B1315

What Does Code B1315 Mean?

The diagnostic trouble code B1315, “Battery Saver Relay Coil Circuit Short To Battery,” indicates a specific electrical fault within the vehicle’s battery saver system. This system is typically managed by the Body Control Module (BCM) or, in some applications, the Powertrain Control Module (PCM/ECM). Its primary function is to de-energize non-essential electrical accessories (e.g., radio, interior lights, power windows, accessory outlets) after a predetermined time or when the ignition is switched off and certain conditions are met (e.g., door closed) to prevent parasitic battery drain.

The BCM/PCM monitors the control circuit for the Battery Saver Relay. When code B1315 is set, it signifies that the control circuit for the relay’s coil is continuously receiving battery positive voltage (B+) when it should not be. This can occur if the control wire from the BCM/PCM to the relay coil has chafed and shorted to a constant 12-volt power source, or if there is an internal fault within the relay or the controlling module that creates this unintended B+ condition. The BCM/PCM detects this by observing a voltage on the control circuit that is inconsistent with its commanded state, interpreting it as a direct short to the battery supply. This compromises the module’s ability to properly cycle the relay, leading to a failure of the battery saver function.

Common Symptoms

• Non-essential accessories remain active: Components like the radio, interior lights, power windows, or accessory power outlets may not turn off after the ignition is switched off, even after a significant delay or after doors are closed.
• Parasitic battery drain: Due to accessories remaining on, the vehicle’s battery may drain over time, leading to slow cranking or a no-start condition after the vehicle has been parked for a period.
• Intermittent operation of controlled accessories: While less common for a “short to battery,” the constant activation of the relay might cause erratic behavior of the accessories it controls.
• No visible symptoms (initially): Depending on the specific fault and vehicle design, the symptom of parasitic drain might only manifest after several hours or overnight, making the immediate detection of the issue challenging without a scan tool.

What Causes the Code B1315?

• Faulty Battery Saver Relay: The relay itself may have an internal short circuit within its coil windings, or between the coil control terminal and an internal B+ connection, causing it to be continuously energized or to exhibit a short on its control circuit.
• Damaged Wiring Harness: A short to battery positive voltage in the wiring between the BCM/PCM and the battery saver relay coil. This commonly occurs due to chafing, pinching, or insulation breakdown, allowing the control wire to contact a constant 12V power supply.
• Corrosion or Water Intrusion: Electrical corrosion or water ingress into connectors, fuse boxes, or the relay’s socket can create unintended conductive paths (a “bridge”) between the relay’s control circuit and a constant battery voltage source.
• Faulty Body Control Module (BCM) / Power Control Module (PCM): While less frequent, an internal failure within the BCM or PCM, such as a shorted relay driver circuit, could cause the module to continuously output battery voltage on the battery saver relay control circuit, thereby setting the code.

How to Diagnose and Troubleshoot

Diagnosing B1315 requires careful electrical circuit testing, typically involving a digital multimeter (DMM) and a vehicle-specific wiring diagram.

