* Driving in inappropriate conditions: The most common way 4WD stresses a transmission is by using it on dry pavement. 4WD systems are designed for low-traction situations like snow, mud, or ice. Using 4WD on dry pavement causes binding in the drivetrain, which puts extra strain on the transmission, transfer case, and other components. This extra stress can lead to premature wear and tear, eventually causing transmission failure.
* Shifting issues: Improper shifting while in 4WD can also damage the transmission. For example, shifting into 4WD while the vehicle is moving at high speeds or failing to properly engage or disengage 4WD can cause significant stress.
* Lack of maintenance: General lack of transmission maintenance (fluid changes, etc.) makes it *more* vulnerable to damage regardless of 4WD use. If the transmission is already weak, the added stress of 4WD use will accelerate its failure.
In short, it's not the *act* of using 4WD that hurts the transmission, but rather *how and when* it's used, and the overall condition of the vehicle. Used properly, 4WD shouldn't cause transmission problems. Used improperly or on a poorly maintained transmission, it will significantly increase the risk of damage.
