The engine control module (ECM) determines engine misfire by detecting variations in crankshaft speed. Crankshaft speed variations can also occur when a vehicle is operating over a rough road. The wheel speed sensor produces an alternating current (AC) voltage that increases with wheel speed. The ECM receives a rough road signal from an electronic brake control module (EBCM), if equipped with the anti-lock brake system (ABS). The ABS can determine if the vehicle is on a rough road surface based on the signal from each wheel speed sensor. On vehicles with Non-ABS, a rough road signal is sent from a wheel speed sensor located at the left front wheel to the ECM. The ECM can determine if the vehicle is on a rough road surface based on the signal from the wheel speed sensor. When the ECM detects a rough road condition, misfire diagnostics will be disabled.

P1390

The ECM detects the voltage on the rough road signal circuit of the wheel speed sensor is less than 1 volt, greater than 5 volts, or difference between the minimum and maximum voltage is greater than 5 volts.

P1396

The ECM detects an invalid rough road signal from the EBCM.

DTC P1390 and P1396 are Type C DTCs.

DTC P1390 and P1396 are Type C DTCs.

Schematic Reference

Engine Controls Schematics

Electrical Information Reference

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

1. Engine running, observe the DTC information with a scan tool. DTCs P1390 or P1396 should not set.
2. 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.

Important: If the ABS lamp is illuminated, the ABS system must be diagnosed prior to using this diagnostic procedure. Refer to Diagnostic System Check - Vehicle.

1. Ignition OFF, disconnect the harness connector at the wheel speed sensor or the EBCM - if equipped with ABS.
2. Ignition OFF, test for less than 5 ohms of resistance between the rough road low reference circuit terminal 2 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.

3. Ignition ON, test for 4.8-5.2 volts on the rough road signal circuit terminal 1.

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

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

4. If all circuits test normal, test or replace the wheel speed sensor or EBCM - if equipped with ABS.

Important: This procedure is for vehicles without ABS. If the wheel speed sensor harness is damaged in any way, DO NOT repair the harness.

1. Ignition OFF, disconnect the harness connector at the wheel speed sensor.

Notice: Support the lower control arms in the normal horizontal position in order to avoid damage to the drive axles. Do not operate the vehicle in gear with the wheels hanging down at full travel.

2. Raise and support the drive wheels.
3. Engine running, place the transmission into 2nd gear or drive for an automatic.
4. Test for 70 mV AC between the rough road signal terminal 1 and low reference terminal 2.

⇒	If less than the specified range, replace the wheel speed sensor.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.