B1305

What Does Code B1305 Mean?

DTC B1305 indicates a detected electrical fault within the control circuit of the Accessory Delay Relay coil, specifically a “Short To Ground.” This code is typically set by the Body Control Module (BCM), Smart Junction Box (SJB), Generic Electronic Module (GEM), or in some integrated systems, the Powertrain Control Module (PCM), which monitors the electrical integrity of various body electrical components. The Accessory Delay Relay is responsible for providing power to certain accessories (e.g., power windows, radio, sunroof) for a brief period after the ignition is turned off, usually until a door is opened. When the controlling module attempts to energize the relay coil by supplying voltage or ground to its control circuit, it expects a specific current draw and voltage characteristic. If the module detects an abnormally low resistance or an unintended direct path to chassis ground on the relay coil’s control circuit, which results in excessive current draw or an incorrect voltage state, it interprets this as a short to ground and sets code B1305. This fault prevents the module from properly controlling the relay, leading to potential malfunction of the accessory delay function or related components.

Common Symptoms

• Loss of Accessory Delay Functionality: Accessories like power windows, radio, or sunroof may not operate for the typical short duration after the ignition is switched off.
• Inoperative Accessories: In some cases, if the short prevents the relay from ever activating, the affected accessories may not work at all.
• Repeatedly Blown Fuses: A short to ground can cause the fuse protecting the relay control circuit or the module’s driver circuit to blow repeatedly.
• Battery Drain: If the short causes a parasitic draw, it can lead to a dead battery over time, although this is less common for a coil short as it typically prevents activation.
• No Audible Click from Relay: The accessory delay relay may not produce an audible “click” when the ignition is cycled or a door is opened, indicating it’s not being properly energized.

What Causes the Code B1305?

• Faulty Accessory Delay Relay: An internal short within the relay coil windings, causing an unintended path to ground.
• Damaged Wiring Harness: The control wire for the accessory delay relay coil is chafed, pinched, or otherwise damaged, causing it to make direct contact with the vehicle’s chassis ground.
• Corroded Connectors or Relay Socket: Moisture intrusion or corrosion in the relay socket or associated wiring connectors can create a low-resistance path to ground.
• Aftermarket Electrical Modifications: Improperly installed aftermarket accessories or wiring splices that introduce a short into the relay’s control circuit.
• Faulty Control Module: (Less common) An internal fault within the BCM, SJB, GEM, or PCM where the relay driver circuit itself has an internal short to ground.

How to Diagnose and Troubleshoot

Diagnosis of B1305 requires systematic electrical testing with a digital multimeter (DMM) and potentially an OBD-II scan tool capable of accessing body control module data.

1. Visual Inspection: Begin by performing a thorough visual inspection of the accessory delay relay, its socket, and the wiring harness leading to it. Look for any signs of physical damage, chafing, bare wires, corrosion, or burnt insulation. Pay close attention to areas where wiring passes through firewalls or body panels.
2. Retrieve Freeze Frame Data: Use an OBD-II scanner to read freeze frame data associated with B1305. This information can provide valuable context regarding vehicle conditions (e.g., voltage, engine state) at the time the code was set. Clear the DTC after recording the data.
3. Isolate and Test the Relay:
   • Locate the accessory delay relay, often found in the under-hood fuse box, an interior fuse panel, or behind the dash. Consult the vehicle’s service manual for its exact location.
   • Remove the relay from its socket.
   • Using a DMM, measure the resistance across the relay coil terminals (typically terminals 85 and 86 on a Bosch-style relay). Compare this reading to manufacturer specifications (usually between 60-120 ohms). A very low resistance (e.g., near 0 ohms) or infinite resistance (open circuit, though not the cause of a short to ground) indicates an internal fault.
   • Bench test the relay by applying fused 12V and ground to the coil terminals. Listen for an audible click and then verify continuity across the switch terminals (e.g., 30 and 87) with the DMM. If the relay doesn’t click or fails continuity testing, it’s faulty.
4. Test the Relay Control Circuit (Wiring Inspection):
   • With the relay still removed, identify the control circuit terminal in the relay socket that is driven by the control module. Consult the vehicle’s wiring diagram.
   • Using a DMM set to continuity or resistance mode, measure the resistance between this control circuit terminal in the socket and a known good chassis ground. The resistance should be high or infinite when the control module is not actively supplying ground (or power, depending on the relay type). If you measure low resistance or continuity to ground, there is a short in the wiring between the control module and the relay socket.
   • If the control module supplies voltage to activate the coil, use the DMM to check for 12V on the power-supply side of the coil control circuit when commanded ON. If it’s a ground-side switched relay, connect a test light to battery positive and probe the control wire terminal. The test light should illuminate when the module commands the relay ON (e.g., ignition ON, before a door is opened). If it stays lit continuously or never lights, investigate the wiring back to the module.
5. Inspect Related Fuses: Although the code points to the coil circuit, check all fuses related to the accessory delay system and the control module’s power supply, as a severe short could cause secondary fuse failures.
6. Control Module Verification: If the relay and all associated wiring test good, and no external short is found, the fault may lie within the control module itself (e.g., a shorted internal driver). This is typically confirmed by process of elimination and often requires specialized diagnostic equipment or module replacement for verification.

Recommended Repairs and Solutions

The appropriate repair depends directly on the root cause identified during diagnosis.

• Replace the Accessory Delay Relay: If diagnostic steps confirm the relay itself has an internal short to ground in its coil, replacement with a new, OEM-quality relay is necessary. Ensure the replacement relay matches the original specifications.
• Repair or Replace Damaged Wiring: If the wiring harness between the control module and the relay socket is found to be chafed, cut, or shorted to ground, the damaged section must be professionally repaired. This involves stripping back insulation, soldering new wire sections with heat-shrink tubing, or replacing the entire sub-harness if damage is extensive. Ensure proper routing and securement of the wiring to prevent future issues.
• Clean Corroded Connectors/Socket: If corrosion is identified in the relay socket or wiring connectors, thoroughly clean the affected terminals using an appropriate electrical contact cleaner and a small brush. Apply dielectric grease to the terminals to prevent future corrosion once cleaned.
• Correct Aftermarket Wiring Issues: If aftermarket accessories are determined to be the cause of the short, they should be disconnected. If they are to remain installed, their wiring must be corrected and properly isolated to prevent any unintended short circuits.
• Replace the Control Module: If all other components and wiring have been thoroughly tested and verified as functional, and the control module’s internal driver is suspected to be faulty and causing the short to ground, the control module (BCM, SJB, GEM, or PCM) may require replacement. Note that module replacement often necessitates programming or flashing by a dealership or a shop with factory diagnostic tools.
• Mechanic’s Tip: Always disconnect the battery’s negative terminal before performing any electrical repairs to prevent accidental shorts or damage to electronic components. After completing repairs, clear the DTC with an OBD-II scanner and cycle the ignition to verify that the accessory delay function operates correctly and the code does not return. Pay close attention to vehicle-specific wiring diagrams, as relay configurations and control module interactions can vary significantly between makes and models.