Introduction:
The P0979 diagnostic trouble code (DTC) indicates a problem within the control circuit for Shift Solenoid (SS) 3 in your vehicle's transmission control module (TCM). This code signifies that the TCM has detected a lower-than-expected voltage signal on the circuit responsible for controlling SS3, which can lead to shifting problems and potential damage to the transmission. Understanding the causes, symptoms, and diagnostic procedures associated with P0979 is crucial for proper diagnosis and repair.
| Topic | Description | Possible Actions |
|---|---|---|
| Definition | P0979 indicates the Transmission Control Module (TCM) has detected a low voltage condition in the control circuit for Shift Solenoid 3. The TCM uses solenoids to control fluid pressure to engage different gears within the automatic transmission. | Review vehicle-specific repair manual for component locations and wiring diagrams. |
| Symptoms | Harsh or erratic shifting. Transmission slipping. Transmission not shifting into certain gears. Stuck in a gear. Check Engine Light (CEL) illuminated. Possible limp mode (reduced engine power and limited gear selection). * Increased fuel consumption. | Note the specific shifting problems experienced. Record when the problem occurs (e.g., cold start, warm engine, specific speed). |
| Possible Causes | Faulty Shift Solenoid 3. Wiring harness issues (damaged, corroded, or loose connections). Short to ground in the SS3 control circuit. Open circuit in the SS3 control circuit. Faulty Transmission Control Module (TCM). Low transmission fluid level or contaminated fluid. Internal transmission damage. Battery voltage issues. | Check battery voltage. Inspect wiring harness for damage and corrosion. Check transmission fluid level and condition. |
| Diagnostic Tools | OBD-II scanner. Digital multimeter (DMM). Wiring diagrams for the vehicle's transmission system. Hydraulic pressure gauge (for transmission). | Gather necessary tools before starting the diagnostic process. |
| Diagnostic Procedure | 1. Verify the code with an OBD-II scanner. 2. Check transmission fluid level and condition. 3. Inspect wiring and connectors to SS3 for damage. 4. Test SS3 resistance with a DMM. 5. Check for shorts to ground in the SS3 circuit. 6. Test SS3 control circuit voltage at the TCM. 7. Perform a transmission hydraulic pressure test. 8. If all other tests pass, suspect a faulty TCM. | Follow a logical diagnostic procedure to isolate the root cause. Refer to the vehicle-specific repair manual for detailed instructions. |
| Shift Solenoid Function | Shift solenoids are electromechanical devices that control the flow of hydraulic fluid within an automatic transmission. They are controlled by the TCM, which activates or deactivates the solenoids to direct fluid pressure to different clutches and bands, enabling gear changes. SS3 specifically controls fluid flow for a particular gear or shift transition. | Understand the role of shift solenoids in the transmission's operation. |
| TCM Role | The Transmission Control Module (TCM) is the electronic control unit that manages the automatic transmission. It receives input from various sensors (e.g., vehicle speed, engine speed, throttle position) and uses this information to determine the optimal gear and shift timing. The TCM then activates the appropriate shift solenoids to achieve the desired gear. | Understand the TCM's central role in transmission control. |
| Wiring Diagram Importance | Wiring diagrams are essential for tracing the SS3 control circuit and identifying potential wiring problems. They show the location of connectors, wire colors, and the path of the circuit from the TCM to the SS3 solenoid. | Obtain the correct wiring diagram for your specific vehicle model and year. |
| Resistance Testing | Measuring the resistance of the SS3 solenoid helps determine if the solenoid coil is open, shorted, or within the acceptable range specified by the manufacturer. An out-of-range resistance reading indicates a faulty solenoid. | Consult the vehicle-specific repair manual for the correct resistance specifications. |
| Voltage Testing | Voltage testing at the TCM and at the SS3 solenoid helps determine if the TCM is sending the correct voltage signal to the solenoid. A low voltage reading can indicate a wiring problem, a faulty TCM, or a short to ground. | Compare the measured voltage to the expected voltage specified in the repair manual. |
| Hydraulic Pressure Test | A hydraulic pressure test measures the fluid pressure within the transmission at various points. This test can help identify internal transmission problems, such as faulty clutches or valves, that may be affecting the operation of SS3. | This test requires specialized equipment and knowledge. It is typically performed by a professional transmission technician. |
