Introduction
The B1585 Diagnostic Trouble Code (DTC) indicates a fault within the dimming control circuit for the instrument panel, specifically related to the "decrease" input signal. This code signifies that the vehicle's computer, often the Body Control Module (BCM), has detected an unintended voltage supply (short to battery) on the wire responsible for reducing the instrument panel brightness. This can lead to the instrument panel lights being stuck at maximum brightness, intermittent dimming issues, or even a complete failure of the dimming function. Addressing this issue promptly is crucial for driver safety and comfort, as improper instrument panel illumination can affect visibility, especially during nighttime driving.
Comprehensive Information Table
| Topic | Description | Potential Causes |
|---|---|---|
| B1585 Code Definition | Dim Panel Decrease Input Circuit Short To Battery. This means the BCM sees battery voltage on the dimming decrease input signal when it shouldn't. | Short to Voltage: The dimming decrease signal wire is contacting a wire carrying battery voltage (12V). |
| Affected Systems | Instrument Panel Illumination, Dimming Control System, Body Control Module (BCM). | Wiring Damage: Frayed, cracked, or damaged insulation on the dimming decrease wire, allowing it to contact a positive voltage source. |
| Symptoms | Instrument panel lights stuck at maximum brightness, inability to dim the instrument panel lights, intermittent dimming issues, blown fuses (related to lighting circuit). | Faulty Dimmer Switch: Internal short within the dimmer switch assembly, causing the decrease signal wire to receive constant battery voltage. |
| Possible Diagnostic Steps | Visual inspection of wiring, multimeter testing, scan tool diagnostics, component testing. | BCM Malfunction: In rare cases, the BCM itself may be faulty and incorrectly interpreting the dimming decrease signal. |
| Affected Components | Dimmer Switch, Wiring Harness, Instrument Panel, Body Control Module (BCM). | Corrosion: Corrosion on connectors or wiring terminals, creating a path for voltage to leak onto the dimming decrease signal wire. |
| Severity | Moderate to High. Impacts driver visibility and comfort. | Aftermarket Accessories: Improperly installed aftermarket accessories (e.g., audio systems, lighting modifications) that tap into the vehicle's electrical system and interfere with the dimming circuit. |
| Common Vehicle Brands | Applicable to various vehicle brands and models. (Specific brand information will vary). | Rodent Damage: Rodents chewing on the wiring harness, exposing the dimming decrease wire and causing it to short to a nearby voltage source. |
| Related DTCs | B1586 (Dim Panel Increase Input Circuit Short To Battery), B1584 (Dim Panel Decrease Input Circuit Open). | Improper Grounding: While less likely to directly cause a "short to battery," a poor ground connection in the lighting system can sometimes contribute to erratic behavior and potentially exacerbate the symptoms of a short to battery on the dimming control circuit. |
| Repair Costs | Variable, depending on the cause. Could range from minor wiring repair to component replacement. | Previous Repairs: Damage to the wiring harness or components during previous repairs that were not properly addressed. |
| Safety Implications | Reduced visibility, driver distraction. | Water Intrusion: Water leaking into the vehicle's interior and affecting the electrical system, leading to corrosion and shorts in the dimming circuit. |
Detailed Explanations
B1585 Code Definition: The B1585 code specifically indicates that the vehicle's computer has detected an unexpected high voltage on the wire responsible for signaling the instrument panel to decrease its brightness. This is interpreted as a "short to battery" because the voltage on that wire should be low or zero when the dimmer switch is attempting to reduce the panel's illumination.
Affected Systems: The B1585 code directly impacts the instrument panel illumination, which is controlled by the dimming control system. The Body Control Module (BCM) is usually the central computer responsible for managing this system, receiving input from the dimmer switch and adjusting the voltage supplied to the instrument panel lights.
Symptoms: The most common symptom is the instrument panel lights being stuck at their maximum brightness, making it impossible to dim them. Other symptoms can include intermittent dimming issues where the lights flicker or behave erratically, and in some cases, a blown fuse related to the lighting circuit due to the excessive current draw caused by the short.
