* Weak or failing alternator: The defroster draws a significant amount of current. If your alternator is weak, it can't keep up with the increased demand, causing the voltage to sag. This voltage drop can cause the engine's computer (PCM) to shut down to protect itself, resulting in a stall.
* Bad battery: A weak or failing battery can't provide the necessary amperage, especially under load. Similar to a weak alternator, this leads to a voltage drop and potential engine stall. Test the battery's voltage both at rest and while the defroster is on.
* Loose or corroded battery cables: Poor connections at the battery terminals or elsewhere in the electrical system can increase resistance, leading to voltage drops. Inspect the cables for corrosion and looseness. Clean or replace as needed.
* Wiring issues: A faulty wire in the defroster circuit could have high resistance, causing a voltage drop. This is less likely but possible, requiring a more in-depth electrical check.
* Voltage regulator (part of the alternator): The voltage regulator controls the alternator's output. A failing regulator can cause inconsistent voltage output, leading to the problem you're experiencing.
Troubleshooting Steps:
1. Check battery voltage: Test the battery voltage with a multimeter while the engine is running at idle, then turn on the defroster and monitor the voltage. A significant drop (more than a couple of volts) points to a battery or charging system problem.
2. Inspect battery cables: Check for corrosion and tightness at both battery terminals and any major connections in the electrical system. Clean any corrosion with a wire brush and baking soda and water solution. Tighten loose connections.
3. Test the alternator: Have the alternator tested at an auto parts store. They can usually do this for free.
4. Check the voltage regulator (if accessible): Access to this component varies by vehicle. If you can visually inspect it, look for any signs of damage. Replacing it is often more cost-effective than replacing the entire alternator.
5. Check fuses and relays: Inspect fuses related to the defroster and blower motor for blown fuses. Also check any relays associated with the defroster.
If you're not comfortable working on your vehicle's electrical system, take it to a qualified mechanic for diagnosis and repair. Ignoring the problem could lead to more serious damage.
