B1347

What Does Code B1347 Mean?

DTC B1347 signifies a detected electrical fault within the control or power circuit of the heated backlite relay. The heated backlite, commonly known as the rear defroster or rear window defogger, is an essential safety feature for visibility. This code is typically set by the Body Control Module (BCM) or a Smart Junction Box (SJB) – in some architectures, potentially the Powertrain Control Module (PCM) or Engine Control Module (ECM) if it manages body electrical functions – when it monitors the electrical characteristics of the heated backlite relay circuit and finds them outside of manufacturer-specified parameters. This could involve an unexpected voltage reading, an open circuit, a short to ground, or a short to voltage, indicating that the module is unable to properly energize or de-energize the relay, or that the relay itself is not responding as expected due to an internal fault or an issue with its power/ground supply. The subsystem primarily affected is the vehicle’s rear defroster system, compromising its functionality.

Common Symptoms

• Inoperative Rear Defroster: The most direct symptom is that the rear window defroster fails to activate and clear frost or condensation from the rear glass.
• Defroster Indicator Light Malfunction: The dashboard indicator light for the rear defroster may not illuminate when the switch is pressed, or it might flash intermittently.
• No Visible Symptoms (Other than Defroster Failure): This code typically affects only the defroster system and does not usually impact engine performance or other vehicle systems.

What Causes the Code B1347?

• Faulty Heated Backlite Relay: The relay itself may have an internal open circuit, short circuit, or mechanical failure preventing proper switching.
• Open Circuit in Relay Wiring: A break or discontinuity in the wiring leading to or from the relay coil (control circuit) or the relay’s power side (load circuit).
• Short to Ground or Voltage: The relay control circuit or power circuit may be shorted to chassis ground or to a constant voltage source, leading to incorrect electrical signals.
• Corroded Relay Socket or Connector Pins: Environmental exposure can cause corrosion on the relay pins or within its socket, creating high resistance or intermittent connections.
• Blown Fuse: A fuse protecting the heated backlite relay’s power supply or its control circuit may have blown, effectively creating an open circuit.
• Faulty Body Control Module (BCM) / Smart Junction Box (SJB): While less common, an internal failure within the BCM/SJB’s relay driver circuit or monitoring logic can trigger this code.

How to Diagnose and Troubleshoot

Diagnosis of B1347 requires a systematic approach, often utilizing a digital multimeter (DMM) and a comprehensive wiring diagram.

1. Retrieve and Document Freeze Frame Data: Use an OBD-II scanner to read the B1347 code and any associated freeze frame data. This provides snapshots of vehicle operating conditions when the fault occurred, which can be valuable for intermittent issues. Check for any other related B-codes that might indicate a broader electrical issue.
2. Visual Inspection:
   • Locate the heated backlite relay (consult a service manual for its exact location, often in an underhood fuse box or interior fuse panel).
   • Inspect the relay, its socket, and surrounding wiring for signs of damage, corrosion, melting, or loose connections.
   • Check the fuse(s) associated with the heated backlite circuit and the relay control circuit. A blown fuse often indicates an underlying short circuit.
3. Electrical Testing of the Relay Circuit (using DMM):
   • Relay Coil Control Circuit: With the ignition on and the rear defroster switch activated, use a DMM to test for appropriate voltage and ground signals at the relay coil terminals. Depending on the circuit design (power-side or ground-side switched), you should see battery voltage (12V) and ground at the respective terminals when the relay is commanded ON.
   • Relay Load Circuit (Power In): Verify battery voltage at the relay’s power input terminal (usually a larger gauge wire) with the ignition on. This supplies power to the defroster grid when the relay activates.
   • Relay Output Circuit (Power Out): With the relay installed and commanded ON, check for battery voltage at the relay’s output terminal, which should feed the rear defroster grid. If power is present at the input but not the output when commanded ON, the relay is suspect.
   • Continuity and Resistance Checks: With the battery disconnected and the relay removed, use the DMM to check for continuity on all associated wires from the BCM/SJB to the relay, and from the relay to the defroster grid. Check for any unwanted shorts to ground or voltage in these circuits. Measure the resistance of the relay coil itself; compare to specifications (typically 60-120 ohms).
4. Bench Test the Relay: If possible, remove the relay and apply fused battery voltage and ground directly to its coil terminals. Use the DMM in continuity mode to check if the switch contacts close (normally open relay) or open (normally closed relay). Listen for an audible click. A non-clicking relay or one that fails continuity tests is faulty.
5. Bidirectional Control (if available): Some advanced scan tools allow for bidirectional control of the rear defroster system. This can be used to command the relay on and off while monitoring voltage and current parameters, helping to isolate if the command signal from the BCM/SJB is present.

Recommended Repairs and Solutions

The specific repair will depend on the root cause identified during diagnosis:

• Replace Faulty Heated Backlite Relay: If the relay itself is determined to be defective after thorough testing, replace it with a new, OEM-specified relay.
• Repair or Replace Damaged Wiring: If an open circuit, short to ground, or short to voltage is found in the wiring, repair the damaged section using appropriate automotive wiring repair techniques (e.g., soldering and heat shrink) or replace the entire affected harness segment if damage is extensive.
• Clean Corroded Terminals: If corrosion is present at the relay socket or connector pins, carefully clean the terminals using electrical contact cleaner and a small brush. Apply dielectric grease to prevent future corrosion.
• Replace Blown Fuse: If a blown fuse is discovered, replace it with a fuse of the correct amperage. It is crucial to identify and rectify the underlying cause of the fuse blowing to prevent recurrence.
• Replace Body Control Module (BCM) / Smart Junction Box (SJB): If all other components and wiring test good and the BCM/SJB is confirmed to be failing to provide the correct control signal or monitoring the circuit incorrectly, it may require replacement. BCM/SJB replacement often necessitates programming or flashing to the vehicle’s specific options and VIN, which typically requires specialized diagnostic equipment.

Mechanics’ Tips:

• Always Start with Fuses: A quick check of relevant fuses can save significant diagnostic time.
• Verify Ground Integrity: Ensure that the ground paths for both the relay coil and the defroster grid are clean, tight, and free of corrosion. A poor ground can mimic an open circuit.
• OEM Parts Recommended: For critical components like relays, using OEM (Original Equipment Manufacturer) parts can prevent compatibility issues or premature failure often seen with some aftermarket components.
• Clear Code and Test: After completing any repair, clear the DTC B1347 using an OBD-II scanner and then operate the rear defroster system several times to ensure the fault does not return and the system functions correctly.