The B1790 diagnostic trouble code (DTC) indicates an issue with the input circuit of the auto lamp sensor, also commonly known as the ambient light sensor or twilight sensor. This sensor plays a crucial role in automatically controlling the vehicle's headlights and other lighting features based on the surrounding light levels. A malfunctioning circuit can lead to erratic headlight behavior, impacting both safety and driver convenience.
Understanding the potential causes, symptoms, and troubleshooting steps associated with the B1790 code is essential for accurate diagnosis and effective repair. This article aims to provide a comprehensive overview of the B1790 code, covering its definition, causes, symptoms, diagnostic procedures, and potential solutions.
| Component/Aspect | Description | Possible Causes |
|---|---|---|
| Definition of B1790 | Auto Lamp Sensor Input Circuit Failure - Indicates a malfunction in the electrical circuit connected to the ambient light sensor. The Powertrain Control Module (PCM) or Body Control Module (BCM) has detected an abnormal voltage or signal from the sensor. | Open circuit in the wiring, Short to ground in the wiring, Short to voltage in the wiring, Faulty ambient light sensor, Corroded connectors, PCM/BCM failure (rare). |
| Function of Auto Lamp Sensor | Measures the ambient light level and sends a corresponding voltage signal to the PCM/BCM. This signal is used to automatically control the headlights, parking lights, and dashboard illumination. | Inaccurate light readings leading to improper headlight activation, Headlights remaining on during the day, Headlights not activating at night, Erratic dashboard illumination. |
| Common Symptoms | Headlights not turning on automatically at dusk, Headlights remaining on during daylight hours, Intermittent headlight operation, "Auto" headlight setting not functioning, Warning light illuminated on the dashboard (e.g., check engine light or a dedicated lighting system warning). | Driver inconvenience, Reduced visibility at night, Potential safety hazard (if headlights fail to activate), Battery drain (if headlights remain on). |
| Wiring Diagram Overview | Typically involves three wires: power (usually 5V), ground, and signal. The signal wire carries a voltage proportional to the amount of light detected by the sensor. Refer to the vehicle's specific wiring diagram for accurate pin locations and wire colors. | Damaged or frayed wires, Loose or corroded connectors, Incorrect wiring modifications. |
| Diagnostic Tools | Scan tool (capable of reading manufacturer-specific codes), Multimeter, Wiring diagram, Test light, Circuit tester. | Incorrect or incomplete diagnostic information, Potential for misdiagnosis, Damage to electrical components. |
| Diagnostic Procedure | 1. Verify the B1790 code is present. 2. Inspect the wiring and connectors for damage. 3. Test the sensor's power, ground, and signal wires. 4. Use a scan tool to monitor the sensor's output signal. 5. Perform a sensor resistance test (if applicable). 6. Check for short circuits to ground or voltage. 7. If the wiring and sensor are good, suspect PCM/BCM failure. | Time-consuming process, Requires electrical troubleshooting skills, May require specialized tools. |
| Troubleshooting Steps | 1. Repair or replace damaged wiring. 2. Clean or replace corroded connectors. 3. Replace the faulty ambient light sensor. 4. Reprogram or replace the PCM/BCM (if necessary). | Incorrect repair leading to recurring issues, Damage to other components, Increased repair costs. |
| Common Mistakes | Replacing the sensor without thoroughly inspecting the wiring, Failing to check for short circuits, Neglecting to verify the PCM/BCM functionality. | Unnecessary parts replacement, Wasted time and money, Recurring problems. |
| Related DTCs | B1791 (Auto Lamp Sensor Circuit Range/Performance), B1792 (Auto Lamp Sensor Circuit Low), B1793 (Auto Lamp Sensor Circuit High) | Provides a broader understanding of potential lighting system issues. Aids in narrowing down the specific problem. |
| Sensor Location | Typically located on the dashboard, near the windshield, or in the rearview mirror housing. Refer to the vehicle's service manual for the exact location. | Damage during dashboard repairs, Obstruction of the sensor by objects on the dashboard. |
| Voltage Testing | With the ignition on, the power wire should have approximately 5V. The ground wire should have continuity to ground. The signal wire voltage will vary depending on the ambient light level. | Incorrect voltage readings indicating wiring issues or sensor malfunction. |
| Resistance Testing | Some sensors have a resistance value that can be tested with a multimeter. Refer to the vehicle's service manual for the correct resistance range. | Out-of-range resistance readings indicating a faulty sensor. |