1. Initial Inspection and Verification:
   • Begin with a thorough visual inspection of the battery and charging system to ensure adequate voltage (at least 12.6V static).
   • Locate the battery saver relay (consult the vehicle’s service manual for its exact location, often in a fuse box or junction block).
   • Visually inspect the relay and its immediate wiring harness for any signs of physical damage, corrosion, melted insulation, or loose connections.
   • With the ignition off and the key removed, attempt to verify the reported symptom (e.g., do accessories remain active when they should not?).
2. Isolate and Test the Battery Saver Relay Circuit:
   • Disconnect the battery saver relay from its socket.
   • Using a DMM, identify the relay coil’s control wire terminal within the relay socket (consult wiring diagram).
   • With the ignition off and the key out, measure the voltage between the identified control wire terminal and a known good chassis ground. If you measure battery voltage (e.g., 12V or higher), it confirms a short to battery exists either in the wiring harness or from the controlling module.
   • If the DMM shows 0V (or very low voltage) at this point, the short may be internal to the relay itself, or the controlling module is at fault.
3. Test the Wiring Harness:
   • If a constant B+ was found on the control wire at the relay socket with the relay disconnected, then the short is in the wiring or the BCM/PCM.
   • To differentiate, disconnect the BCM/PCM connector that contains the battery saver relay control wire.
   • Re-measure the voltage on the control wire at the relay socket. If the B+ voltage is now gone, the short is internal to the BCM/PCM. If B+ is still present, the short is within the wiring harness between the BCM/PCM connector and the relay socket.
   • Using a DMM, check for continuity between the control wire and all other B+ wires in the harness while the BCM/PCM and relay are disconnected.
4. Test the Battery Saver Relay:
   • Using a DMM, measure the resistance across the relay’s coil terminals (typically pins 85 and 86, but refer to the specific relay diagram). Compare this reading to the manufacturer’s specifications (usually 60-120 ohms). An open circuit (OL) indicates a broken coil, while an extremely low resistance might suggest an internal short.
   • Apply 12V (from an external power source) across the coil terminals (85 and 86, observing polarity if indicated). Listen for an audible click, indicating the coil is energizing and the internal contacts are moving.
   • While 12V is applied to the coil, test for continuity between the switched power terminals (e.g., 30 and 87). There should be continuity. Remove power; continuity should cease.
   • Crucially, with no power applied to the coil, check for any continuity between the coil terminals and the switched power terminals (30, 87, 87a) or the constant B+ input terminal. Any continuity here indicates an internal short within the relay.
5. Test Body Control Module (BCM) / Power Control Module (PCM):
   • If the wiring and relay test good, the BCM/PCM may be at fault. This typically requires an advanced scan tool with bi-directional control capabilities.
   • With the BCM/PCM connected and the relay reinstalled, use the scan tool to command the battery saver relay ON and OFF (if supported).
   • While commanding, use a DMM or oscilloscope to monitor the voltage on the relay’s control circuit. Verify that the BCM/PCM is properly switching the voltage (e.g., providing ground or 12V when commanded, and 0V or open when not commanded). If the BCM/PCM consistently outputs B+ when it should be off, the module is likely faulty.

Recommended Repairs and Solutions

Once the root cause of B1315 has been accurately identified through systematic diagnosis, the following repairs are typically recommended:

• Replace the Battery Saver Relay: If the relay itself is found to have an internal short, an open coil, or stuck contacts, replacement with a new, OEM-specified relay is the correct solution. Ensure the new relay matches the original specifications exactly.
• Repair or Replace Damaged Wiring Harness: If the diagnostic steps reveal a short to battery within the wiring harness, the damaged section must be repaired. This involves carefully locating the compromised area, splicing in new wire of the correct gauge, using solder and heat-shrink tubing for durable connections, and re-routing/securing the harness to prevent future damage. In cases of extensive damage or multiple shorts, replacing the entire affected harness segment may be more practical.
• Clean and Repair Corroded Connectors/Sockets: If corrosion or water intrusion is the cause, thoroughly clean all affected terminals and connectors using specialized electrical contact cleaner and a small brush. Repair or replace any severely corroded or damaged terminals. Apply dielectric grease to protect against future moisture. Identify and seal the source of water ingress if possible.
• Replace Body Control Module (BCM) / Power Control Module (PCM): If all other components (relay, wiring harness, connectors) test good and the BCM/PCM is definitively identified as outputting a constant B+ on the battery saver relay control circuit when it should not, then replacement of the module is necessary. It is crucial to note that BCM/PCM replacement often requires specialized programming and calibration with a factory-level scan tool, which should be performed by a qualified technician.

Important Mechanics’ Tips:

• Always consult the vehicle’s specific wiring diagrams and service manual for accurate pinouts, component locations, and resistance/voltage specifications. These vary significantly between manufacturers and models.
• Before performing any electrical repairs, always disconnect the negative terminal of the battery to prevent short circuits, module damage, or personal injury.
• After completing any repair, clear the DTC B1315 using an OBD-II scanner and perform a functional test or extended drive cycle to confirm the battery saver function operates correctly and the code does not return.
• Consider checking for other related B-codes or C-codes, as a shared power or ground issue could potentially trigger multiple diagnostic trouble codes.