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 provide 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 Voltage | P1392 | P1392 | P1393 | 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 Voltage | 0.0 Volts | 0.0 Volts | 5.0 Volts¹
Rough Road Sensor Signal | 0.0 Volts | 0.0 Volts | 5.0 Volts¹
Low Reference | -- | 0.0 Volts | 5.0 Volts¹
¹ 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 0.5-4.5 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 more than 4 volts.

### Conditions for Running the DTC

#### P1391

• | The engine is operating.

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

| OR

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

• | The vehicle speed is between 50-112 km/h (30-70 mph).

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

#### P1392 and P1393

• | The engine has been running for more 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 for more than 22.5 seconds.

#### 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 output is more than 4.9 volts.

### Action Taken When the DTC Sets

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

### Conditions for Clearing the MIL/DTC

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

### Reference Information

#### Schematic Reference

Engine Controls Schematics

#### Connector End View Reference

• | Engine Control Module Connector End Views

• | Engine Controls 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

• | Scan Tool Data List

• | Scan Tool Data Definitions

• | Scan Tool Output Controls

### Circuit/System Verification

1. Start the engine, observe the rough road sensor voltage parameter. The reading should be between 1.1-3.7 volts.
2. If the vehicle passes the Circuit/System Verification Test, then operate the vehicle within the conditions for running the DTC. You may also operate the vehicle within the conditions that are captured in the Freeze Frame/Failure Records List.

### Circuit/System Testing

1. Ignition OFF, disconnect the harness connector of the rough road sensor.
2. Ignition ON, verify that the rough road sensor parameter is less than 0.1 volt.

⇒ | If greater than 0.1 volt, test the signal circuit of the rough road sensor for a short to voltage. If the circuit/connections test normal, replace the ECM.

Important: The test lamp is used to load the circuit and may not illuminate.

3. Ignition ON, connect a test lamp between the 5-volt reference circuit of the rough road sensor and ground.

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.

4. Test for 4.8-5.2 volts between the 5-volt reference circuit of the rough road sensor and ground.

⇒ | If less than 4.8 volts, test the 5-volt reference circuit of the rough road sensor for a short to ground or open/high resistance. If the circuit/connections test normal, replace the ECM.

⇒ | If greater than 5.2 volts, test the 5-volt reference circuit of the rough road sensor for a short to voltage. If the circuit/connections test normal, replace the ECM.

5. Ignition ON, test for less than 0.2 volt between the low reference circuit of the rough road sensor and ground.

⇒ | If greater than 0.2 volt, test the low reference circuit of the rough road sensor for a short to voltage. If the circuit/connections test normal, replace the ECM.

6. Ignition OFF, test for less than 5 ohms of resistance between the low reference circuit of the rough road sensor and ground.

⇒ | If greater than 5 ohms, test the low reference circuit of the rough road sensor for an open/high resistance. If the circuit/connections test normal, replace the ECM.

7. Ignition ON, install a 3-amp fused jumper wire between the signal circuit and the 5-volt reference circuit of the rough road sensor to verify that the rough road sensor parameter is greater than 4.8 volts.

⇒ | If less than 4.8 volts, test the signal circuit of the rough road sensor for a short to ground or an open/high resistance. If the circuit/connections test normal, replace the ECM.

8. If all circuits/connections test normal, 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 ECM replacement, setup, and programming