| Effect of Aftermarket Accessories | Installation of aftermarket accessories (e.g., remote starters, alarm systems) can sometimes interfere with the auto lamp sensor circuit. | Erratic headlight behavior, False DTCs, Interference with other vehicle systems. |
| PCM/BCM Programming | In some cases, replacing the PCM/BCM may require programming to ensure proper communication with the auto lamp sensor and other vehicle systems. | Malfunctioning auto lamp system, Communication errors, Potential damage to the PCM/BCM. |
| Impact of Tinted Windows | Darkly tinted windows can reduce the amount of light reaching the sensor, potentially causing the headlights to activate prematurely. | Premature headlight activation, Driver inconvenience. |
| Ambient Light Sensor Types | Photodiodes, Phototransistors, and Integrated Light Sensors. Each type has a different electrical characteristic. | Understanding the sensor type can help in proper testing and diagnostics. |
| Connector Types | Molex, Delphi, and other common automotive connectors. Identifying the connector type is important for proper removal and reinstallation. | Damaged connectors, corroded terminals, improper connections. |
| CAN Bus Communication | Some vehicles use the CAN (Controller Area Network) bus to communicate sensor data to the PCM/BCM. | CAN bus issues can affect the sensor data and cause false DTCs. |
| Impact of Battery Voltage | Low battery voltage can affect the sensor's performance and cause inaccurate readings. | Unreliable sensor readings, false DTCs. |
| Temperature Effects | Extreme temperatures can affect the sensor's accuracy and performance. | Inaccurate sensor readings, especially in very hot or cold weather. |
Detailed Explanations
Definition of B1790: The B1790 code signifies that the vehicle's computer (PCM or BCM) has detected a problem within the electrical circuit connected to the auto lamp sensor. This typically involves an abnormal voltage reading, indicating an open, short to ground, or short to voltage in the circuit. The computer monitors the signal from the sensor, and if the signal falls outside the acceptable range, the B1790 code is triggered.
Function of Auto Lamp Sensor: The auto lamp sensor, also known as the ambient light sensor or twilight sensor, measures the intensity of surrounding light. It then sends a voltage signal to the vehicle's computer (PCM/BCM). This signal is used to automatically control the headlights, parking lights, and sometimes even the dashboard illumination, ensuring optimal visibility in varying light conditions.
Common Symptoms: Several symptoms can indicate a B1790 code. These include headlights not turning on automatically at dusk, headlights remaining on during daylight hours, intermittent headlight operation, the "auto" headlight setting not functioning, and the illumination of a warning light on the dashboard (check engine light or a dedicated lighting system warning).
Wiring Diagram Overview: The auto lamp sensor circuit usually consists of three wires: a power wire (typically supplying 5V), a ground wire, and a signal wire. The signal wire carries a voltage that varies proportionally with the amount of light detected by the sensor. Consulting the vehicle's specific wiring diagram is crucial for identifying the correct pin locations and wire colors for accurate testing and troubleshooting.
Diagnostic Tools: Diagnosing a B1790 code requires several tools. A scan tool capable of reading manufacturer-specific codes is essential for confirming the code and accessing related data. A multimeter is needed to test voltage, continuity, and resistance in the circuit. A wiring diagram provides valuable information about the circuit layout. A test light or circuit tester can also be helpful for identifying power and ground issues.
Diagnostic Procedure: The diagnostic procedure for B1790 involves several steps: First, verify that the B1790 code is actually present. Next, carefully inspect the wiring and connectors associated with the sensor for any signs of damage or corrosion. Then, test the sensor's power, ground, and signal wires using a multimeter. Use a scan tool to monitor the sensor's output signal in real-time. If applicable, perform a sensor resistance test according to the vehicle's service manual. Finally, check for short circuits to ground or voltage. If the wiring and sensor appear to be functioning correctly, suspect a possible PCM/BCM failure, although this is less common.
Troubleshooting Steps: Troubleshooting a B1790 code may involve several steps. Repairing or replacing any damaged wiring is essential. Clean or replace corroded connectors to ensure proper electrical connections. If the sensor is found to be faulty, replace it with a new one. In rare cases, reprogramming or replacing the PCM/BCM may be necessary, but this should only be considered after thoroughly investigating all other potential causes.
