Here's a breakdown of why and what to consider:
* Governor: The governor senses vehicle speed and tells the transmission when to shift. If the governor is malfunctioning (e.g., sticking, faulty sensor), it may not be sending the correct signal to downshift at higher speeds when you accelerate for passing. The transmission may be receiving the signal that a lower gear is unnecessary.
* Transmission Solenoids/Valve Body: The governor's signal is used to activate solenoids within the transmission's valve body. These solenoids control the flow of fluid, which dictates shifting. A faulty solenoid (or a problem within the valve body itself) could prevent the proper engagement of lower gears under acceleration. This is a common cause of this type of shifting issue.
* Throttle Position Sensor (TPS): While less likely, a faulty TPS can provide incorrect information to the transmission's control system. This could lead to inappropriate shifting behavior.
* Computer/PCM (Powertrain Control Module): In rare cases, a problem within the vehicle's computer could misinterpret signals from the governor, TPS, or other sensors, leading to the described issue.
Troubleshooting Steps (in order of easiest to most difficult):
1. Check the Transmission Fluid: Make sure the transmission fluid level is correct and the fluid itself is clean and red (not brown or burnt). Low fluid can cause significant problems.
2. Scan for Diagnostic Trouble Codes (DTCs): Use an OBD-II scanner (available at auto parts stores) to check for any transmission-related trouble codes stored in the computer. These codes will often give clues to the problem.
3. Inspect the Governor (Advanced): This requires removing the transmission, which is a significant undertaking and best left to a professional transmission mechanic.
4. Professional Inspection: The most practical approach is to have a qualified mechanic inspect the transmission. They can diagnose the problem more accurately and recommend the most appropriate repair.
Do not attempt major transmission repairs yourself unless you have significant mechanical experience. Improper repair can lead to catastrophic transmission failure and costly repairs.
The fact that it shifts up normally suggests the transmission itself might not be completely failing, but a faulty sensor or electrical component is much more likely the problem. Get a professional diagnosis to avoid wasting money on unnecessary parts.