| Fluid Level and Condition | Low or contaminated transmission fluid can affect the performance of the shift solenoids and other transmission components. Low fluid can cause the solenoids to operate erratically, while contaminated fluid can clog the solenoids and prevent them from functioning properly. | Check the fluid level regularly and change the fluid according to the manufacturer's recommendations. |
| Potential Repair Costs | Repair costs can vary depending on the cause of the P0979 code. Replacing a shift solenoid is generally less expensive than replacing the TCM or rebuilding the transmission. Wiring repairs can also be relatively inexpensive. A full transmission rebuild is the most expensive option. | Obtain estimates from multiple repair shops before authorizing any repairs. |
| Preventive Maintenance | Regular transmission fluid changes, proper vehicle maintenance, and avoiding harsh driving habits can help prevent transmission problems and extend the life of the transmission. | Follow the manufacturer's recommended maintenance schedule for your vehicle. |
| Related DTCs | P0976: Shift Solenoid (SS) 2 Control Circuit Low Voltage (TCM). P0977: Shift Solenoid (SS) 2 Control Circuit High Voltage (TCM). P0978: Shift Solenoid (SS) 3 Control Circuit High Voltage (TCM). Other transmission-related DTCs. | Understanding related DTCs can help narrow down the possible causes of the problem. |
Detailed Explanations:
Definition:
The P0979 code, as mentioned, directly points to an issue with the control circuit of Shift Solenoid 3. This circuit is the electrical pathway that the TCM uses to send signals to SS3, instructing it to open or close and thus control the flow of transmission fluid. A "low voltage" condition means the TCM is not receiving the expected voltage feedback from this circuit, indicating a potential fault. This fault will prevent the TCM from properly controlling SS3, leading to shifting problems.
Symptoms:
The symptoms associated with a P0979 code are directly related to the improper functioning of the transmission due to the faulty SS3 control. Erratic or harsh shifting is a common symptom as the transmission struggles to engage the correct gear. Transmission slipping can occur if the clutches are not fully engaged due to incorrect fluid pressure. The vehicle might also fail to shift into certain gears or become stuck in a single gear. The Check Engine Light (CEL) will illuminate, and the vehicle may enter limp mode to protect the transmission from further damage. This can also lead to increased fuel consumption as the engine works harder to compensate for the inefficient gear selection.
Possible Causes:
Several factors can contribute to a P0979 code. A faulty Shift Solenoid 3 itself is a primary suspect. Wiring harness issues, such as damaged, corroded, or loose connections, can disrupt the electrical signal. A short to ground in the SS3 control circuit can cause the voltage to drop. Conversely, an open circuit can also prevent the proper voltage signal from reaching the TCM. A faulty TCM can also be the culprit, as it is responsible for controlling the solenoids. Low transmission fluid level or contaminated fluid can affect the hydraulic pressure needed for the solenoids to function correctly. Internal transmission damage can also indirectly affect the solenoid's operation. Finally, battery voltage issues can impact the entire electrical system, including the TCM and the solenoid circuits.
Diagnostic Tools:
Diagnosing a P0979 code requires specific tools. An OBD-II scanner is essential for reading the DTC and other relevant data. A digital multimeter (DMM) is needed to test the voltage, resistance, and continuity of the SS3 circuit. Wiring diagrams are crucial for tracing the circuit and identifying potential problem areas. A hydraulic pressure gauge can be used to assess the overall health of the transmission and the function of the solenoids.
Diagnostic Procedure:
The diagnostic procedure should follow a logical sequence. First, verify the code using an OBD-II scanner. Next, check the transmission fluid level and condition. Then, inspect the wiring and connectors to SS3 for any signs of damage or corrosion. Test the SS3 resistance using a DMM to check for internal faults within the solenoid. Check for shorts to ground in the SS3 circuit. Test the SS3 control circuit voltage at the TCM to determine if the TCM is sending the correct signal. A transmission hydraulic pressure test can further assess the transmission's internal condition. If all other tests pass, a faulty TCM should be suspected.
Shift Solenoid Function:
Shift solenoids are vital components in automatic transmissions. They act as electrically controlled valves that regulate the flow of hydraulic fluid. When the TCM sends a signal, the solenoid opens or closes, directing fluid pressure to specific clutches and bands within the transmission. This action engages or disengages these components, resulting in a gear change. SS3, in particular, controls the fluid flow necessary for a specific gear or shift transition. Its proper operation is essential for smooth and accurate shifting.
