* Dimming Switch/Rheostat: This is a common issue. The switch that controls the brightness of the dash lights may be faulty and failing to provide sufficient power when the headlights are on (which draws significant power). The headlights are higher priority, so the dash lights lose out.
* Faulty Ground: A poor ground connection somewhere in the electrical system can cause voltage drops, affecting the dash lights more significantly when the headlights' higher current draw is introduced.
* Wiring Issues: Damaged, corroded, or loose wiring, particularly in the area of the headlight switch or dimmer switch, could interrupt the flow of electricity to the dash lights.
* Blown Fuse (less likely): While less probable, a blown fuse related to the instrument panel illumination could also be responsible. However, this usually would mean complete darkness, not just dimming when the headlights are on.
* Alternator Problems (less likely, but serious): A failing alternator might not be providing sufficient power, leading to voltage drops that affect the dash lights more when other high-power consumers (like headlights) are active. This is less likely to cause *complete* shutdown of dash lights but can contribute to dimming.
What to do:
1. Check the fuses: Start by checking the fuses related to the instrument panel and headlight circuits in the fuse box (usually under the hood and inside the cabin). Refer to your owner's manual for the fuse locations and amperage.
2. Inspect the dimmer switch: Carefully examine the dimmer switch for any signs of damage or looseness. This might require partial disassembly depending on your comfort level.
3. Professional Diagnosis: If you're not comfortable troubleshooting electrical systems yourself, take your Tahoe to a qualified mechanic or auto electrician. They have the tools and expertise to diagnose the problem accurately and safely. The problem is likely in the dimmer switch circuit itself.
Ignoring this issue could lead to further electrical problems, so it's best to address it promptly.
