What could cause a transmission to not operate properly in overdrive while all other ranges operate fine?

The proper operation of an automatic transmission in overdrive is affected by various components, one of which is the overdrive brake (denoted as BO). The overdrive brake is responsible for holding the output shaft during overdrive operation, allowing the vehicle to take advantage of the gear ratio that improves fuel efficiency and reduces engine RPM at higher speeds.

If the overdrive brake is defective, it can result in the transmission failing to engage the overdrive gear correctly while still permitting functionality in other ranges. This can lead to symptoms such as the vehicle not going into overdrive, having insufficient speed with the overdrive engaged, or experiencing a lack of power on acceleration when overdrive should be used.

In contrast, while the forward clutch and the 2nd brake both play critical roles in the operation of the transmission, their failures would typically result in broader operational issues across multiple gear ranges rather than isolating the issue specifically to overdrive. Therefore, identifying a defective overdrive brake as the cause of the specific problem helps in clearly understanding that the malfunction relates directly to the function of the overdrive transmission operation.