P1639

Diagnostic Chart 1 Of 2:

Diagnostic Chart 2 Of 2:

Schematic:

CIRCUIT DESCRIPTION
The PCM uses the 5 Volt Reference B circuit as a sensor feed for the A/C refrigerant 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 engine is running.

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 10 seconds.

ACTION TAKEN WHEN THE DTC SETS
^ The PCM does not illuminate the malfunction indicator lamp (MIL).
^ The PCM stores the conditions that were present when the DTC set as Fail 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 or by disconnecting the PCM battery feed.

DIAGNOSTIC AIDS

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

Check for the following conditions
^ A faulty 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. Use a corresponding mating terminal to test for proper terminal tension.
^ 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 connectors and 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.

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

TEST DESCRIPTION
Number(s) below refer to the step number(s) on the Diagnostic Table.
11. This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is replaced, the new PCM must be programmed.