Q: How to replace the transmission speed sensors on Nissan Altima?A: The topmost part of the transmission holds the transmission speed sensors. Primary speed sensor senses the speed of primary pulley while secondary sensor senses the speed of output shaft. Disconnect electrical connector from sensor to remove it. Then, remove the mounting bolt and discard off the o-ring then remove it. Spread some transmission fluid onto a new O-ring and put it on a sensor. It is as simple as that when installation is concerned; just do everything opposite than during removal. However, this procedure only applies to automatic transaxles only.