Introduction:
The P0708 diagnostic trouble code (DTC) indicates a problem with the Transmission Range Sensor (TRS), also known as the Neutral Safety Switch or Inhibitor Switch, circuit. This sensor is crucial for informing the vehicle's computer (PCM/TCM) of the gear selector's position, enabling functions like starting the engine, engaging the correct gear, and activating reverse lights. A high input signal in this circuit suggests a potential open circuit or wiring issue that requires diagnosis and repair to ensure proper vehicle operation.
Comprehensive Table: P0708 - Transmission Range Sensor Circuit High Input
| Topic | Description | Possible Causes |
|---|---|---|
| Definition | Indicates the PCM/TCM has detected a voltage reading on the TRS circuit that is higher than expected. This suggests an open circuit or a short to voltage. | Open circuit in the TRS signal wire, Short to voltage in the TRS signal wire, Faulty Transmission Range Sensor (TRS), PCM/TCM failure (rare), Corroded or damaged connectors and wiring, Improper TRS adjustment. |
| Symptoms | Inability to start the vehicle, Engine starts in gear other than Park or Neutral, Incorrect gear selection, Reverse lights not working, Transmission shifting problems, Limp mode (reduced engine power and limited gear selection), Check engine light illuminated. | |
| TRS Function | The Transmission Range Sensor (TRS) monitors the position of the gear selector and sends a corresponding voltage signal to the PCM/TCM. This signal informs the computer about the driver's gear selection (Park, Reverse, Neutral, Drive, etc.). The PCM/TCM uses this information to control various transmission functions, including starting, shifting, and safety features. | |
| Voltage Measurement | The PCM/TCM typically provides a reference voltage (usually 5 volts) to the TRS. The sensor then grounds this voltage through a series of resistors, creating a voltage divider. The voltage returned to the PCM/TCM varies depending on the gear selected. A "high input" indicates that the voltage returned is higher than the acceptable range for any given gear. | |
| Diagnostic Steps | Visual inspection of wiring and connectors, Voltage testing at the TRS connector, Continuity testing of the TRS circuit, TRS adjustment check, TRS resistance testing, PCM/TCM testing (as a last resort). | |
| Common Mistakes | Replacing the TRS without thorough diagnosis, Neglecting wiring and connector issues, Overlooking TRS adjustment, Assuming PCM/TCM failure without proper testing. | |
| Wiring Diagram Analysis | Understanding the specific wiring diagram for your vehicle is crucial. Identify the wires related to the TRS signal, ground, and reference voltage. Use a multimeter to check for voltage, continuity, and shorts according to the diagram. Pinpoint any breaks, shorts, or corrosion in the wiring. | |
| TRS Adjustment Procedure | The TRS must be properly adjusted to ensure it accurately reflects the gear selector position. The adjustment procedure varies depending on the vehicle make and model. Typically, it involves aligning specific marks on the TRS with corresponding marks on the transmission or gear selector linkage. Incorrect adjustment can lead to inaccurate signals and shifting problems. | |
| Resistance Testing | Using a multimeter, measure the resistance between the terminals of the TRS. Refer to the vehicle's service manual for the correct resistance values for each gear position. Deviations from these values indicate a faulty TRS. Conduct this test with the sensor disconnected from the wiring harness. | |
| Related Codes | P0705 (Transmission Range Sensor Circuit Malfunction), P0706 (Transmission Range Sensor Circuit Range/Performance), P0707 (Transmission Range Sensor Circuit Low Input), P0709 (Transmission Range Sensor Circuit Intermittent). These codes can provide additional information about the nature of the TRS problem. | |
| Tools Required | Multimeter, Wiring diagram for your vehicle, Socket set, Screwdriver set, Scan tool (for reading and clearing codes), Wire stripper/crimper, Test light, Penetrating oil (for loosening corroded connectors). | |
| Safety Precautions | Disconnect the negative battery cable before working on electrical components. Use caution when working around the transmission and moving parts. Wear safety glasses to protect your eyes. Consult the vehicle's service manual for specific safety instructions. | |
