Technical Description of OBD-II Trouble Code
The circuit for the Four Wheel Drive (4WD) Switch is registering a high voltage.
What does that mean?
This is a generic diagnostic trouble code (DTC) for the powertrain and is applicable to many OBD-II vehicles (1996-newer). This includes vehicles from various manufacturers such as Chrysler, Ford, Jeep, General Motors, Dodge, Chevrolet, Mercedes, etc. Although it is a generic code, the specific repair steps may vary depending on the year, make, model, and powertrain configuration of the vehicle.
The OBD-II trouble code P0839 and related codes P0836, P0837, and P0838 are associated with the circuit for the four wheel drive (4WD) switch. This circuit is also known as the transfer case control circuit.
The purpose of the 4WD switch circuit is to allow the driver to select the activation of the 4WD system and change the gear ratios of the transfer case as needed, including two wheel high, two wheel low, neutral, four wheel high, and four wheel low.
When the PCM or TCM detects a voltage or resistance that is significantly higher than the normal expected range within the 4WD switch circuit, it will set code P0839 and may illuminate the check engine light, 4WD malfunction light, or both.
A four wheel drive selector switch:
What is the severity of this DTC?
The severity of this code is typically moderate, but problems with the 4WD switch circuit can be severe if the transfer case is stuck in low gear. This can cause excessive stress on internal transmission and engine components, requiring immediate attention. In some cases, the vehicle may not move or shift into gear at all.
What are some of the symptoms of the code?
Symptoms of a P0839 trouble code may include:
What are some of the common causes of the code?
Possible causes for this P0839 code may include:
What are some P0839 troubleshooting steps?
The first step in troubleshooting any malfunction is to research the Technical Service Bulletins (TSBs) specific to the vehicle’s year, model, and power plant. This can save a significant amount of time by providing guidance in the right direction.
The second step is to check the transmission and transfer case fluid to ensure it is at the appropriate level and in good condition. Then, locate all components associated with the 4WD switch circuit and inspect them for any obvious physical damage. Depending on the specific vehicle, this circuit may include several components such as the transfer case, switches, solenoids, the PCM, and TCM. Conduct a thorough visual inspection of the associated wiring to check for any obvious defects such as scraping, rubbing, bare wires, or burn spots. Next, check the connectors and connections for security, corrosion, and damaged pins. This process should include all wiring connectors and connections to all components, including the PCM and TCM. Consult the specific technical data for the vehicle to verify the circuit configuration and determine if a fuse or fuse-able link is part of the circuit.
Advanced Steps
The advanced steps are highly specific to the vehicle and require advanced equipment for accurate execution. These procedures involve using a digital multimeter and referencing the specific technical documentation for the vehicle.
Voltage Checks
The reference voltage and acceptable ranges may vary depending on the specific vehicle and circuit configuration. The technical data will include troubleshooting charts and the appropriate sequence to follow for an accurate diagnosis.
If this process reveals a lack of power source or ground, continuity testing may be necessary to check the integrity of the wiring, connectors, and other components. Continuity tests should always be performed with the power removed from the circuit, and the normal readings for wiring and connections should be 0 ohms of resistance. Resistance or lack of continuity indicates faulty wiring that is either open or shorted and must be repaired or replaced. A continuity test from the PCM or TCM to the frame can confirm the serviceability of ground straps and ground wires. The presence of resistance indicates a loose connection or possible corrosion.
What are some common repairs for this code?
Common Mistake
We hope that the information in this article has been helpful in guiding you towards resolving the problem with your 4WD switch circuit fault code. Please note that this article is strictly informational, and the specific technical data and service bulletins for your vehicle should always take precedence.
