B1304

What Does Code B1304 Mean?

DTC B1304 signifies a fault within the Accessory Delay Relay (ADR) coil control circuit, specifically indicating a short to battery voltage. This is typically a manufacturer-specific “B” code, meaning it relates to the vehicle’s body, comfort, or convenience systems, and is usually detected and managed by the Body Control Module (BCM), rather than the Engine Control Module (ECM) or Powertrain Control Module (PCM).

The Accessory Delay Relay is a critical component responsible for providing power to various convenience accessories (such as the radio, power windows, sunroof, or interior lighting) for a predetermined period after the ignition is turned off, or until a specific event occurs, like opening a driver’s door. The BCM actively monitors the voltage on the control circuit for the ADR coil. When the BCM commands the relay coil to be de-energized (i.e., accessories should turn off), it expects to see a voltage near 0V on this circuit. If the BCM detects a constant or excessively high voltage, specifically battery voltage (B+) or a significantly elevated voltage, present on the relay coil’s control circuit when it should be open or grounded, it interprets this as a “short to battery” condition and sets code B1304. This often implies an unintended electrical path supplying voltage to the coil, causing it to remain energized or report an incorrect state.

Common Symptoms

• Accessories Remain On: The most common symptom is that accessories powered by the ADR (e.g., radio, power windows, sunroof) remain active indefinitely even after the ignition is turned off and the delay period should have expired, or a door has been opened.
• Parasitic Battery Drain: Due to accessories staying energized, the vehicle’s battery may experience a parasitic draw, leading to a dead battery, especially after the vehicle has been parked for an extended period.
• Inoperative Accessories: In some cases, if the BCM detects a persistent fault, it might disable the circuit altogether as a protective measure, leading to the affected accessories not working at all.
• Warning Lights/Messages: While less common for this specific code, related system faults could trigger a “Service Engine Soon” or specific warning message on the instrument cluster, although B-codes typically do not illuminate the MIL directly.

What Causes the Code B1304?

• Damaged Wiring Harness: The most frequent cause involves the wiring harness for the Accessory Delay Relay coil control circuit chafing, rubbing through its insulation, and making contact with a constant 12-volt (B+) power wire or a fused power distribution source. This unintended contact directly supplies battery voltage to the relay coil circuit.
• Corroded Electrical Connectors: Moisture intrusion or exposure to corrosive elements can lead to corrosion within the ADR connector, the BCM connector, or inline connectors. This corrosion can bridge adjacent terminals, creating a short circuit path to battery voltage.
• Faulty Accessory Delay Relay: Although the code points to the coil circuit, an internal short within the relay itself, particularly between the coil terminals and an internal constant power supply, can cause the BCM to detect a short to battery on the control side.
• Faulty Body Control Module (BCM): Less common, but a defective BCM with an internal short or a failed driver circuit could erroneously supply continuous voltage to the Accessory Delay Relay coil control circuit, even when it should be off.
• Improper Aftermarket Accessory Installation: Poorly installed aftermarket electronics, particularly those tapping into existing vehicle wiring, can inadvertently create a short to battery voltage on the ADR coil control circuit.

How to Diagnose and Troubleshoot

Diagnosing B1304 requires a systematic approach, utilizing a digital multimeter (DMM), an OBD-II scan tool capable of reading B-codes, and vehicle-specific wiring diagrams.

