* Transmission Fluid Level and Condition: Low or dirty transmission fluid is a very common cause. Low fluid prevents proper lubrication and pressure regulation, leading to premature overdrive engagement. Dirty fluid compromises the fluid's ability to lubricate and transmit pressure effectively. Check your transmission fluid level and condition first. If it's low, add the correct type of fluid. If it's dirty or burnt-smelling, a complete fluid and filter change is needed.
* Transmission Solenoids: These electrically controlled valves regulate fluid flow within the transmission. A faulty solenoid (especially the overdrive solenoid) can cause the transmission to shift into overdrive too early or not at all. These are relatively inexpensive to replace, but it requires some mechanical skill.
* Throttle Position Sensor (TPS): This sensor tells the computer the throttle position. If the TPS is faulty and reporting a lower throttle position than actual, the computer might shift into overdrive sooner than it should to save fuel. A bad TPS can also cause other drivability issues.
* Transmission Computer (PCM or TCM): In some cases, the Powertrain Control Module (PCM) or Transmission Control Module (TCM) (depending on your Aerostar's setup) itself might be malfunctioning. This is less likely but possible. Diagnosis would require a professional scan tool.
* Vacuum Leaks: While less common in electronic transmissions, vacuum leaks can sometimes affect shift points. This is more likely if you're experiencing other vacuum-related issues.
* Governor Pressure Sensor/Switch: This sensor measures the transmission fluid pressure, and a faulty one can cause incorrect shift points.
Troubleshooting Steps:
1. Check the transmission fluid: This is the absolute first step. Check the level (with the engine running and warmed up) and the condition (color, smell).
2. Scan the codes: Use an OBD-II scanner (available at auto parts stores) to check for any diagnostic trouble codes (DTCs) stored in the computer. These codes can point to the problem.
3. Inspect the wiring: Visually check the wiring harness for any damage, corrosion, or loose connections to the transmission, TPS, and related components.
If you're not comfortable working on your transmission yourself, take it to a qualified mechanic. They have the tools and expertise to diagnose the problem accurately and perform the necessary repairs. Trying to diagnose and fix transmission problems without the right knowledge can lead to more extensive and costly damage.
