Records data only. This information will not be stored as Freeze Frame data. |
Conditions for Clearing the MIL/DTC
| • | A History DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction. |
| • | The DTC can be cleared by using the scan tool Clear Info function. |
Diagnostic Aids
Check for the following conditions:
| • | Poor connection at PCM. Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection. Refer to Intermittents and Poor Connections Diagnosis . |
| • | 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 connectors and wiring harnesses related to the Fuel Tank pressure sensor. Refer to Wiring Repairs . |
| • | Inspect PCM and engine grounds for being secure and clean. Refer to Power and Grounding Component Views . |
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:
-
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 Powertrain Control Module Replacement/Programming .
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Was the Powertrain OBD System Check performed? | -- | ||
2 | Visually/physically check the PCM and engine grounds. Ensure that the grounds are clean and secure. Was a problem found? | -- | ||
3 | Select DTC info, Last Test Fail and note if any other DTCs are set. Are there any EVAP codes set? | -- | Go to Applicable DTC | |
4 |
Is the voltage greater than the specified value indicated? | 5.5V | ||
5 | Monitor DMM connected between the 5 Volt Reference B circuit and the PCM ground circuit while disconnecting the Fuel Tank pressure sensor, and A/C Pressure Sensor. Is voltage greater than the specified value still indicated with the Fuel Tank pressure sensor, and A/C Pressure Sensor disconnected? | 5.5V | ||
6 |
Was a problem found? | -- | ||
7 | Locate and repair short to volt Fuel Tank or A/C pressure sensor signal circuit or ground circuit wiring. Refer to Wiring Repairs . Is the action complete? | -- | -- | |
8 | Is voltage less than the specified value indicated? | 4.5V | ||
9 |
Was a problem found? | -- | ||
|
Important: : Replacement PCM must be programmed Refer to Powertrain Control Module Replacement/Programming . Replace PCM. Is the action complete? | -- | -- | ||
11 |
Note test result, does scan tool indicate DTC P1639 failed this ignition? | -- | Repair complete |
