Here's why:
* Shared Circuit/Ground: The most likely culprit is a shared power source or ground between the automatic headlight system and the heater blower motor. When the blower motor draws a significant current (especially on higher fan speeds), it might cause a voltage drop sufficient to disrupt the less robust circuit controlling the automatic headlights. This drop could be due to insufficient wiring gauge, a corroded connection, or a failing ground. The automatic headlight system might be more sensitive to voltage fluctuations than the manual operation.
* Faulty Control Module: The automatic headlight control module itself might be failing or have a faulty internal connection. The increased current draw from the blower motor could exacerbate an existing problem within the module, leading to a malfunction.
* Wiring Harness Damage: A damaged or corroded wire in the wiring harness, connecting the headlight control module, the blower motor, and the battery, could be causing the issue. Wires can be chafed, corroded, or even have a short circuit. The additional current draw from the blower could increase the effect of the damage.
* Insufficient Battery Power: While less likely, a weak or failing battery could struggle to supply the combined power demand of the headlights and blower motor. The automatic system, being more sensitive, might shut down first.
Troubleshooting Steps:
1. Check the fuses: Ensure that all fuses related to the headlights and the blower motor are intact and not blown.
2. Inspect Wiring: Carefully inspect the wiring harness for any visible damage, corrosion, or loose connections, particularly in areas where the wiring might be subjected to vibration or movement (near the blower motor and headlight switch).
3. Battery Voltage Test: Check the battery voltage while the blower motor is running at its highest speed. A significant voltage drop indicates a problem with the battery or charging system.
4. Ground Connection: Locate the ground connection points for both the headlight system and the blower motor. Clean these connections thoroughly with a wire brush and ensure they have a solid, clean connection.
5. Professional Diagnosis: If you're not comfortable performing these checks yourself, it's best to take your Grand Prix to a qualified mechanic or auto electrician. They can perform a more thorough diagnostic check, identify the faulty component, and carry out the necessary repairs. They may have access to specialized diagnostic equipment.
The fact that the headlights work manually points towards a problem within the automatic control system rather than a complete headlight failure. Focusing your investigation on power, grounding, and the control module's integrity is the best approach.
