P1642

Circuit Description

The PCM contains chips called Output Driver Modules that provide 7 switchable ground outputs for operating solenoids, relays, telltales, and other devices. Each of these ground outputs has an internal fault line that feeds back to the PCM's microprocessor. When the output is OFF, the output driver should receive the 12 volts fed to the solenoid, relay, etc. When the output is ON, it should receive 0 volt since it is grounding the output. If it receives 0 volt when the output is OFF or 12 volts when the output is ON, it sets the fault line low. DTC P1642 is used to monitor the fault line of the vehicle speed output signal circuit. The Instrument Panel Cluster sends 12 volts on the vehicle speed output circuit to the PCM. The PCM then sends a pulse width modulated signal that the Instrument Panel Cluster (IPC), cruise control module and the Road Sensing Suspension (RSS) module or Continuously Variable Road Sensing Suspension (CVRSS) module use to determine vehicle speed.

Conditions for Running the DTC

^ DTC P1640 not set.
^ Engine running at least 10 seconds.

Conditions for Setting the DTC

Fault line is low for 2 seconds.

Action Taken When the DTC Sets

^ The Malfunction Indicator Lamp (MIL) will not illuminate.
^ No message will be displayed.

Conditions for Clearing the DTC

^ A History DTC will clear after forty consecutive warm-up cycles with no failures of any non-emission related diagnostic test.
^ A Last Test Failed (current) DTC will clear when the diagnostic runs and does not fail.
^ Use a scan tool to clear DTCs.
^ Interrupting PCM battery voltage may or may not clear DTCs. This practice is not recommended.

Diagnostic Aids

Reviewing the Failure Rec. may help diagnose an intermittent failure by showing how long ago and under what conditions the DTC set.

Test Description

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

2. With the key ON and the vehicle not moving, 0 volts should be received at the PCM displaying OK.
3. By creating a vehicle speed signal a open/short can be observed, if present. If a fault is present it will only be displayed on the scan tool intermittently due to the pulse width modulated signal.