B1585: Dimmer Signal Circuit Short to Battery
The B1585 code indicates that the Instrument Cluster Module (ICM) or lighting control module has detected a short to battery voltage in the dimmer signal input circuit. This circuit allows the driver to adjust dashboard brightness using a potentiometer or variable resistor. A short to battery means the voltage stays high (around 12V) instead of varying with the dimmer control position, preventing the panel lights from dimming.
1. Commonly Affected Makes
2. Most Common Causes
- Short circuit in the dimmer signal wiring to battery positive (B+)
- Faulty panel dimmer potentiometer (internal short)
- Corroded or bridged pins at the dimmer switch connector
- Defective input circuit in the instrument cluster module
- Harness damage causing contact between signal wire and a power wire
3. Symptoms You Will Notice
- Dashboard lights stuck at full brightness
- Inability to dim the instrument panel lights
4. Step-by-Step Diagnosis
Check for battery voltage on the dimmer signal wire with the dimmer control in various positions
Inspect the dimmer switch connector for corrosion or bent pins
Test the dimmer potentiometer resistance across its range
Check for shorts between the signal wire and B+ in the harness
If wiring and switch are good, suspect a faulty instrument cluster module
5. Frequently Asked Questions
Is it safe to drive?▼
Yes, the vehicle is safe to drive. The issue only affects interior lighting brightness and does not impact safety systems.
How much does it cost to fix?▼
Repair costs typically range from $50 to $200 for a faulty dimmer switch or wiring repair, up to $500 or more if the instrument cluster module needs replacement.