Chapter 8 Manual Transmission Construction and Operation 171 Copyright by Goodheart-Willcox Co., Inc. The speedometer gear mates with a worm gear on the output shaft, and the pinion gear shaft revolves inside the adapter. The adapter usually has both internal and external seals. The speedometer assembly drives a cable, which runs through a housing. This cable assembly extends from the speedometer adapter in the extension housing to the speedometer head, or speed indicator assembly, in the dash. Whenever the output shaft turns, the worm gear drives the pinion gear, and the speedometer cable turns. Transmission identification tag Case Fill plug Drain plug Hole for speedometer assembly Rear seal Extension housing Backup lamp switch Vent Access cover Shifter assembly Figure 8-26. Accessory parts may be bolted to or installed in the transmission case or extension housing. Access covers, fill and drain plugs, identification tags, shift levers, and boots are some common examples. In addition, the transmission usu- ally contains a vent somewhere at the top. The vent relieves pressure caused as the transmission lubricant heats up and expands. (Chrysler) This makes the speedometer head register the road speed of the vehicle. On many modern vehicles, the speedometer cable has been replaced with an electronic speed sensor, Figure 8-28. The rotation of the output shaft induces a small alternating (AC) current in the speed sensor. The vehicle computer reads this current as vehicle speed. The speed sensor signal is also used to operate the speedometer. Switches, sensors, and solenoids Most late-model manual transmissions are equipped with one or more switches, sensors, and sometimes, sole- noids. Most of these devices will be screwed into tapped holes in the transmission case or in the extension hous- ing. Solenoids help control transmission shifting. Switches are sometimes installed on the transmission to indicate to other vehicle systems that the transmission is in a par- ticular gear. For example, one switch is used to turn on the backup lights when the vehicle is shifted into reverse. Another component is used to prevent starter operation if the transmission is in gear. Some sensors tell the on-board com- puter when the transmission is in a particular gear so the computer can make adjustments to the engine fuel and igni- tion systems. Examples of some of these components can be seen in Figure 8-22 and Figure 8-26. Some late-model transmissions have solenoids that prevent reverse engagement when the vehicle is moving Retainer bolt and washer Seal retainer Adapter oil seal Adapter retainer Speedometer adapter Adapter O-ring Speedometer pinion gear To cable assembly Rotating teeth create a magnetic field in the pickup coil of a conventional electronic system, sending a voltage signal to the module S N S Module B A Figure 8-27. The speedometer pinion gear and adapter are usually installed in the extension housing. The adapter in this illustration is held in place by a single bolt and retainer. The speedometer pinion rotates inside of the adapter. (Chrysler) Figure 8-28. A—The relationship of the speed sensor and the shaft-mounted rotor. Rotation of the rotor induces a magnetic field in the sensor. This creates a voltage signal that is sent to the vehicle’s computer. B—This photo shows the relative positions of one sensor and rotor.