B1691 — Autolamp Delay Circuit Short To Battery #

Note: The description for DTC B1691 may vary depending on the vehicle manufacturer (OEM). Always refer to your vehicle’s service manual for the most accurate terminology.

• Autolamp Delay Circuit Short To Battery (Ford, Lincoln, Mercury)
• Autolamp Circuit High Input (Mazda)
• Automatic Headlamp Delay Circuit Shorted to B+ (General term, various OEMs)

Summary of DTC B1691 #

Diagnostic Trouble Code B1691 indicates that the vehicle’s autolamp delay circuit—the part of the system that keeps the headlights on for a preset period after the ignition is turned off—has detected an electrical short directly to the battery’s positive voltage (B+). In practical terms, this means the system recognizes an abnormal, uninterrupted supply of power to the autolamp delay circuit, which can disrupt its expected timing function. This circuit is managed by the Body Control Module (BCM), which monitors and controls various vehicle functions related to lighting, security, and comfort. This code is most often found in vehicles equipped with automatic headlamp features, a convenience system designed to improve safety and usability. If left unresolved, a B1691 code could lead to issues like battery drain or headlamps that do not turn off as intended.

Common Symptoms for DTC B1691 #

When the B1691 code is triggered, drivers may notice irregular behavior with their vehicle’s exterior lighting system, particularly the headlamps. The symptoms often range from subtle quirks to more obvious malfunctions, depending on the severity of the electrical issue. Recognizing these signs early can help prevent further complications, such as a dead battery or unexpected lighting failure.

• Headlamps remain on after the ignition is turned off (longer than normal or indefinitely)
• Headlamps do not turn off even when the autolamp delay timer should have expired
• Unusual battery drain, especially after parking the vehicle overnight
• Warning chime or message related to exterior lighting on the dashboard
• In some cases, other lighting features (interior lights, parking lights) may also malfunction

Probable Causes for Code B1691 #

The B1691 trouble code is typically set off when the BCM detects a constant voltage supply where there should only be a timed or controlled voltage. This can be attributed to several underlying issues, but some are more common than others. Addressing the most frequent causes first will often lead to a quicker, more efficient repair.

1. Short to battery in the autolamp delay circuit wiring — Damaged, pinched, or exposed wires making contact with a power source
2. Faulty autolamp relay or control module — Internal failure causing unintended power flow
3. Malfunctioning Body Control Module (BCM) — Software or hardware defects in the module itself
4. Incorrectly installed aftermarket accessories — Non-OEM wiring or devices interfering with the autolamp circuit
5. Corroded or shorted connectors — Moisture or debris causing electrical bridges across pins

Step-by-Step Diagnostic Procedure for B1691 #

Diagnosing a B1691 code demands a systematic approach, blending observation with precise electrical testing. A methodical process ensures that you identify the root cause without replacing unnecessary parts or overlooking hidden issues. Here’s a reliable workflow for technicians and advanced DIYers:

1. Verify the code with a professional-grade scan tool and record any related codes.
2. Perform a visual inspection of the autolamp wiring harness, especially near the headlamp switch and BCM.
3. Check for signs of damaged, pinched, or exposed wires that might be contacting a 12V battery source (B+).
4. Inspect connectors at the headlamp switch, autolamp relay, and BCM for corrosion, moisture, or debris.
5. Test the autolamp relay and related control modules for internal shorts or stuck contacts using a multimeter.
6. If equipped, disconnect aftermarket accessories from the lighting circuit and retest system operation.
7. Use wiring diagrams to trace the circuit from the autolamp relay back to the BCM, confirming continuity and absence of shorts to B+.
8. If no faults are found in wiring or relays, consider performing a BCM self-test or re-flash (with OEM software and procedures).

Possible Repairs for DTC B1691 #

Once the source of the short is pinpointed, repairs should be made with OEM-grade components and procedures to guarantee reliability. Address each cause methodically, keeping in mind that a clean and secure electrical connection is vital for modern vehicle electronics.

• Repair or replace any damaged or shorted wiring in the autolamp delay circuit
• Replace a faulty autolamp relay or control module as specified by the service manual
• Clean, dry, and secure any corroded or contaminated connectors
• Remove or rewire non-OEM accessories that interfere with the autolamp system
• If BCM failure is confirmed, replace or reprogram the module per OEM instructions

Is It Safe to Drive With Code B1691? #

Limited: While it is technically possible to drive with a B1691 code present, it is not advisable for an extended period. The main risk is unintended headlamp operation—either staying on and draining the battery or, in rare cases, failing to illuminate when needed. If your headlamps remain on after parking, your battery could discharge unexpectedly, potentially leaving you stranded. For safety and reliability, the issue should be addressed promptly.

Vehicles Most Commonly Affected by B1691 #

Although B1691 can appear in a variety of models, it is most frequently reported in vehicles from manufacturers that use similar autolamp delay systems. If you drive one of the following, you might be more likely to encounter this code:

• Ford (F-150, Explorer, Taurus, Focus, Escape)
• Lincoln (Town Car, Navigator, MKZ)
• Mercury (Grand Marquis, Mountaineer)
• Mazda (models sharing platforms with Ford, e.g., Tribute)

Common Mistakes & Best Practices for B1691 Diagnosis #

Avoiding missteps during diagnosis and repair is essential for a successful outcome. Here are some frequent pitfalls and how to steer clear of them:

• Overlooking aftermarket wiring or accessories that may have been spliced into the lighting circuit
• Replacing the BCM or relay without first checking for simple wiring faults
• Assuming the code is caused by a bad headlamp bulb (the bulb itself rarely triggers this code)
• Neglecting to check connectors for corrosion or shorting from moisture intrusion
• Not clearing the code after repairs and verifying system operation under real conditions

Quick FAQ for DTC B1691 #

To wrap up, here are answers to some of the most common questions about B1691:

• Can a weak battery cause B1691?
  
  No, a weak battery is unlikely to trigger this code; B1691 refers specifically to a short circuit supplying constant power to the autolamp delay circuit.
• Will resetting the BCM clear the code permanently?
  
  Not unless the underlying electrical fault is repaired. The code will likely return if the short to battery condition persists.