GM Service Manual Online
For 1990-2009 cars only

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.

  1. The rough road sensor will give correct voltages only if it is level and mounted securely to its bracket.

  2. 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

Go to Diagnostic System Check - Engine Controls

2

Inspect for the following conditions and repair as needed:

    • The rough road sensor connector seal is missing or a damaged sensor.
    • The rough road sensor mounting flanges are cracked, missing, or incorrectly installed.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 3

3

  1. Start the engine.
  2. Allow the engine to idle.
  3. Observe the rough road sensor voltage parameter with a scan tool.

Is the voltage within the specified range?

1.1-3.7 V

Go to Step 4

Go to Step 5

4

  1. Observe the Freeze Frame/Failure Records for this DTC.
  2. Turn OFF the ignition for 30 seconds.
  3. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Does the DTC fail this ignition?

--

Go to Step 5

Go to Intermittent Conditions

5

  1. Turn OFF the ignition.
  2. Disconnect the rough road sensor.
  3. Turn ON the ignition, with the engine OFF.
  4. Observe the rough road sensor voltage parameter with a scan tool.

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

12

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

  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

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?

--

Go to Diagnostic Trouble Code (DTC) List

System OK