DTC P1642 Vehicle Speed Output Circuit. Cadillac Concours 1996

For 1990-2009 cars
Engine Controls Components
VSS/ Vehicle Speed Output/ SMCC
OBD II Symbol Description Notice
ESD Notice
ESD Notice
ESD Notice
ESD Notice

### 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 IPC, cruise control module and the RSS or CVRSS module use to determine vehicle speed.

### Conditions for Setting the DTC

#### Test Conditions

• | DTC P1640 not set.

• | Engine running at least 10 seconds.

#### Failure Condition

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. Refer to Clearing Diagnostic Trouble Codes in PCM Description and Operation.

### 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.

1. With the key ON and the vehicle not moving, 0 volts should be received at the PCM displaying OK.

2. 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.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | Go to Step 2 | Go to A Powertrain On Board Diagnostic (OBD) System Check

2

1. Turn the key ON engine OFF.
2. Using a scan tool select Vehicle Speed Output Signal display.

Does the display indicate OK?

--

Go to Step 3

Go to Step 6

3

1. Start the engine.
2. With the vehicle wheels off the ground generate a vehicle speed signal.
3. Check the Vehicle Speed Output Signal display.

Does the display EVER indicate Open/Short?

--

Go to Step 4

Fault not present. Refer to Diagnostic Aids

4

1. Record the fluid life indexes, turn the key to LOCK and wait 30 seconds.
2. Disconnect PCM connector C2.
3. Turn the key ON.
4. Using DMM J 39200 measure the voltage to ground at PCM connector C2 terminal 25.

Is the voltage the same or more than the value specified?

10.0 volts

Go to Step 7

Go to Step 5

5 | Repair the open or short to ground on the Vehicle Speed Output circuit. Is the repair complete? | -- | Go to Powertrain Control Module Diagnosis for Verify Repair | --
6 | Repair the short to voltage on the Vehicle Speed Output circuit. Is the repair complete? | -- | Go to Powertrain Control Module Diagnosis for Verify Repair | --

7

1. Check the terminal contact at the PCM.
2. Repair terminal contact if needed.

Was the terminal contact repaired?

--

Go to Powertrain Control Module Diagnosis for Verify Repair

Go to Step 8

8 | Replace the PCM. Refer to PCM Replacement/Programming . Is the replacement complete? | -- | Go to Powertrain Control Module Diagnosis for Verify Repair | --