1. Verify the Code and Review Freeze Frame Data: Connect an OBD-II scan tool and confirm B1304 is present. Note any associated freeze frame data, as it can provide valuable information on vehicle operating conditions when the fault was set. Check for other related B-codes that might point to a broader electrical issue.
2. Visual Inspection of Wiring and Connectors:
   • Locate the Accessory Delay Relay (refer to a service manual for its exact location, often in the fuse box or under the dash).
   • Carefully inspect the wiring harness leading to and from the relay for any signs of chafing, pinching, melting, or corrosion. Pay close attention to areas where the harness passes through firewalls, along frame rails, or near moving components.
   • Inspect the relay connector and its terminals for corrosion, bent pins, or signs of overheating. Disconnect the connector and check both male and female terminals.
3. Accessory Delay Relay Test:
   • With the ignition OFF and key removed, remove the Accessory Delay Relay from its socket.
   • Using a DMM, measure the resistance across the relay coil terminals (typically terminals 85 and 86 on a standard Bosch-style relay). Compare this reading to manufacturer specifications (usually between 70-100 ohms). An open circuit (infinite resistance) or a very low resistance (approaching 0 ohms) indicates an internal relay fault.
   • If possible, apply 12V and ground to the coil terminals to verify the relay clicks, and then measure continuity across the switch terminals (typically 30 and 87) to ensure proper operation.
4. Voltage Measurement at Relay Socket (Control Circuit):
   • With the Accessory Delay Relay removed, turn the ignition to the “OFF” position. Ensure all doors are closed and wait for the accessory delay period to expire (or simulate by opening and closing the driver’s door).
   • Using a DMM set to DC volts, probe the relay socket terminal that corresponds to the BCM’s control wire for the coil (consult the wiring diagram to identify this specific terminal).
   • You should read approximately 0 volts. If battery voltage (12V or more) is present, this confirms a short to battery voltage on the control circuit when it should be off.
5. Continuity Test for Short to B+:
   • Disconnect the vehicle’s battery for safety.
   • Disconnect the BCM’s main connector (or the specific connector containing the ADR coil control wire).
   • With the ADR removed and the BCM disconnected, identify the specific ADR coil control wire at the relay harness connector.
   • Using the DMM set to continuity or resistance, check for continuity between this control wire and a known constant 12V source (e.g., a fused B+ terminal in the fuse box, or the positive battery cable). There should be no continuity.
   • Also, check for continuity between the control wire and other adjacent power wires within the harness.
   • A low resistance reading indicates a short. Carefully trace this wire back towards the BCM, inspecting for the point of contact with a power source.
6. BCM Output Test (If Wiring is Confirmed Good):
   • If all wiring to and from the ADR is confirmed to be free of shorts and open circuits, and yet voltage is still detected on the control wire when the BCM is connected and the ignition is off (and delay expired), the BCM itself may be at fault.
   • This typically requires further advanced diagnostic procedures, potentially including scope analysis of the BCM’s output driver, or swapping with a known good BCM if available, though BCMs often require programming.

Recommended Repairs and Solutions

Once the root cause of B1304 has been accurately identified through systematic diagnosis, the following repairs are typically recommended:

• Repair or Replace Damaged Wiring: The most common solution involves locating and repairing the specific section of the wiring harness that has shorted to battery voltage. This usually requires carefully stripping back the harness loom, isolating the damaged wire, repairing it using appropriate soldering and heat-shrink tubing techniques, and re-routing it to prevent future damage. If the damage is extensive, replacing the entire wiring sub-harness may be necessary.
• Replace Corroded Connectors or Terminals: If corrosion is found to be bridging terminals, the affected connector or individual pins should be replaced. Ensure the new connections are sealed to prevent future moisture intrusion.
• Replace Accessory Delay Relay: If diagnostic steps confirm an internal fault within the relay coil itself, replace the Accessory Delay Relay with a new, OEM-specified part.
• Replace Body Control Module (BCM): If all wiring and relay components are verified to be in good working order and the BCM is definitively diagnosed as the source of the erroneous voltage, it will need replacement. Be aware that BCM replacement often requires specialized programming and calibration using a factory-level scan tool to integrate it with the vehicle’s other electronic modules (e.g., key fobs, immobilizer system).
• Rectify Aftermarket Accessory Installation: If aftermarket accessories are found to be the cause of the short, they should be properly disconnected or rewired following best practices to ensure they do not interfere with vehicle electrical systems.

Mechanic’s Tip: Always disconnect the vehicle’s battery before performing any electrical repairs to prevent accidental shorts or damage to sensitive electronic components. After completing repairs, clear the DTCs with a scan tool and perform a functional test of all affected accessories to confirm the repair.