Here's a breakdown of the most common causes and how to approach troubleshooting:
1. Blend Door Actuator(s): This is the most likely culprit. The blend door actuators are small motors that control the flow of hot and cold air to each side. One of them is likely malfunctioning, failing to move the blend door to the correct position.
* Troubleshooting: You'll need to access the actuators (usually located behind the dashboard, requiring significant disassembly). With the system running, carefully observe the blend doors (if accessible) to see if they're moving properly. A visual inspection of the actuators themselves might reveal broken gears or other obvious damage.
* Repair: Replacing a faulty actuator is usually the solution. You'll need to purchase the correct replacement part (specify your Jeep's trim level when ordering). This involves removing the old actuator, installing the new one, and carefully reassembling the dashboard. This is a time-consuming and somewhat complex job.
2. Vacuum Lines: The blend doors are often controlled by vacuum lines. A leak or blockage in these lines can prevent proper operation.
* Troubleshooting: Inspect all vacuum lines connected to the HVAC system for cracks, holes, or disconnections. Use a vacuum pump or your mouth (carefully!) to test for leaks.
* Repair: Repair or replace damaged vacuum lines. You might need to use specialized vacuum line connectors.
3. Mode Door Actuator: This actuator controls whether air comes from the vents, defrost, or floor. A malfunctioning mode door actuator can sometimes cause issues similar to a blend door problem.
* Troubleshooting and Repair: Similar to the blend door actuator. Access, inspect, and potentially replace.
4. Control Module/Head Unit: While less likely, a problem with the climate control module itself could cause this issue.
* Troubleshooting: This is the hardest to troubleshoot. You might need a diagnostic scan tool to check for error codes related to the HVAC system. A malfunctioning module often requires replacement.
* Repair: Replacement of the head unit is expensive and best left to a professional unless you have significant electronics experience.
Before you start:
* Consult a repair manual: A shop manual specific to your 1999 Jeep Grand Cherokee will be invaluable. It will provide diagrams, torque specifications, and detailed instructions for disassembling the dashboard.
* Disconnect the battery: Always disconnect the negative battery terminal before working on any electrical components in your vehicle.
* Take pictures: Take lots of pictures as you disassemble the dashboard. This will help you remember how to put everything back together.
This is a complex repair. If you're not comfortable tackling this yourself, the cost of professional diagnosis and repair will likely be less than the cost of potentially damaging your vehicle further trying to do it yourself. A mechanic with experience working on Jeep Grand Cherokees will have the tools and knowledge to diagnose and fix the problem efficiently.
