When a wiper motor operates but not when connected to the linkage, what is the most likely issue?

When the wiper motor operates when disconnected from the linkage but fails to function when the linkage is attached, this suggests that the motor itself is functioning properly. The most likely issue in this scenario is binding linkage.

Binding occurs when the wiper arms or linkage components are obstructed or overly tight, preventing them from moving freely. This can be caused by various factors, such as debris or corrosion accumulating in the wiper mechanism, misalignment, or wear and tear on the components. Because the motor is capable of running without the added load of the wiper blades, the linkage not being able to move indicates a physical obstruction or mechanical resistance that the motor cannot overcome.

This situation indicates that the motor is not receiving an adequate load when the linkage is not connected, as it operates without resistance. However, when reconnected, if the linkage is binding, the motor may stall because it has to exert more force than it is designed to handle, which leads to failure in operation even though the motor is otherwise functional.

In contrast, a bad relay or a power supply issue would typically affect the motor's ability to run in the first place, and a short circuit would usually result in a non-functional motor regardless of how the linkage is connected. Therefore