* Tow/Haul Mode: This is the most common and simplest method. Activating tow/haul mode (usually a button on the shifter or dashboard) alters the transmission's shift points and prevents upshifts into overdrive. This keeps the engine in a lower gear at higher RPMs, providing more power for climbing hills and maintaining speed while towing.
* Transmission's Internal Logic: Even without tow/haul mode engaged, the transmission's computer constantly monitors engine load, throttle position, and vehicle speed. If it detects a significant load (like a trailer), it will likely prevent upshifting into overdrive automatically to maintain adequate power and prevent lugging the engine. This is particularly true on modern vehicles with sophisticated electronic control units (ECUs).
* Throttle Position: If the driver keeps the accelerator pedal depressed, maintaining a high engine speed, the transmission will usually avoid shifting into overdrive as this indicates a need for more power.
* Grade Sensing: Some vehicles have grade sensing systems that detect uphill inclines. This will prevent overdrive engagement automatically, ensuring sufficient engine power to maintain speed while ascending grades.
In essence, it's a combination of driver input (tow/haul mode), vehicle sensors (load, speed, incline), and the transmission's control system working together to prevent overdrive when towing and ensure optimal performance and engine protection.
