Circuit 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 2.35-2.65 volts. During a rough road condition, the voltage output can vary between 0.5 volts 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 more than 4 volts. If the ECM detects that the rough road sensor input is not within a calibrated limit of the estimated value, this DTC sets.
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P1391 Rough Road Sensor Performance
Conditions for Running the DTC
| • | The engine is running. |
| • | The vehicle speed is less than 5 km/h (3.1 mph). |
| OR |
| • | The engine is running. |
| • | The engine is running more than 10 seconds and the vehicle speed is between 50-112 km/h (30-70 mph). |
| • | DTC P1391 runs continuously once the above conditions are met. |
Conditions for Setting the DTC
The ECM detects that the rough road sensor output is not within range for more than 22.5 seconds.
Action Taken When the DTC Sets
| • | The control module stores the DTC information into memory when the diagnostic runs and fails. |
| • | The malfunction indicator lamp (MIL) will not illuminate. |
| • | The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Failure Records. |
| • | The driver information center, if equipped, may display a message. |
Conditions for Clearing the DTC
| • | A current DTC Last Test Failed clears when the diagnostic runs and passes. |
| • | A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other non-emission related diagnostic. |
| • | Clear the DTC with a scan tool. |
Diagnostic Aids
| • | Check for the following conditions: |
| - | A poor connection at the ECM--Inspect the harness connections for backed-out terminals, improper mating, broken locks, improperly formed or damaged terminals, and a poor terminal-to-wire connection. |
| - | A damaged harness--Inspect the wiring harness for damage. If the harness appears to be OK, observe the rough road sensor display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the display will indicate the location of the fault. |
| • | Since the rough road sensor shares the ECM 5-volt reference and ground terminals with the A/C pressure sensor, a damaged A/C pressure sensor harness or sensor could cause a rough road sensor DTC to set. |
| • | The rough road sensor will give correct voltages only if it is level and mounted securely to its bracket. |
| • | Reviewing the Failure Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
The rough road sensor will give correct voltages only if it is level and mounted securely to its bracket.
-
This step tests for poor connection at the ECM or the harness connections for backed-out terminals, improper mating, broken locks, improperly formed or damaged terminals, and a poor terminal-to-wire connection.
Step | Action | Values | Yes | No | ||||
|---|---|---|---|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics Connector End View Reference: Engine Controls Connector End Views | ||||||||
1 | Did you perform the Diagnostic System Check - Engine Controls? | -- | Go to Step 2 | |||||
Inspect for the following conditions and repair as needed:
Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 3 | |||||
3 |
Is the voltage within the specified range? | 1.1-3.7 V | Go to Step 4 | Go to Step 5 | ||||
4 |
Does the DTC fail this ignition? | -- | Go to Step 5 | Go to Intermittent Conditions | ||||
5 |
Is the voltage less than the specified value? | 0.1 V | Go to Step 6 | Go to Step 9 | ||||
6 | Measure the voltage from the 5-volt reference circuit of the rough road sensor to a good ground with a DMM. Is the voltage less than the specified value? | 5.2 V | Go to Step 7 | Go to Step 10 | ||||
7 | Measure the resistance between the low reference circuit of the rough road sensor and the ECM housing with a DMM. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Is the resistance less than the specified value? | 2 ohms | Go to Step 11 | Go to Step 8 | ||||
8 | Test the low reference circuit of the rough road sensor for a high resistance or an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 12 | ||||
9 | Test the signal circuit of the rough road sensor for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 12 | ||||
10 |
Important: The 5-volt reference circuits are internally and externally connected at the controller. Other sensors that share the 5-volt reference circuit may also have DTCs set. Disconnecting a sensor on the shared 5-volt reference circuit may isolate a shorted sensor. Review the electrical schematic and diagnose the shared circuits and sensors. Test the 5 volt reference circuit of the rough road sensor for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 12 | ||||
11 | Test for shorted terminals and for poor connection at the rough road sensor. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 15 | Go to Step 13 | |||||
Test for shorted terminals and for poor connections at the engine control module (ECM). Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 14 | |||||
13 | Replace the rough road sensor. Did you complete the replacement? | -- | Go to Step 15 | -- | ||||
14 | Replace the ECM. Refer to Engine Control Module Replacement . Did you complete the replacement? | -- | Go to Step 15 | -- | ||||
15 |
Did the DTC fail this ignition? | -- | Go to Step 2 | Go to Step 16 | ||||
16 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | System OK | |||||
