Introduction:

The P1520 diagnostic trouble code (DTC) indicates a problem with the Transmission Range Switch (TRS) circuit, also known as the Neutral Safety Switch or Inhibitor Switch on some vehicles. This switch is crucial for identifying the gear selected by the driver and relaying this information to the engine control unit (ECU) or transmission control module (TCM). A malfunctioning TRS circuit can lead to various drivability issues, including starting problems, incorrect gear selection, and even transmission damage.

Comprehensive Transmission Range Switch (TRS) Information

| Topic | Description the TRS (Transmission Range Switch) is a vital component of the vehicle's transmission system. Its primary role is to detect the position of the gear selector lever (e.g., Park, Reverse, Neutral, Drive, Low) and transmit this information to the vehicle's computer systems, particularly the engine control unit (ECU) and transmission control module (TCM). This information is crucial for various functions, including:

  • Starting the Engine: The TRS prevents the engine from starting unless the gear selector is in Park or Neutral, acting as a safety mechanism.
  • Transmission Control: The TCM uses the TRS signal to determine the appropriate gear to engage and to manage shift points.
  • Reverse Lights Activation: The TRS activates the reverse lights when the gear selector is in Reverse.
  • Cruise Control Functionality: In some vehicles, the TRS provides input to the cruise control system, disabling it when the gear selector is moved out of Drive.
  • Displaying Gear Indication: The TRS signal is used to illuminate the correct gear indicator on the instrument panel.

Failure of the TRS or its circuit can lead to a range of problems, from the vehicle not starting to erratic shifting and safety concerns. The P1520 code specifically indicates an issue with the electrical circuit associated with the TRS, meaning the ECU or TCM is not receiving the expected signals from the switch. This could be due to a variety of factors, including a faulty switch, damaged wiring, poor connections, or even a problem within the ECU or TCM itself.

Detailed Explanations

1. Function of the Transmission Range Switch (TRS):

The TRS, also known as the Neutral Safety Switch or Inhibitor Switch, is an electromechanical device that senses the position of the gear selector lever. It typically consists of a rotating or sliding contact assembly that aligns with different sets of electrical contacts based on the selected gear. These contacts complete specific circuits, sending voltage signals to the ECU/TCM indicating the selected gear.

2. Location of the Transmission Range Switch (TRS):

The TRS is usually located on the transmission housing, where the gear selector linkage connects to the transmission. On some vehicles, it might be found inside the transmission, requiring removal and disassembly to access. Specific location varies depending on the vehicle make, model, and year. Consulting a repair manual or online resources specific to the vehicle is crucial for pinpointing its exact location.

3. Symptoms of a Faulty TRS Circuit:

  • Engine won't start: The most common symptom. The engine may crank but not start, or it may not crank at all.
  • Engine starts in gear: A serious safety hazard. The engine may start when the gear selector is in Drive or Reverse.
  • Reverse lights don't work: The reverse lights may not illuminate when the gear selector is in Reverse.
  • Erratic shifting: The transmission may shift improperly, skip gears, or shift harshly.
  • Gear indicator malfunction: The gear indicator on the instrument panel may not display the correct gear.
  • Cruise control inoperative: The cruise control system may not function.
  • Stalling: The engine may stall when shifting gears.
  • Transmission stuck in gear: The transmission may become stuck in a particular gear.

4. Possible Causes of P1520 Code:

  • Faulty Transmission Range Switch (TRS): The switch itself may be worn, damaged, or have internal shorts or opens.
  • Wiring problems: Damaged, corroded, or shorted wiring in the TRS circuit. This includes wires leading to and from the TRS.
  • Connector issues: Loose, corroded, or damaged connectors at the TRS or at the ECU/TCM.
  • Grounding problems: Poor or missing ground connection for the TRS circuit.
  • ECU/TCM failure: In rare cases, the ECU or TCM itself may be faulty, preventing it from properly interpreting the TRS signal.
  • Mechanical misalignment: If the TRS is not properly aligned with the gear selector linkage, it may not accurately detect the selected gear.
  • Low battery voltage: A weak or failing battery can sometimes cause erratic electrical signals, potentially triggering the P1520 code.

