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Tahoe / Yukon Service Manual

DTC B1918

Diagnostic Instructions Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. Review Strategy Based Diagnosis for an overview of the diagnostic approach. Diagnostic Procedure Instructions provides an overview of each diagnostic category. DTC Descriptors DTC B1918 Headlamp Switch Signal Refer to Symptom Byte List for symptom byte information. Diagnostic Fault Information

Circuit Circuit Function Short to Ground Open/High Resistance Short to Voltage Signal Performan
7555 Signal B1918 16 1 B1918 17 1
7556 Low Reference — 1 — —
1. Interior Accent Lighting Malfunction

Scan Tool Functions K9 Body Control Module – Data Display – Interior Lighting Data

Data Parameter Description
Dimmer Switch This data parameter will increase/decrease in 5% increments as the dimmer switch +/- buttons are pressed.

Circuit/System Description The S30 Headlamp Switch is used to control the headlights of the vehicle, as well as interior backlight dimming, and fog lights when equipped (T79/T40). The K9 Body Control Module (BCM) provides 5 V signal circuit 7555 and low reference circuit 7556 to the S30 Headlamp Switch for backlight dimming and fog light controls. When a dimming/fog light button is pressed, the signal circuit becomes grounded through the appropriate resistor internal to the S30 Headlight Switch and voltage from the BCM will decrease accordingly. The BCM interprets the signal and responds with the appropriate control function. Conditions for Running the DTC Battery voltage must be between 9-16 V. Conditions for Setting the DTC DTC B1918 16 The BCM detects a short to ground on signal circuit 7555. DTC B1918 17 The BCM detects a short to voltage or an open/high resistance on signal circuit 7555. Actions Taken When the DTC Sets The BCM will not dim any interior backlighting. All interior backlighting will default to full bright. Conditions for Clearing the DTC The condition responsible for setting the DTC no longer exists. A history DTC will clear once 40 consecutive malfunction-free ignition cycles have occurred. Reference Information Schematic Reference Interior Lights Dimming Schematics Connector End View Reference Master Electrical Component List Description and Operation Interior Lighting Systems Description and Operation Electrical Information Reference Circuit Testing Connector Repairs Testing for Intermittent Conditions and Poor Connections Wiring Repairs Scan Tool Reference Control Module References for scan tool information Circuit/System Testing

  1. Ignition/vehicle OFF, disconnect the connector at the S30 Headlamp Switch, ignition/vehicle ON.
  2. Verify a test lamp illuminates between low reference terminal 12 and B+. ⇒ If the test lamp does not illuminate 2.1. Test circuit 7556 for an open/high resistance. ⇒ If an open/high resistance is found, repair the fault on the circuit. ⇓ If an open/high resistance is not found. 2.2. Replace the K9 Body Control Module. ⇓ If the test lamp illuminates
  3. Test for 4.8 V – 5.2 V between signal circuit terminal 11 and ground. ⇒ If less than 4.8 V 3.1. Test circuit 7555 for a short to ground. ⇒ If a short to ground is found, repair the fault on the circuit. ⇓ If a short to ground is not found 3.2. Test circuit 7555 for an open/high resistance. ⇒ If an open/high resistance is found, repair the fault on the circuit. ⇓ If an open/high resistance is not found. 3.3. Replace the K9 Body Control Module. ⇒ If greater than 5.2 V 3.1. Test circuit 7555 for a short to voltage. ⇒ If a short to voltage is found, repair the fault on the circuit. ⇓ If a short to voltage is not found. 3.2. Replace the K9 Body Control Module. ⇓ If 4.8 V – 5.2 V
  4. Replace the S30 Headlamp Switch. Repair Instructions Perform the Diagnostic Repair Verification after completing the repair. Lighting Component Replacement Reference Control Module References for BCM replacement, programming, and setup