Reverse Inhibit Control Circuit Malfunction
The PCM has detected a fault in the reverse inhibit control circuit. The reverse inhibit control circuit includes the system wiring, connectors, and components designed to prevent the driver from shifting the manual transmission into reverse if the vehicle is in forward motion.
Code Set Parameters
If the PCM detects an abnormal voltage or resistance value from the reverse inhibit control circuit, a code will be stored and a malfunction indicator lamp may be illuminated. If the traction or transmission controller (or other controller/s) detects the presence of a brake lamp voltage signal, and the transmission control system (usually the PCM) fails to detect the proper amount of voltage or resistance, this type of code will be stored and a service engine soon lamp illuminated. The CAN Bus system is used to transfer data between individual control modules.
These may include the inability to place the shifter in reverse or the ability to place the shifter in reverse when the vehicle is in motion. Other symptoms may include an illuminated service engine soon lamp, and a stored code.
The most common causes of this type of malfunction code being stored are due to a defective reverse inhibit switch or a blown fuse. Other possible causes include shorted, open, or corroded wiring and/or electrical connectors.
Technicians report that this condition is often attributed to a mechanical shifter malfunction when reverse inhibit system electrical components, circuitry, or connectors are at fault.
- A scanner (or code reader) and a digital volt/ohmmeter will be helpful in successfully diagnosing the potential causes of this code.
In most modern OBD-II applications the reverse inhibit system is used to prevent the driver from shifting into reverse when the vehicle is in forward motion (usually at speeds above 5 mph)
While reverse inhibit switch design may vary dramatically from one manufacturer to another, it is usually of the “on/off” contact variety and is stationary mounted in the transmission housing near the shifter assembly
- Regardless of design, constant voltage is typically present on one side of the switch and when the contacts in the switch are closed, voltage is transferred across the contacts
- This activates the reverse inhibitor and it protrudes, thus interfering with the shifter and preventing shifting into reverse, inadvertently
Begin with a visual inspection of all wiring and connectors
- Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
- Always retest the system after repairs are completed to ensure success.
If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
- This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
- Continue by clearing the code and operating the vehicle to see if it returns
- This will help to determine whether or not the malfunction is intermittent
After the codes are cleared, test drive the vehicle to see if the code returns
- If the code fails to immediately return, you may have an intermittent condition
- Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can even be attempted
If the code immediately returns, check for battery voltage on the input side of the reverse inhibitor switch using the digital volt/ohmmeter
- If there is no voltage present on the input side of the switch, disconnect any related control modules and check for continuity between the system fuse and the input circuit at the reverse inhibitor switch
- Repair open or shorted circuits as required
If voltage is present on the input side of the reverse inhibitor switch, use the scanner to command the reverse inhibitor “ON” and check for voltage on the output side of the switch
- If you have voltage on the input side and none on the output side (when the reverse inhibitor is commanded on by the scanner) suspect a defective reverse inhibitor switch
- Replace the switch and retest the system afterward to ensure a successful repair.
Should you determine that voltage is present on both the input and output sides of the reverse inhibitor switch observe inhibitor operation to make sure that it works properly
- If it does not, Replace the reverse inhibitor switch and retest the system to ensure a successful repair.
If there is voltage present on the output circuit of the brake lamp switch and the reverse inhibitor operates normally, check circuit continuity and resistance between the reverse inhibitor switch and the PCM (or transmission/traction controller)
- Again, refer to the vehicle wiring diagram to ensure that all circuits are tested
- Repair or replace system open or shorted circuits/connectors as required
If all circuits coincide with manufacturer’s specifications, suspect a defective PCM
- Remember that PCM failure is rare and replacement will require reprogramming