* Alternator: This is the most likely suspect. The alternator charges the battery. When you use power-hungry accessories like the power windows, the alternator has to work harder. If the alternator is failing or weak, it can't keep up with the demand. This leads to a voltage drop, causing the engine to stumble or stall, especially under load. A failing alternator may also show a low battery voltage warning light on the dash.
* Battery: A weak or failing battery can't supply enough power, exacerbating the problem when accessories are used. Similar to a weak alternator, this will cause voltage drops leading to rough running and stalling. Have the battery tested for its cranking amps and overall health.
* Grounding Issues: Poor electrical grounds can cause voltage fluctuations throughout the system. A bad ground connection could cause the engine to misfire or stall when the power windows (a significant electrical draw) are activated. This is less likely to be the *sole* cause but could contribute to the problem.
* Wiring Harness: Damaged or corroded wiring, particularly in the area of the power window switch or related circuits, could cause intermittent electrical problems. This could lead to voltage drops and irregular engine behavior.
* PCM (Powertrain Control Module): While less likely, a failing PCM could misinterpret voltage drops or other electrical signals, resulting in erratic engine performance. This is usually diagnosed through advanced diagnostics.
Why it's worse with the power windows:
Power windows draw a substantial amount of current. When they're used, the already stressed electrical system (due to the failing alternator or battery, for example) is pushed beyond its capacity, resulting in the noticeable rough running and stalling.
Troubleshooting Steps:
1. Battery and Alternator Test: Have your battery and alternator tested at an auto parts store or mechanic. This is the most crucial first step.
2. Visual Inspection: Check the battery terminals for corrosion and clean them if necessary. Inspect wiring harnesses, especially those near the power window switches and related components, for damage or corrosion.
3. Voltage Check: Use a multimeter to check the voltage at the battery while the engine is running, both at idle and with the power windows operating. A significant voltage drop indicates a charging system problem.
4. Ground Connection Inspection: Check the main ground connections to the engine and chassis for tightness and corrosion.
If these steps don't identify the problem, you'll likely need to take your Bonneville to a qualified mechanic for more advanced diagnostics. They have the tools to properly test the PCM and other electrical components.