5. Diagnostic Steps for P1520 Code:

  1. Verify the code: Use an OBD-II scanner to confirm the presence of the P1520 code and check for any other related codes.
  2. Inspect the TRS: Visually inspect the TRS for any signs of damage, such as cracks, broken connectors, or corrosion.
  3. Check the wiring: Carefully examine the wiring harness and connectors associated with the TRS for damage, shorts, or corrosion. Use a multimeter to check for continuity and voltage.
  4. Test the TRS: Use a multimeter to test the TRS according to the vehicle manufacturer's specifications. This typically involves checking for continuity between specific terminals in different gear positions.
  5. Check the ground: Ensure that the TRS has a good ground connection.
  6. Check the ECU/TCM: If all other components appear to be functioning correctly, the ECU or TCM may be the problem. This typically requires professional diagnosis and testing.
  7. Check for mechanical issues: Ensure the TRS is properly aligned with the gear selector linkage.

6. Common Mistakes During Diagnosis:

  • Replacing the TRS without proper testing: It's essential to test the TRS and the wiring before replacing the switch.
  • Ignoring wiring problems: Damaged wiring is a common cause of TRS circuit problems.
  • Neglecting grounding issues: A poor ground connection can cause a variety of electrical problems.
  • Assuming the ECU/TCM is faulty without proper testing: The ECU/TCM is rarely the cause of the P1520 code.
  • Not consulting a vehicle-specific repair manual: Repair manuals provide detailed information on testing and troubleshooting the TRS circuit for specific vehicles.

7. Tools Needed for Diagnosis and Repair:

  • OBD-II Scanner: To read and clear diagnostic trouble codes.
  • Multimeter: To test voltage, continuity, and resistance.
  • Wiring diagrams: To understand the wiring layout of the TRS circuit.
  • Socket set: To remove and install the TRS.
  • Wire stripper/crimper: To repair damaged wiring.
  • Contact cleaner: To clean corroded connectors.
  • Vehicle-specific repair manual: Provides detailed instructions and specifications.
  • Jack and jack stands: To safely access the TRS on some vehicles.

8. Repair Procedures:

  • TRS Replacement:
    1. Disconnect the negative battery cable.
    2. Locate the TRS on the transmission housing.
    3. Disconnect the electrical connector from the TRS.
    4. Remove the TRS retaining bolts or screws.
    5. Remove the old TRS.
    6. Install the new TRS, ensuring it is properly aligned.
    7. Tighten the retaining bolts or screws to the specified torque.
    8. Connect the electrical connector to the TRS.
    9. Reconnect the negative battery cable.
    10. Clear the P1520 code and test the system.
  • Wiring Repair:
    1. Identify the damaged wiring.
    2. Cut out the damaged section of wire.
    3. Strip the ends of the wires.
    4. Connect the wires using a butt connector or solder.
    5. Insulate the connection with heat shrink tubing or electrical tape.
  • Connector Repair:
    1. Clean the connector with contact cleaner.
    2. Repair any damaged pins or terminals.
    3. Ensure the connector is securely connected.

9. Variations of the P1520 Code:

While P1520 is a general code, some manufacturers may use slightly different codes or descriptions for similar issues. Always consult a vehicle-specific repair manual to confirm the exact meaning of the code.

10. Preventative Maintenance:

  • Regular visual inspection: Periodically inspect the TRS, wiring, and connectors for any signs of damage or corrosion.
  • Keep the engine bay clean: A clean engine bay can help prevent corrosion and damage to electrical components.
  • Address any transmission problems promptly: Transmission problems can sometimes put stress on the TRS, leading to premature failure.

Frequently Asked Questions

  • What does the P1520 code mean? The P1520 code indicates a problem with the Transmission Range Switch (TRS) circuit, meaning the ECU/TCM is not receiving the expected signals.

  • Can I drive with a P1520 code? It's not recommended. A faulty TRS can cause unpredictable shifting and create a safety hazard, and may leave you stranded.

  • How much does it cost to fix a P1520 code? The cost depends on the cause. Replacing the TRS is typically less expensive than repairing damaged wiring or replacing the ECU/TCM. Expect to pay between $100 and $500 for the repair.

  • Is the Transmission Range Switch the same as the Neutral Safety Switch? Yes, the Transmission Range Switch and Neutral Safety Switch are often used interchangeably. They perform the same function. The term "Inhibitor Switch" is also used.

  • Can a bad battery cause a P1520 code? Potentially, yes. Low battery voltage can sometimes cause erratic electrical signals that may trigger the P1520 code.

Conclusion

The P1520 code can indicate a range of issues within the Transmission Range Switch circuit, from a faulty switch to damaged wiring. Proper diagnosis and repair are crucial to ensure the safe and reliable operation of the vehicle. Always consult a vehicle-specific repair manual and follow safe repair practices when working on electrical systems.