Here's a breakdown of why and what to check:
* Blend Door Actuator: This small motor is responsible for controlling the direction of airflow. It opens and closes a blend door that directs air to either the upper or lower vents. If the actuator is faulty (stuck, broken gear, or electrical problem), it can't properly position the blend door, resulting in air flowing to both.
* Vacuum System (if applicable): Some older vehicles, including some models around that era, use a vacuum system to control blend doors. A leak in the vacuum lines or a malfunctioning vacuum control unit could prevent the blend door from moving correctly. This is less likely on a '90 Riviera, but possible.
* Wiring: A short circuit or broken wire to the blend door actuator could also prevent it from receiving the correct signal to change the airflow.
* Control Panel/Switch: While less likely, a faulty switch or a problem within the HVAC control panel itself could be sending the wrong signal to the actuator. However, this is usually indicated by other erratic behavior beyond just the top/bottom issue.
Troubleshooting Steps:
1. Check the Fuse: Start by making sure the fuse for the HVAC system isn't blown. Consult your owner's manual for the fuse box diagram.
2. Listen Carefully: Turn the system on and off, listening carefully near the dashboard area. You might hear a clicking sound (possibly faint) from the blend door actuator as it tries (and fails) to move. This can help pinpoint its location.
3. Visual Inspection (Advanced): If you're comfortable working on your car, you might try accessing the blend door actuator. This usually requires partial dashboard disassembly, which can be challenging. You can look for signs of damage, binding, or anything obviously wrong. *Only attempt this if you have experience with automotive repair; otherwise, seek professional help.*
4. Professional Diagnosis: The most reliable approach is to take your Riviera to a qualified mechanic specializing in older vehicles. They have the tools and experience to diagnose the problem accurately, and determine if it's a faulty actuator, a vacuum issue, wiring, or something else.
Don't attempt extensive disassembly unless you are experienced in auto repair. Incorrectly reassembling components can cause further damage. The safest and often most cost-effective approach is to seek professional assistance.
