B1347 Heated Backlight Relay Circuit Failure

Introduction:

The B1347 Diagnostic Trouble Code (DTC) indicates a problem within the heated backlight relay circuit. This circuit is responsible for activating the rear window defogger, a crucial safety feature in cold weather climates. A malfunctioning heated backlight can lead to reduced visibility and potential safety hazards, making prompt diagnosis and repair essential.

B1347 Heated Backlight Relay Circuit Failure: A Comprehensive Guide

Detailed Explanations

Definition of B1347:

The B1347 DTC specifically points to a failure within the electrical circuit that controls the heated backlight relay. This relay is the switch that provides power to the rear window defogger grid. When the PCM or BCM detects an open circuit (no current flow) or a short circuit (unintended current flow) in the relay's control or power circuit, it sets the B1347 code. This indicates the system can't reliably turn on the defogger. The control circuit is typically the low current side used to activate the relay's coil, while the power circuit is the high current side that feeds power to the defogger grid.

Symptoms:

The most obvious symptom is a non-functional rear window defogger. The heated backlight indicator light on the dashboard may not illuminate when the defogger button is pressed, or it may illuminate intermittently. The Check Engine Light (CEL) or a specific warning light related to the defogger system will likely be illuminated. Depending on the vehicle's electrical architecture, other seemingly unrelated electrical issues might also surface if the heated backlight circuit shares a common power supply or ground with other systems. For example, the radio, power windows, or other interior functions could be affected.

Common Diagnostic Steps:

1. Visual Inspection: Begin with a thorough visual inspection of all wiring, connectors, and the relay itself. Look for any signs of damage, corrosion, melted insulation, or loose connections. Pay close attention to the relay socket and any wiring harnesses near the rear window.

2. Fuse Check: Locate the fuse that protects the heated backlight circuit (consult the vehicle's owner's manual or a wiring diagram). Use a fuse puller to remove the fuse and visually inspect it for a broken filament. A blown fuse indicates a potential short circuit downstream. Replace the fuse with one of the correct amperage rating after addressing the underlying cause of the blown fuse.

3. Relay Testing: The heated backlight relay can be tested using a multimeter. Disconnect the relay and check for continuity across the coil terminals. If there's no continuity, the coil is open and the relay needs to be replaced. You can also apply 12V to the coil terminals and listen for a click, indicating the relay is switching. Use the multimeter to check for continuity between the normally open terminals when the relay is energized.

4. Voltage and Ground Checks: With the relay removed, use a multimeter to check for voltage at the power supply terminal of the relay socket. You should see battery voltage when the ignition is on. Also, check for a good ground connection at the ground terminal of the relay socket. A poor ground can prevent the relay from operating correctly.

5. Wiring Continuity Test: Use a multimeter to check the continuity of the wiring between the relay, PCM/BCM, and ground. This will help identify any open circuits or shorts to ground. Refer to a wiring diagram to identify the correct wires to test.

6. PCM/BCM Testing: If all other components check out okay, the PCM or BCM may be the source of the problem. These modules control the relay's activation based on inputs from the defogger switch and other sensors. Testing these modules typically requires specialized equipment and expertise, so it's best left to a qualified technician.

7. Scan Tool Data: Connect a scan tool to the vehicle's diagnostic port (OBD-II port). Retrieve any stored trouble codes and freeze frame data, which can provide valuable clues about the conditions that triggered the B1347 code. The scan tool can also be used to monitor the relay activation signal from the PCM/BCM to see if the module is commanding the relay to turn on.

Tools Required:

• Multimeter: Essential for testing voltage, continuity, and resistance.
• Wiring Diagram: A vehicle-specific wiring diagram is crucial for identifying the correct wires and components in the heated backlight circuit.
• Scan Tool: While not always necessary, a scan tool can retrieve trouble codes, freeze frame data, and monitor relay activation signals, making diagnosis easier.
• Fuse Puller: A small tool designed to safely remove fuses without damaging them.
• Socket Set and Wrenches: Needed for removing and installing components.
• Wire Strippers and Crimpers: Used for repairing or replacing damaged wiring.
• Test Light: A simple tool for checking for voltage at various points in the circuit.

Potential Repair Costs:

The cost to repair a B1347 code can vary significantly depending on the root cause. A faulty relay is the least expensive fix, typically costing between $20 and $50. Wiring repairs can range from $50 to $200, depending on the extent of the damage and the labor involved. If the PCM or BCM is faulty, replacement or reprogramming can be significantly more expensive, potentially costing between $300 and $1000 or more. Labor costs will also add to the total repair bill.

Severity:

The severity of a B1347 code is considered moderate. While the vehicle is still drivable, the lack of a functioning rear window defogger can significantly reduce visibility in cold or foggy conditions, posing a safety risk. It's important to address this issue promptly to ensure safe driving conditions.

Related DTCs:

Other DTCs that may be related to a B1347 code include:

• B1346 (Heated Backlight Circuit Open): This code indicates an open circuit in the heated backlight circuit, which could be caused by a broken wire, a faulty relay, or a blown fuse.
• B1348 (Heated Backlight Circuit Short to Ground): This code indicates a short circuit to ground in the heated backlight circuit, which could be caused by damaged wiring or a faulty relay.
• Codes related to the PCM/BCM: If the PCM or BCM is faulty, it may generate other codes related to its internal functions or its ability to control other circuits.
• Codes relating to other components on the same circuit: If the heated backlight circuit shares a power source or ground with other systems, a fault in those systems could trigger the B1347 code.

Frequently Asked Questions

• What does the B1347 code mean? The B1347 code indicates a problem with the heated backlight relay circuit, meaning the system is unable to reliably activate the rear window defogger. This can be due to a faulty relay, wiring issues, or a problem with the PCM/BCM.

• Can I drive with a B1347 code? Yes, but it's not recommended, especially in cold or foggy conditions. The lack of a functioning rear window defogger can significantly impair visibility and increase the risk of an accident.

• How do I fix a B1347 code? Start by inspecting the wiring, connectors, and relay for damage. Check the fuse and test the relay. If the problem persists, you may need to test the wiring continuity or have the PCM/BCM tested by a professional.

• Is the B1347 code expensive to fix? The cost varies depending on the cause. A faulty relay is relatively inexpensive, while wiring repairs or PCM/BCM replacement can be significantly more costly.

• Can a bad ground cause a B1347 code? Yes, a poor or corroded ground connection can prevent the relay from operating correctly and trigger the B1347 code. Checking and cleaning ground connections is a crucial step in diagnosing this issue.

Conclusion

The B1347 DTC signals a malfunction in the heated backlight relay circuit, a critical system for maintaining visibility in adverse weather conditions. Thorough diagnosis, starting with simple checks like fuse and relay inspection, is essential to pinpoint the root cause and restore the defogger's functionality, ensuring safe driving.