| Impact on Driving | The P0708 code can significantly impact driving by preventing the engine from starting, causing incorrect gear selection, resulting in harsh shifting, and potentially triggering limp mode. These issues can compromise safety and drivability. | |
| PCM/TCM Location | The Powertrain Control Module (PCM) or Transmission Control Module (TCM) location varies depending on the vehicle make and model. Common locations include under the dashboard, under the seats, or in the engine compartment. Consult the vehicle's service manual for the specific location of the PCM/TCM in your vehicle. | |
| Connector Types | TRS connectors can vary in design but typically feature multiple pins to accommodate the different gear position signals. Common connector types include rectangular and square connectors with locking mechanisms to ensure a secure connection. It's crucial to inspect the connector for corrosion, damage, and loose pins. | |
| Troubleshooting Flowchart | 1. Verify the P0708 code with a scan tool. 2. Perform a visual inspection of the TRS, wiring, and connectors. 3. Check the TRS adjustment. 4. Test the TRS circuit for voltage and continuity. 5. Perform TRS resistance testing. 6. If all other tests fail, consider PCM/TCM testing. 7. Repair or replace components as needed. 8. Clear the code and retest the system. | |
| Repair Strategies | Repairing a P0708 often involves fixing wiring issues, replacing damaged connectors, adjusting the TRS, or replacing the TRS itself. In rare cases, PCM/TCM replacement may be necessary. Proper diagnosis is crucial to identify the root cause of the problem and implement the appropriate repair strategy. | |
| Aftermarket Parts | When replacing the TRS, consider using a high-quality aftermarket part from a reputable brand. While OEM parts are often preferred, reliable aftermarket options can provide a cost-effective alternative. Ensure the replacement part is compatible with your vehicle's make and model. | |
| Preventative Measures | Regularly inspect wiring and connectors for corrosion and damage. Protect wiring from exposure to harsh elements. Ensure the TRS is properly adjusted. Address any transmission shifting problems promptly. Following these preventative measures can help reduce the risk of TRS failure and related issues. |
Detailed Explanations:
Definition: The P0708 code signifies that the vehicle's computer has detected an abnormally high voltage signal in the Transmission Range Sensor (TRS) circuit. This voltage exceeds the expected range for any gear position, indicating a fault within the circuit that needs investigation.
Symptoms: The symptoms associated with P0708 can range from minor inconveniences to major drivability problems. These include difficulty starting the car, the engine starting in the wrong gear, incorrect gear selection, non-functional reverse lights, shifting difficulties, and potentially triggering limp mode, which limits engine power and gear selection.
TRS Function: The Transmission Range Sensor (TRS) acts as the communication bridge between the gear selector and the vehicle's computer. It monitors the position of the gear shift lever and transmits this information to the PCM/TCM through a varying voltage signal. This data is vital for proper engine starting, gear engagement, and other transmission-related functions.
Voltage Measurement: The PCM/TCM typically sends a 5-volt reference signal to the TRS. The TRS then acts as a variable resistor, grounding the voltage and returning a specific voltage signal back to the computer depending on the gear selected. A high input reading (P0708) means the returned voltage is abnormally high, usually indicating an open circuit or a short to voltage somewhere in the circuit.
Diagnostic Steps: Diagnosing a P0708 code requires a systematic approach. Begin with a visual inspection of the wiring and connectors. Next, perform voltage and continuity tests on the TRS circuit. Check the TRS adjustment and perform resistance testing. Only consider PCM/TCM testing as a last resort, after ruling out all other possibilities.
Common Mistakes: Avoid the common pitfall of replacing the TRS without thorough diagnosis. Wiring and connector problems are frequent causes of P0708. Also, ensure the TRS is properly adjusted before assuming it is faulty. Finally, don't jump to conclusions about PCM/TCM failure without rigorous testing.
