B1590 — Autolamp Delay Increase Circuit Short To Ground #

The wording of this DTC (Diagnostic Trouble Code) may vary depending on the vehicle manufacturer (OEM). Always refer to your service manual for the exact description used by your vehicle.

Known label variations for B1590:

• Autolamp Delay Increase Circuit Short To Ground (Ford, Lincoln, Mercury)
• Autolamp Delay Circuit – Short to Ground (Mazda, select models)
• Automatic Headlight Delay Circuit Fault (Jaguar/Land Rover)

Summary for code B1590 #

B1590 is a Body Control Module (BCM) related trouble code that indicates a short-to-ground condition in the circuit controlling the autolamp delay increase function. In practical terms, this system manages the period your headlights remain on after the ignition is turned off—a feature widely known as “autolamp delay.” When the BCM detects electrical current flowing directly to ground instead of through its intended path, it sets this code. This malfunction may interfere with your headlight delay operation, potentially affecting nighttime visibility and vehicle security. The detection is generally specific to vehicles equipped with automatic lighting control, and a persistent B1590 should prompt a thorough inspection of the affected circuit.

Common symptoms for code B1590 #

When B1590 is present, you may notice changes in how your vehicle’s automatic headlight system behaves. Some symptoms are subtle, while others are more pronounced, depending on the severity of the short.

• Autolamp delay feature does not operate (headlights turn off immediately or stay on indefinitely)
• Headlights may flicker or fail to respond to the autolamp setting
• Possible warning messages or indicator lights on the dashboard
• Occasional battery drain if headlights remain on unexpectedly
• BCM stores additional related lighting system codes

Most likely causes for code B1590 #

This fault is typically caused by a direct electrical short within the autolamp delay circuit. The most frequent culprits involve wiring insulation breakdown or connector problems, but component failure is also possible. Here are the probable causes, ranked by likelihood:

1. Damaged wiring harness: Chafing, cuts, or pinched wires in the autolamp delay circuit touching the vehicle’s chassis ground.
2. Corroded or loose connectors: Moisture intrusion or vibration may cause terminals to corrode or disconnect, creating a path to ground.
3. Faulty autolamp relay or module: Internal short within the relay or the BCM’s output driver circuit.
4. Incorrect aftermarket wiring: Poorly installed accessories (alarm, remote start, lighting upgrades) tapping into the autolamp circuit.

Step-by-step diagnosis for B1590 #

A systematic approach is vital for accurately diagnosing B1590. The following steps use standard tools (multimeter, test lamp) and assume basic familiarity with automotive electrical systems:

• 1. Verify the code: Use a scan tool to confirm B1590 is active and note any related codes.
• 2. Inspect fuse and relay: Check the autolamp fuse and related relay for signs of melting, corrosion, or incorrect installation.
• 3. Visual inspection: Examine the wiring harnesses from the headlamp switch, BCM, and relay for visible damage, especially near pinch points or recent repairs.
• 4. Check connectors: Disconnect connectors in the autolamp circuit; inspect for bent pins, corrosion, or loose fit. Clean and reconnect as needed.
• 5. Test for short-to-ground: With the circuit powered off, use a multimeter to measure resistance between the autolamp delay wire and chassis ground. A reading near zero ohms confirms a short.
• 6. Isolate the fault: If a short is detected, disconnect sections of the harness or module until the reading changes, identifying the location of the fault.
• 7. Inspect the BCM: If wiring and connectors pass inspection, the issue may be internal to the BCM or autolamp relay; consult OEM test procedures.

Possible repairs for code B1590 #

Once the root cause is identified, repairs should be targeted and durable. Always clear the code and retest the system after any work.

• Repair or replace damaged wiring: Remove and replace any section of the autolamp circuit harness with insulation damage or confirmed short.
• Clean or replace connectors: Treat corroded terminals with contact cleaner, or replace connectors as needed.
• Replace faulty relay or module: If the relay or BCM autolamp output is shorted internally, install a new OEM part.
• Correct aftermarket wiring: Safely reroute or remove non-OEM wiring spliced into the affected circuit.

Is it safe to drive with code B1590? #

Driving with B1590 is generally safe, but with limitations. The main risk is decreased convenience or safety if your headlights do not operate automatically as intended—especially if they fail to turn on or off in low-light situations. There is also a risk of battery drain if the headlights stay on. It is wise to address this code promptly, but short-term driving is possible if you manually control the lights and monitor battery health.

Vehicles most commonly affected by B1590 #

B1590 is most frequently seen on vehicles equipped with factory autolamp systems, primarily from North American and select European OEMs. Here are typical models:

• Ford F-150, Explorer, Expedition, Taurus (various years)
• Lincoln Town Car, Navigator, MKZ
• Mercury Grand Marquis, Mountaineer
• Mazda Tribute (select years, shared platform with Ford)
• Jaguar XJ, Land Rover Range Rover (with Ford-derived electronics)

Common mistakes & best practices for B1590 #

Technicians and DIYers sometimes overlook key steps or misinterpret symptoms when troubleshooting B1590. Avoid these pitfalls and follow best practices for a reliable repair:

• Assuming the problem is always the BCM—wiring faults are more common
• Overlooking aftermarket accessories spliced into lighting circuits
• Failing to inspect connectors for hidden corrosion or moisture
• Not performing a proper continuity test to confirm the exact location of the short
• Neglecting to check TSBs (Technical Service Bulletins) for known issues on your vehicle model

Quick FAQ for code B1590 #

Q: Will disconnecting the battery clear B1590?
A: Disconnecting the battery may reset the code temporarily, but if the root cause remains, B1590 will return as soon as the system detects the fault again.

Q: Can a failing headlight bulb cause B1590?
A: No, B1590 is specifically related to the delay circuit control, not the bulbs themselves. However, incorrect bulb installation or wiring near the headlamp area could contribute to a short if it damages the harness.