Possible Causes:
* Weak or Failing Alternator: This is the most common cause. The defrost system draws a lot of power, especially the rear window defroster. If the alternator is weak or failing, it can't supply enough power to both the engine and the defrost system simultaneously, leading to a stall. This is especially true if the battery is also weak.
* Bad Battery: A weak or failing battery can't hold enough charge to handle the sudden high demand of the defrost system. Combined with a marginally functioning alternator, this can easily cause a stall.
* Loose or Corroded Battery Terminals: Poor connections at the battery terminals create resistance, preventing sufficient power flow. This is exacerbated when a high current draw (like the defrost) is added.
* Faulty Wiring or Connections: A short circuit or a loose connection somewhere in the defrost system's wiring can draw excessive current, overloading the system.
* Excessive Electrical Draw from Another System: While less likely to directly cause the stall, a problem with another high-draw system (like the blower motor) running concurrently with the defrost could put too much strain on the electrical system.
* Parasitic Drain: A constant drain on the battery even when the car is off can leave the battery too low to handle the defrost's demand.
Troubleshooting Steps:
1. Check the Battery and Terminals: Inspect the battery terminals for corrosion and tighten them if necessary. Have the battery tested at an auto parts store to ensure it's holding a sufficient charge.
2. Check the Alternator: Have the alternator tested at an auto parts store. They can usually do this while the alternator is still in the vehicle. A load test is particularly useful.
3. Inspect Wiring: Visually inspect all wiring related to the defrost system, looking for any signs of damage, fraying, or loose connections. Pay close attention to the connections at the blower motor, rear defroster element, and any relays associated with the defrost system.
4. Check for Parasitic Drain: This is more involved, but if the above steps don't reveal the problem, a parasitic drain could be the issue. This requires a multimeter to measure current draw while the car is off.
5. Test Other Electrical Systems: While less likely, consider whether other electrical components are malfunctioning, contributing to the overall load.
Important Note: If you're not comfortable working on your car's electrical system, take it to a qualified mechanic. Incorrectly diagnosing and attempting to repair electrical problems can lead to further damage or even injury.
