Understanding the System:
Your AC blend door is controlled by vacuum. When the engine is running and providing sufficient vacuum, the actuator moves the blend door to direct air as you've selected (face, defrost, etc.). Under load, engine vacuum drops, and if there's a leak in the vacuum lines, the actuator may not have enough power to maintain the door's position, causing it to close (usually resulting in only defrost).
Troubleshooting Steps:
1. Check Vacuum Lines: This is the most likely culprit. Inspect all vacuum lines connected to the HVAC system. Look for:
* Cracks or holes: Carefully examine the lines for any visible damage.
* Loose connections: Ensure all connections are securely attached. A simple loose clamp can cause a significant leak.
* Kinks or pinched lines: Check for any areas where the lines might be bent or compressed, restricting airflow.
* Correct routing: Make sure the lines are routed correctly and aren't rubbing against anything that could damage them. A shop manual will be helpful here.
2. Vacuum Reservoir (if equipped): Some vehicles have a vacuum reservoir to help maintain vacuum during high engine loads. Check its condition and ensure it's properly connected. A failing reservoir can't store enough vacuum, leading to this problem.
3. Blend Door Actuator: While less likely than a leak, a failing blend door actuator could be the issue. If the vacuum lines are all good, the actuator itself might be weak or failing to hold its position under low vacuum. Testing this requires more advanced diagnostics, possibly involving a vacuum gauge.
4. Check for Vacuum Leaks Elsewhere: A large vacuum leak in another part of the engine will also affect the HVAC system. Listen for hissing sounds near the engine when the vacuum drops.
5. Consult a Repair Manual: A repair manual specific to your 1999 Dodge Ram 1500 is invaluable for identifying the vacuum lines and their routing. This will make troubleshooting much easier.
Important Considerations:
* Safety First: Always disconnect the negative battery terminal before working on any electrical or vacuum systems.
* Professional Help: If you're not comfortable working on your vehicle's HVAC system, it's best to take it to a qualified mechanic. Incorrect repairs can lead to further damage.
By systematically checking these areas, you should be able to pinpoint the source of the problem and get your AC blend door working correctly under all engine loads. Remember to replace any damaged vacuum lines with new ones of the correct size and material.