Wiring Diagram Analysis: A wiring diagram is your roadmap to understanding the TRS circuit. Identify the signal, ground, and reference voltage wires. Use a multimeter to trace the circuit, checking for voltage, continuity, and shorts according to the diagram. Pay close attention to any breaks, shorts, or corrosion in the wiring.
TRS Adjustment Procedure: Proper TRS adjustment is critical for accurate gear selection. Consult the vehicle's service manual for the specific adjustment procedure for your make and model. This usually involves aligning marks on the TRS with corresponding marks on the transmission or gear selector linkage.
Resistance Testing: Use a multimeter to measure the resistance between the terminals of the TRS while it is disconnected. Compare the readings to the values specified in the vehicle's service manual for each gear position. Deviations from the expected values indicate a faulty TRS.
Related Codes: Be aware of other related trouble codes, such as P0705, P0706, P0707, and P0709. These codes can provide additional clues about the nature of the TRS problem and help narrow down the diagnosis.
Tools Required: Essential tools for diagnosing and repairing P0708 include a multimeter, wiring diagram, socket set, screwdriver set, scan tool, wire stripper/crimper, test light, and penetrating oil.
Safety Precautions: Always disconnect the negative battery cable before working on electrical components. Exercise caution when working around the transmission and moving parts. Wear safety glasses for eye protection. Refer to the vehicle's service manual for specific safety guidelines.
Impact on Driving: The P0708 code can have a significant impact on driving, potentially preventing the engine from starting, causing erratic shifting, and even triggering limp mode. Addressing this code promptly is crucial for maintaining safe and reliable vehicle operation.
PCM/TCM Location: The location of the PCM/TCM varies depending on the vehicle's make and model. Common locations include under the dashboard, under the seats, or in the engine compartment. Consult your vehicle's service manual for the exact location.
Connector Types: TRS connectors typically have multiple pins to accommodate the different gear position signals. Inspect the connector carefully for corrosion, damage, and loose pins, ensuring a secure connection.
Troubleshooting Flowchart: Follow a logical troubleshooting flowchart: Verify the code, visually inspect wiring and connectors, check TRS adjustment, test voltage and continuity, perform resistance testing, and consider PCM/TCM testing only if necessary.
Repair Strategies: Repairing a P0708 often involves fixing wiring problems, replacing damaged connectors, adjusting the TRS, or replacing the TRS itself. In rare cases, PCM/TCM replacement may be required.
Aftermarket Parts: When replacing the TRS, consider using a high-quality aftermarket part from a reputable brand. Make sure the replacement part is compatible with your vehicle's make and model for optimal performance.
Preventative Measures: Regularly inspect wiring and connectors for corrosion and damage, protect wiring from the elements, ensure proper TRS adjustment, and address any transmission shifting problems promptly.
Frequently Asked Questions:
Question: What does the P0708 code mean? Answer: It signifies a high voltage input in the Transmission Range Sensor circuit, indicating a potential open circuit or short to voltage.
Question: Can I drive my car with a P0708 code? Answer: It's not recommended, as it can lead to shifting problems and potential damage to the transmission. It might also prevent the car from starting.
Question: Is it expensive to fix a P0708 code? Answer: The cost varies depending on the cause. Fixing a wiring issue is typically less expensive than replacing the TRS or, in rare cases, the PCM/TCM.
Question: Can a bad battery cause a P0708 code? Answer: While a failing battery can sometimes cause unusual electrical issues, it is less likely to directly cause a P0708 code. The focus should be on the TRS circuit itself.
Question: How do I clear the P0708 code after fixing the problem? Answer: Use a scan tool to clear the code. You may need to drive the vehicle for a short period for the code to clear completely.
Conclusion:
The P0708 code points to a problem within the Transmission Range Sensor circuit, often related to wiring or the sensor itself. Thorough diagnosis, including visual inspection, voltage testing, and resistance checks, is crucial for identifying and resolving the underlying cause and restoring proper transmission functionality.