TCM Role:
The Transmission Control Module (TCM) is the brain of the automatic transmission. It constantly monitors various sensors, such as vehicle speed, engine speed, and throttle position, to determine the optimal gear for the current driving conditions. Based on this information, the TCM activates the appropriate shift solenoids to achieve the desired gear. The TCM also monitors the performance of the solenoids and other transmission components, setting diagnostic codes if any problems are detected.
Wiring Diagram Importance:
Wiring diagrams are indispensable tools for diagnosing electrical problems in the transmission system. They provide a detailed map of the SS3 control circuit, showing the location of connectors, wire colors, and the path of the circuit from the TCM to the SS3 solenoid. This information is crucial for tracing the circuit, identifying potential breaks or shorts, and verifying the integrity of the wiring.
Resistance Testing:
Resistance testing is a key step in diagnosing a faulty shift solenoid. By measuring the resistance of the solenoid coil with a DMM, you can determine if the coil is open, shorted, or within the acceptable range specified by the manufacturer. An open circuit indicates a broken coil, while a shorted circuit indicates a short between the coil windings. An out-of-range resistance reading suggests that the solenoid is faulty and needs to be replaced.
Voltage Testing:
Voltage testing is another important diagnostic step. By measuring the voltage at the TCM and at the SS3 solenoid, you can determine if the TCM is sending the correct voltage signal to the solenoid. A low voltage reading can indicate a wiring problem, a faulty TCM, or a short to ground. Comparing the measured voltage to the expected voltage specified in the repair manual can help pinpoint the source of the problem.
Hydraulic Pressure Test:
A hydraulic pressure test is a more advanced diagnostic procedure that measures the fluid pressure within the transmission at various points. This test can help identify internal transmission problems, such as faulty clutches or valves, that may be affecting the operation of SS3. It requires specialized equipment and knowledge and is typically performed by a professional transmission technician.
Fluid Level and Condition:
The transmission fluid plays a critical role in the proper functioning of the transmission. Low fluid levels can cause the solenoids to operate erratically, while contaminated fluid can clog the solenoids and prevent them from functioning properly. Regular fluid changes, using the correct type of fluid, are essential for maintaining the health of the transmission.
Potential Repair Costs:
The cost of repairing a P0979 code can vary depending on the cause of the problem. Replacing a shift solenoid is generally less expensive than replacing the TCM or rebuilding the transmission. Wiring repairs can also be relatively inexpensive. However, a full transmission rebuild is the most expensive option. Obtaining estimates from multiple repair shops before authorizing any repairs is always recommended.
Preventive Maintenance:
Preventive maintenance is crucial for extending the life of the transmission and preventing problems like the P0979 code. Regular transmission fluid changes, using the correct type of fluid, are essential. Proper vehicle maintenance, such as addressing other engine and electrical issues promptly, can also help prevent transmission problems. Avoiding harsh driving habits, such as frequent hard acceleration and braking, can also reduce stress on the transmission.
Related DTCs:
Understanding related DTCs can provide valuable clues about the underlying cause of the P0979 code. For example, codes related to other shift solenoids (e.g., P0976, P0977, P0978) may indicate a common problem, such as a faulty TCM or a wiring issue affecting multiple solenoids. Other transmission-related DTCs can also provide insights into the overall health of the transmission and the potential causes of the P0979 code.
Frequently Asked Questions:
What does the P0979 code mean?
It indicates a low voltage condition in the control circuit for Shift Solenoid 3, which can lead to shifting problems.
Can I drive with a P0979 code?
It's not recommended, as it can cause further transmission damage; get it checked as soon as possible.
What's the first thing I should check?
Start by checking the transmission fluid level and condition, as low or contaminated fluid can cause issues.
Is it just the solenoid that's bad?
Not always; wiring, the TCM, or even internal transmission issues can trigger this code.
Can I fix this myself?
Depending on your mechanical skills, you might be able to diagnose and repair wiring issues or replace the solenoid, but complex diagnoses or TCM replacement should be done by a professional.
How often should I change my transmission fluid?
Refer to your vehicle's owner's manual for the recommended service interval, typically every 30,000 to 60,000 miles.
Conclusion:
The P0979 code signals a potential problem with the Shift Solenoid 3 control circuit, which can lead to various transmission issues. Thorough diagnostics, including checking the fluid, wiring, solenoid, and TCM, are crucial for accurate identification and effective repair to prevent further transmission damage.