Possible Diagnostic Steps: Diagnosing the B1585 code typically involves a systematic approach. A visual inspection of the wiring harness and connectors related to the dimmer switch and instrument panel is crucial. A multimeter can be used to test for voltage on the dimming decrease wire and to check for continuity and shorts to ground. A scan tool can provide valuable information from the BCM, including live data and the ability to perform component tests.
Affected Components: The primary components involved are the dimmer switch itself, the wiring harness connecting the switch to the BCM and instrument panel, the instrument panel lights, and the Body Control Module (BCM). A failure in any of these components can potentially trigger the B1585 code.
Severity: The severity of the B1585 code is generally considered moderate to high. While it may not immediately disable the vehicle, it significantly impacts driver visibility, particularly at night. The inability to dim the instrument panel lights can cause glare and driver fatigue, potentially increasing the risk of accidents.
Common Vehicle Brands: The B1585 code is a generic code, meaning it can appear in a wide range of vehicle brands and models. The specific wiring diagrams and component locations will vary depending on the make and model of the vehicle.
Related DTCs: Related DTCs can provide additional clues about the underlying problem. B1586 (Dim Panel Increase Input Circuit Short To Battery) indicates a similar issue with the "increase" signal. B1584 (Dim Panel Decrease Input Circuit Open) suggests a break in the circuit, rather than a short to voltage.
Repair Costs: Repair costs can vary significantly depending on the root cause of the problem. A simple wiring repair may be relatively inexpensive, while replacing a faulty dimmer switch or BCM can be considerably more costly. Labor costs will also vary depending on the mechanic's hourly rate and the complexity of the repair.
Safety Implications: The primary safety implication is reduced visibility, especially during nighttime driving. The inability to dim the instrument panel lights can cause glare and distract the driver, potentially increasing the risk of accidents.
Potential Causes:
- Short to Voltage: This is the most likely cause. The dimming decrease signal wire is contacting a wire carrying battery voltage (12V).
- Wiring Damage: Frayed, cracked, or damaged insulation on the dimming decrease wire, allowing it to contact a positive voltage source. Look for pinches, abrasions, or signs of overheating.
- Faulty Dimmer Switch: An internal short within the dimmer switch assembly can cause the decrease signal wire to receive constant battery voltage.
- BCM Malfunction: In rare cases, the BCM itself may be faulty and incorrectly interpreting the dimming decrease signal or improperly controlling the voltage on that circuit.
- Corrosion: Corrosion on connectors or wiring terminals can create a path for voltage to leak onto the dimming decrease signal wire.
- Aftermarket Accessories: Improperly installed aftermarket accessories (e.g., audio systems, lighting modifications) that tap into the vehicle's electrical system can interfere with the dimming circuit.
- Rodent Damage: Rodents chewing on the wiring harness can expose the dimming decrease wire and cause it to short to a nearby voltage source.
- Improper Grounding: While less likely to directly cause a "short to battery," a poor ground connection in the lighting system can sometimes contribute to erratic behavior and potentially exacerbate the symptoms.
- Previous Repairs: Damage to the wiring harness or components during previous repairs that were not properly addressed.
- Water Intrusion: Water leaking into the vehicle's interior and affecting the electrical system can lead to corrosion and shorts in the dimming circuit.
Frequently Asked Questions
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What does the B1585 code mean? It means the vehicle's computer has detected a short to battery on the dimming decrease input circuit, causing the instrument panel lights to be overly bright.
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Can I drive with the B1585 code? While you can drive, it's not recommended as the bright instrument panel can be distracting and potentially dangerous, especially at night.
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Is the B1585 code expensive to fix? The cost varies depending on the cause, ranging from minor wiring repairs to component replacements like the dimmer switch or BCM.
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Can I fix the B1585 code myself? If you have experience with automotive electrical systems and are comfortable using a multimeter and scan tool, you may be able to diagnose and repair the issue yourself. However, if you are not experienced, it is best to consult a qualified mechanic.
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What's the first thing I should check? Start with a visual inspection of the dimmer switch wiring and the surrounding harness for any signs of damage or corrosion.
Conclusion
The B1585 DTC signifies a potentially hazardous electrical fault within the instrument panel dimming system. Proper diagnosis and repair are critical for restoring safe and comfortable driving conditions, ensuring optimal visibility at all times. Consulting a qualified technician is recommended if you lack the experience or tools to diagnose and repair the issue yourself.