Introduction: The P1820 diagnostic trouble code (DTC) indicates a fault within the internal mode switch circuit, specifically circuit A, reporting a low voltage condition. This code is generally associated with automatic transmissions and indicates a problem with the transmission control system's ability to correctly identify the current operating mode. Addressing this code promptly is crucial to prevent potential damage to the transmission and ensure proper vehicle operation.
Table: P1820 - Internal Mode Switch Circuit A Low
| Topic | Description | Possible Causes |
|---|---|---|
| Definition | Internal Mode Switch Circuit A Low - Indicates that the voltage signal from the "A" circuit of the internal mode switch (also known as the Transmission Range Sensor or TRS) is lower than expected. This switch informs the PCM/TCM of the gear selector position. | Short to ground in the wiring harness; Faulty Internal Mode Switch/TRS; PCM/TCM failure; Open circuit in the signal wire; Corroded connectors; Low battery voltage. |
| Symptoms | Difficulty shifting gears; Transmission stuck in one gear; Warning lights illuminated (Check Engine, Transmission warning); Erratic shifting behavior; Failure to start; Limp mode (reduced power and speed); Incorrect gear displayed on the instrument panel. | Faulty Internal Mode Switch/TRS; PCM/TCM failure; Transmission problems. |
| Affected Components | Internal Mode Switch (TRS); Wiring harness; PCM (Powertrain Control Module); TCM (Transmission Control Module); Transmission itself. | Wiring harness; PCM/TCM failure; Transmission problems. |
| Diagnostic Tools | OBD-II scanner; Multimeter; Wiring diagrams; Oscilloscope (optional). | Faulty multimeter; Incorrect wiring diagrams. |
| Common Misdiagnoses | Incorrectly blaming the PCM/TCM without properly testing the Internal Mode Switch and wiring; Assuming a transmission problem without verifying the electrical components first; Overlooking corroded connectors or damaged wiring. | Lack of proper testing; Incorrect assumptions about the problem; Overlooking simple issues like corroded connectors. |
| Troubleshooting Steps | 1. Verify the code and record freeze frame data; 2. Inspect the wiring harness and connectors for damage; 3. Test the Internal Mode Switch/TRS for proper resistance and voltage; 4. Check the PCM/TCM inputs and outputs; 5. Perform a wiggle test on the wiring. | Incorrect testing procedures; Misinterpreting test results; Failure to identify intermittent problems. |
| Potential Repair Solutions | Repair or replace damaged wiring; Replace faulty Internal Mode Switch/TRS; Repair or replace corroded connectors; Reprogram or replace PCM/TCM (as a last resort); Address any underlying transmission issues. | Incorrect repair procedures; Using faulty replacement parts; Failing to address the root cause of the problem. |
| Voltage Range | Typically, the Internal Mode Switch/TRS outputs a specific voltage range for each gear position. A "low" voltage reading would be significantly below the expected range for any selected gear. Refer to the vehicle's specific service manual for exact values. | Incorrect voltage measurements due to faulty equipment or improper testing procedures. |
| Resistance Values | The Internal Mode Switch/TRS contains multiple circuits with specific resistance values for each gear position. These values should be checked against the manufacturer's specifications to determine if the switch is functioning correctly. | Incorrect resistance measurements due to faulty equipment or improper testing procedures. |
| Related DTCs | P0705 (Transmission Range Sensor Circuit Malfunction); P0706 (Transmission Range Sensor Circuit Range/Performance); P0707 (Transmission Range Sensor Circuit Low Input); P0708 (Transmission Range Sensor Circuit High Input); Other transmission-related codes. | Misinterpreting related DTCs and focusing on the wrong problem. |
| Wiring Harness Inspection | Focus on chafing, cuts, burns, or corrosion. Pay close attention to areas where the harness passes near exhaust components or sharp edges. Check for loose or broken connectors. | Overlooking minor damage to the wiring harness; Failing to properly secure the harness after repairs. |
| PCM/TCM Testing | Requires specialized equipment and knowledge. Typically involves checking the input and output signals of the PCM/TCM using a multimeter or oscilloscope. Refer to the vehicle's specific service manual for testing procedures. | Attempting to test the PCM/TCM without proper training or equipment; Incorrectly diagnosing a PCM/TCM failure. |
| "A" Circuit Significance | The "A" circuit refers to a specific signal wire within the Internal Mode Switch/TRS wiring. It's crucial to identify the correct wire using a wiring diagram before performing any testing. | Working on the wrong wire due to incorrect information or lack of a wiring diagram. |
Detailed Explanations:
Definition: The P1820 code signifies that the vehicle's computer (PCM or TCM) has detected an abnormally low voltage signal from circuit "A" of the Internal Mode Switch/Transmission Range Sensor (TRS). This switch's function is to communicate the driver's gear selector position (Park, Reverse, Neutral, Drive, etc.) to the vehicle's control systems. A low voltage reading indicates a potential problem within the switch itself, the wiring connecting it to the computer, or the computer itself.
Symptoms: The symptoms associated with a P1820 code can vary depending on the severity of the problem and the vehicle's specific design. Common symptoms include difficulty shifting gears, the transmission becoming stuck in a particular gear, illumination of the check engine light or transmission warning light, erratic or unpredictable shifting behavior, failure to start the engine, entering limp mode (which limits engine power and speed), and an incorrect gear display on the instrument panel.