Common Mistakes: A common mistake is replacing the sensor without first thoroughly inspecting the wiring and connectors for damage. Another error is failing to check for short circuits to ground or voltage, which can damage the new sensor. Neglecting to verify the PCM/BCM functionality can also lead to misdiagnosis and unnecessary repairs.
Related DTCs: Several other DTCs may be related to the auto lamp sensor circuit. These include B1791 (Auto Lamp Sensor Circuit Range/Performance), B1792 (Auto Lamp Sensor Circuit Low), and B1793 (Auto Lamp Sensor Circuit High). Understanding these related codes can provide a broader perspective on potential lighting system issues and aid in narrowing down the specific problem.
Sensor Location: The auto lamp sensor is typically located on the dashboard, near the windshield, or integrated into the rearview mirror housing. Consult the vehicle's service manual or online resources for the exact location, as it can vary depending on the make and model.
Voltage Testing: With the ignition switched on, the power wire to the sensor should typically have approximately 5 volts. The ground wire should exhibit continuity to ground, indicating a good ground connection. The voltage on the signal wire will vary depending on the ambient light level. Use a multimeter to measure these voltages and compare them to the specifications in the vehicle's service manual.
Resistance Testing: Some auto lamp sensors have a specific resistance value that can be tested using a multimeter. Consult the vehicle's service manual for the correct resistance range for the sensor. An out-of-range resistance reading indicates a faulty sensor that needs to be replaced.
Effect of Aftermarket Accessories: Installation of aftermarket accessories, such as remote starters or alarm systems, can sometimes interfere with the auto lamp sensor circuit. These accessories may introduce electrical noise or disrupt the sensor's signal, leading to erratic headlight behavior or false DTCs.
PCM/BCM Programming: In some cases, replacing the PCM/BCM may require programming to ensure proper communication with the auto lamp sensor and other vehicle systems. This programming ensures that the PCM/BCM correctly interprets the sensor's signal and controls the headlights accordingly.
Impact of Tinted Windows: Darkly tinted windows can significantly reduce the amount of light reaching the auto lamp sensor. This can cause the headlights to activate prematurely, even during daylight hours. If you have heavily tinted windows, consider temporarily removing the tint near the sensor to see if it resolves the issue.
Ambient Light Sensor Types: Different types of ambient light sensors are used in vehicles, including photodiodes, phototransistors, and integrated light sensors. Each type has different electrical characteristics, so understanding the sensor type can help with proper testing and diagnostics.
Connector Types: Common automotive connectors used in auto lamp sensor circuits include Molex, Delphi, and other types. Identifying the connector type is important for proper removal and reinstallation, preventing damage to the connector and ensuring a secure electrical connection.
CAN Bus Communication: Some modern vehicles utilize the CAN (Controller Area Network) bus to communicate sensor data to the PCM/BCM. Problems with the CAN bus can affect the sensor data and cause false DTCs, making diagnosis more complex.
Impact of Battery Voltage: Low battery voltage can affect the sensor's performance and cause inaccurate readings. Ensure the vehicle's battery is properly charged and in good condition before troubleshooting the auto lamp sensor circuit.
Temperature Effects: Extreme temperatures can affect the sensor's accuracy and performance. In very hot or cold weather, the sensor may provide inaccurate readings, leading to erratic headlight behavior.
Frequently Asked Questions
What does the B1790 code mean? The B1790 code indicates a problem with the input circuit of the auto lamp sensor, meaning the vehicle's computer is not receiving the expected signal from the sensor.
What are the symptoms of a B1790 code? Common symptoms include headlights not turning on automatically, headlights staying on during the day, and the "auto" headlight setting not working.
Can I fix a B1790 code myself? Depending on your mechanical skills, you might be able to fix it by checking wiring and connectors, but more complex issues may require professional diagnosis.
How much does it cost to fix a B1790 code? The cost varies depending on the cause; replacing the sensor is usually less expensive than repairing damaged wiring or replacing the PCM/BCM.
Is it safe to drive with a B1790 code? It depends on how the code is affecting your headlights. If they are not working correctly, it could be a safety hazard, especially at night.
Conclusion
The B1790 code signals a problem with the auto lamp sensor input circuit, leading to potential headlight malfunctions. Thorough diagnosis and careful troubleshooting are essential for resolving the issue and restoring proper headlight operation. Always consult the vehicle's service manual for specific instructions and wiring diagrams.