If you notice foaming in the oil, drain the transmission and refill it with clean oil. Foaming is evidence that water or some other lubricant that will not mix with the recommended transmission oil is present.

When it becomes necessary to change the trans-mission oil, the following procedure should be used:

1. Before you drain the oil, clean around the drain and fill plugs thoroughly. Both drain and fill plugs should be removed to allow the oil to drain.
2. Drain the transmission immediately after the vehicle has been operated. The oil will then be warm and will readily drain, taking along the suspended contaminants as it drains.
3. Check the drained oil for any uncommon foreign matter, such as large metal particles (steel or brass). This is a good sign of internal damage to the gears, bearings, or synchronizers. If large particles are found, notify your shop supervisor for further instructions.
4. Once the transmission has drained completely and no large metal particles are found, you replace the drain plug and refill the transmission with the proper grade of oil until it reaches the bottom of the fill plug. You then replace the till plug.

Other than the periodic check required on the transmission fluid, drain and refill are performed as prescribed by the manufacturer. You should check the bolts for tightness and inspect the case for damage each scheduled PM.

AUTOMATIC TRANSMISSIONS
Learning Objective: State the operating principles, identify the components, and maintenance procedures of an automatic transmission.

The automatic transmission (fig. 4-23), like the manual transmission, is designed to match the load requirements of the vehicle to the power and speed range of the engine. The automatic transmission, however, does this automatically depending on throttle position, vehicle speed, and the position of the control lever. Automatic transmissions are built in models that have two-, three-, or four-forward speeds and in some that are equipped with overdrive. Operator control is limited to the selection of the gear range by moving a control lever.

The automatic transmission is coupled to the engine through a torque converter. The torque converter is used with an automatic transmission, because it does not have to be manually disengaged by the operator each time the vehicle is stopped. Because the automatic transmission shifts without any inter-ruption of engine torque application, the cushioning effect of the fluid coupling within the torque converter is desirable.

Because the automatic transmission shifts gear ratios independent of the operator, it must do so without the operator releasing the throttle. The automatic transmission does this by using planetary gearsets whose elements are locked and released in various combinations that produce the required forward and reverse gear ratios. The locking of the planetary gearset elements is done through the use of hydraulically actuated multiple-disc clutches and brake bands. The valve body controls the hydraulic pressure that actuates these locking devices. The valve body can be thought of as a hydraulic computer that receives signals that indicate vehicle speed, throttle position, and gearset lever position. Based on this information, the valve body sends hydraulic pressure to the correct locking devices.

The parts of the automatic transmission are as follows:

Torque converter- fluid coupling that connects and disconnects the engine and transmission.

Figure 4-23.- Automatic transmission cross-sectional view.

Input shaft- transfers power from the torque converter to internal drive members and gearsets.

Oil pump- produces pressure to operate hydraulic components in the transmission.

Valve body- operated by shift lever and sensors; controls oil flow to pistons and servos.

Pistons and servos- actuates the bands and clutches.

Bands and clutches- applies clamping or driving pressure on different parts of gearsets to operate them.

Planetary gears- provides different gear ratios and reverse gear.

Output shaft- transfers engine torque from the gearsets to the drive shaft and rear wheels.