Affected Components: The components directly involved in the P1820 code include the Internal Mode Switch (TRS), the wiring harness connecting the switch to the computer, the PCM (Powertrain Control Module) or TCM (Transmission Control Module), and potentially the transmission itself if the problem leads to internal damage.
Diagnostic Tools: Diagnosing a P1820 code requires several diagnostic tools. An OBD-II scanner is essential for retrieving the code and viewing freeze frame data, which provides information about the conditions present when the code was set. A multimeter is necessary for testing the voltage and resistance of the Internal Mode Switch/TRS and the wiring. Wiring diagrams are crucial for identifying the correct wires and components. An oscilloscope can be helpful for analyzing the signal from the Internal Mode Switch/TRS, but it's not always necessary.
Common Misdiagnoses: A common mistake is to immediately suspect the PCM/TCM without thoroughly testing the Internal Mode Switch and its wiring. It's also easy to assume a transmission problem without first verifying the integrity of the electrical components. Overlooking simple issues like corroded connectors or damaged wiring is another frequent error.
Troubleshooting Steps: The troubleshooting process should follow a logical sequence. First, verify the code using an OBD-II scanner and record the freeze frame data. Next, carefully inspect the wiring harness and connectors for any signs of damage, such as chafing, cuts, or corrosion. Then, test the Internal Mode Switch/TRS for proper resistance and voltage readings, comparing the results to the manufacturer's specifications. Check the PCM/TCM inputs and outputs to ensure they are receiving and sending the correct signals. Finally, perform a wiggle test on the wiring to identify any intermittent problems.
Potential Repair Solutions: The appropriate repair will depend on the underlying cause of the P1820 code. Potential solutions include repairing or replacing damaged wiring, replacing a faulty Internal Mode Switch/TRS, repairing or replacing corroded connectors, reprogramming or replacing the PCM/TCM (as a last resort), and addressing any underlying transmission issues that may be contributing to the problem.
Voltage Range: The voltage range expected from the Internal Mode Switch/TRS varies depending on the gear position. The "A" circuit should produce a specific voltage range for each gear. A "low" voltage reading signifies that the voltage is significantly below the expected range for any selected gear. Consulting the vehicle's service manual is vital for obtaining the correct voltage specifications for each gear position.
Resistance Values: Similar to voltage, the Internal Mode Switch/TRS exhibits specific resistance values for each gear position. These resistance values should be measured with a multimeter and compared against the manufacturer's specifications to ascertain the switch's functionality. Deviations from the specified resistance values indicate a potential problem with the switch.
Related DTCs: Several other DTCs can be related to the P1820 code. These include P0705 (Transmission Range Sensor Circuit Malfunction), P0706 (Transmission Range Sensor Circuit Range/Performance), P0707 (Transmission Range Sensor Circuit Low Input), P0708 (Transmission Range Sensor Circuit High Input), and other transmission-related codes. A thorough understanding of these related codes can help in diagnosing the root cause of the problem.
Wiring Harness Inspection: A meticulous inspection of the wiring harness is crucial. Pay close attention to areas where the harness passes near exhaust components or sharp edges, as these are common locations for damage. Look for chafing, cuts, burns, or corrosion. Check for loose or broken connectors. Secure the harness properly after any repairs to prevent future damage.
PCM/TCM Testing: Testing the PCM/TCM requires specialized equipment and expertise. This typically involves checking the input and output signals of the PCM/TCM using a multimeter or oscilloscope. Refer to the vehicle's specific service manual for detailed testing procedures. Incorrectly diagnosing a PCM/TCM failure can lead to unnecessary and expensive repairs.
"A" Circuit Significance: The "A" circuit designates a particular signal wire within the Internal Mode Switch/TRS wiring. It is paramount to accurately identify this wire using a wiring diagram before commencing any testing. Working on the incorrect wire can result in inaccurate diagnoses and potentially damage other components.
Frequently Asked Questions:
What does the P1820 code mean? The P1820 code indicates a low voltage condition in the "A" circuit of the Internal Mode Switch/Transmission Range Sensor. This switch tells the vehicle's computer what gear is selected.
What are the most common symptoms of the P1820 code? Common symptoms include difficulty shifting, transmission stuck in gear, and the check engine light being illuminated. Erratic shifting and failure to start can also occur.
Can I fix the P1820 code myself? Depending on your mechanical skills, you might be able to fix it by inspecting the wiring and connectors. However, proper diagnosis and testing often require specialized tools and knowledge.
Is it safe to drive with the P1820 code? Driving with a P1820 code is not recommended as it can lead to further transmission damage or unexpected shifting. It's best to get it checked by a qualified mechanic as soon as possible.
How much does it cost to fix the P1820 code? The cost to fix the P1820 code can vary depending on the cause of the problem. Replacing the Internal Mode Switch/TRS is typically less expensive than repairing or replacing the PCM/TCM.
Conclusion:
The P1820 code signals a problem with the Internal Mode Switch circuit, specifically circuit A, indicating a low voltage condition. Proper diagnosis and systematic troubleshooting are crucial for identifying the root cause and implementing the correct repair to ensure the proper operation of the transmission and prevent potential damage.