Here's what could be happening:
* Faulty Blend Door Actuator(s): The most likely culprit. These are small electric motors that move the blend doors. One or more might be broken, jammed, or have a faulty electrical connection.
* Vacuum Leak (if vacuum-controlled): While less common in later models, some older vehicles use vacuum to control blend doors. A leak in the vacuum lines could prevent proper operation. This is less likely in a '98 Astro, but still possible.
* Wiring/Electrical Problem: A short, broken wire, or a problem with the control panel itself could prevent the actuators from receiving power or correct signals.
* Control Panel Failure: The control panel might be malfunctioning, sending only the defrost signal to the actuators.
Troubleshooting Steps:
1. Check the Control Panel: Make sure you're actually selecting different vent positions. Try all settings repeatedly.
2. Listen Carefully: When changing the vent settings, listen closely near the dashboard and under the dash. You might hear a clicking sound from the blend door actuators if they are trying to move, but are failing. Absence of any sound points to a likely actuator or electrical problem.
3. Visual Inspection (Difficult): Accessing the blend door actuators in an Astro van can be challenging, requiring some dashboard disassembly. If you're comfortable with car repairs, you could try looking for obvious problems like disconnected wires or severely damaged actuators.
4. Professional Diagnosis: The most reliable method is to take your van to a qualified mechanic. They have the tools and experience to diagnose the problem accurately and repair or replace the faulty component(s). They can test the actuators and their electrical connections efficiently.
Important Note: Before attempting any repairs yourself, disconnect the negative battery terminal to prevent accidental electrical shorts. Blend door actuator replacement can be a time-consuming job, and it's often easier for a professional to handle.
