For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• | Diagnostic Procedure Instructions provides an overview of each diagnostic category.

### DTC Descriptors

DTC P1391: Rough Road Sensor Performance
DTC P1392: Rough Road Sensor Circuit Low Voltage
DTC P1393: Rough Road Sensor Circuit High Voltage

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
5-Volt Reference | P0651, P1392 | P1392 | P0651, P1392 | P1391
Rough Road Sensor Signal | P1392 | P1392 | P1393 | P1391
Low Reference | -- | P1391 | -- | P1391

### Typical Scan Tool Data

#### Rough Road Sensor

Circuit | Short to Ground | Open | Short to Voltage
--- | --- | --- | ---
Operating Conditions: Engine operating in Closed Loop. Parameter Normal Range: 1.1-3.7 Volts.
5-Volt Reference | 0.0 Volts | 0.0 Volts | 5.0 Volts*
Rough Road Sensor Signal | 0.0 Volts | 0.0 Volts | 5.0 Volts*
Low Reference | -- | 0.0 Volts | 5.0 Volts*
* Internal ECM or sensor damage may occur if the circuit is shorted to B+.

### Circuit/System Description

The rough road sensor is a vertical low G-acceleration sensor. By sensing vertical acceleration caused by rough road conditions, the engine control module (ECM) can determine if the changes in crankshaft speed are due to engine misfire or are driveline induced. If the rough road sensor detects a rough road condition, the ECM misfire detection diagnostic will be de-activated. The rough road sensor at rest output should be between 2.30-2.65 volts. During a rough road condition, the voltage output can vary between 0.5 volt and 4.5 volts. The rough road sensor is a capacitive type sensor with 3 circuits:

• | A 5-volt reference circuit

• | A low reference circuit

• | A signal circuit

The ECM provides the rough road sensor with 5 volts on the 5-volt reference circuit and a ground on the low reference circuit. Vibration to the rough road sensor from rough road conditions in the road, provides the ECM with a signal voltage from less than 1 volt to greater than 4 volts.

### Conditions for Running the DTC

#### P1391

• | The engine has been running for greater than 10 seconds.

• | The vehicle speed is less than 5 km/h (3.1 mph).

| OR

• | The engine has been running for greater than 10 seconds.

• | The vehicle speed is between 30-70 km/h (19-43 mph).

• | The DTC runs continuously when the above conditions are met.

#### P1392 and P1393

• | The engine has been running for greater than 10 seconds.

• | The DTCs run continuously when the above condition is met.

### Conditions for Setting the DTC

#### P1391

The ECM detects that the rough road sensor output is not within range.

#### P1392

The ECM detects that the rough road sensor voltage is less than 0.1 volt.

#### P1393

The ECM detects that the rough road sensor voltage is greater than 4.9 volts.

### Action Taken When the DTC Sets

DTC P1391, P1392, and P1393 are type C DTCs.

### Conditions for Clearing the DTC

DTC P1391, P1392, and P1393 are type C DTCs.

### Diagnostic Aids

• | If a shared 5-volt reference circuit is shorted to ground or shorted to a voltage, other 5-volt reference circuits may be affected.

• | The scan tool 5-volt reference parameter should display between 4.8-5.2 volts.

### Reference Information

#### Schematic Reference

Engine Controls Schematics

#### Connector End View Reference

Component Connector End Views

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

#### Scan Tool Reference

Control Module References for scan tool information

### Circuit/System Verification

1. Ignition ON, observe the DTC information with a scan tool. DTC P0651 should not set.

⇒ | If DTC P0651 is set, refer to DTC P0641 or P0651.

2. Engine running, observe the scan tool rough road sensor voltage parameter. The reading should be between 1.1-3.7 volts.
3. Operate the vehicle within the Conditions for Running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

### Circuit/System Testing

1. Ignition OFF, disconnect the harness connector at the rough road sensor.
2. Ignition OFF, test for less than 5 ohms of resistance between the low reference circuit terminal C and ground.

⇒ | If greater than the specified range, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the ECM.

Important: The 5-volt reference circuits are internally and externally connected at the ECM. Other component DTCs may be set. If other DTCs are set, review the electrical schematic and diagnose the applicable circuits and components.

3. Ignition ON, test for 4.8-5.2 volts between the 5-volt reference circuit terminal A and ground.

⇒ | If less than the specified range, test the 5-volt reference circuit for a short to ground or open/high resistance. If the circuit tests normal, replace the ECM.

⇒ | If greater than the specified range, test the 5-volt reference circuit for a short to voltage. If the circuit tests normal, replace the ECM.

4. Verify the scan tool rough road sensor voltage parameter is less than 0.1 volt.

⇒ | If greater than the specified range, test the signal circuit for a short to voltage. If the circuit tests normal, replace the ECM.

5. Install a 3A fused jumper wire between the signal circuit terminal B and the 5-volt reference circuit terminal A. Verify the scan tool rough road sensor voltage parameter is greater than 4.8 volts.

⇒ | If less than the specified range, test the signal circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the ECM.

6. If all circuits test normal, test or replace the rough road sensor.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Rough Road Sensor Replacement

• | Control Module References for engine control module replacement, setup, and programming