Here's why:
* Blend Door Actuator: This small motor controls the mix of hot and cold air coming from your heater core and AC evaporator. When driving, the engine's higher RPMs and increased airflow might be forcing the blend door to partially open, allowing hot air to mix in even if the AC is on. When idling or parked, the lower airflow allows the blend door to stay in a more "cold" position, resulting in cold air. A faulty actuator might be sticking or not responding properly to the climate control settings.
Other possibilities, though less likely given your specific description, include:
* Low Refrigerant: While less probable since you get cold air at idle, low refrigerant could mean insufficient cooling power under load (while driving). The system may struggle to cool the air sufficiently at higher airflow demands.
* Faulty Temperature Sensor: An inaccurate temperature sensor could cause the system to overcompensate, producing more hot air than necessary, especially when the engine is under load (driving).
* Clogged Heater Core: A partially clogged heater core could restrict hot air flow, making it more noticeable when the blower motor is at a lower speed (idle). However, this would typically result in less hot air, not just cold air at idle.
* Wiring/Vacuum Issues: Problems with the wiring or vacuum lines controlling the blend door or other climate control components could intermittently affect the system's function.
Troubleshooting Steps:
1. Check Refrigerant Level: A low refrigerant charge is an easy check. Take your Windstar to a mechanic or auto parts store that can check your AC refrigerant level.
2. Inspect Blend Door Actuator: This requires some mechanical knowledge. You might need to access the actuator (often located under the dashboard) and manually test its movement. If it's sticking or not moving smoothly, it needs replacing. There are many online tutorials available for this, but it's a more involved repair.
3. Diagnose with a Scan Tool: An OBD-II scanner can check for diagnostic trouble codes (DTCs) related to the climate control system. This might pinpoint the faulty component.
4. Professional Diagnosis: If you're not comfortable tackling this yourself, take your Windstar to a qualified mechanic specializing in automotive air conditioning and climate control systems. They have the tools and expertise to properly diagnose and repair the issue.
The blend door actuator is the most likely culprit in your situation, but a proper diagnosis is crucial to ensure the correct repair.
