Range Control System

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Figure 23-12. Typical Torque Converter Feed Circuit.

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TM-9-8000 Principles of Automotive Vehicles Manual

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Figure 23-13. Typical Range Control Circuit.

TM 9-8000

for an automatic shift from first to second gear, but

tubing arranged in rows. The rows of tubing pass

blocks an automatic shift from second to third gear.

through a series of fins. As the oil passes through the

tubing, its latent heat is transferred to the air via

(2) Vehicle Speed Versus Engine Loading.

conduction through the fins. This type of cooler usually

The shifting of the automatic transmission is controlled

is located in front of the engine radiator where it is

by two pressure signals that are indicators of speed and

subjected to airflow. This condition is important for its

engine loading. These signals, as will be described later

efficient operation.

in this para- graph, work against each other to produce a

f. Lubrication. After the oil is passed through

shifting sequence.

the oil cooler, it is piped through internal passages in the

transmission case to the rear section of the transmission

The center of all shift control is the valve body. This unit,

where it provides lubrication for components such as the

which can be thought of as a hydraulic computer,

planetary gearsets and the output shaft bearings. The oil

receives information from the indicators described

then drains back to the sump. (At this time, it should be

above. Based on this information, the valve body

noted that the oil sump also dissipates heat from the

switches line pressure to the proper planetary holding

transmission oil and is considered to be a source of

elements to produce the required gear range. As the

transmission cooling.)

information changes, the valve body changes its line

pressure outputs accordingly. For the sake of simplicity,

23-14. Range Control System (Fig. 23-13).

a two- speed transmission will be used to explain the

operation of all of the typical components. A

a. General. The range control system provides

hypothetical, automatic transmission hydraulic system

automatic or operator-controlled shifting of the

will be constructed in this paragraph . It must be

transmission. The shifting of the transmission is

stressed that this hypothetical system by no means

controlled by the following two indicators:

contains all of the devices necessary in a real

transmission. It is useful for the sake of learning,

(1) Manual Selection. The position of the

because it clearly illustrates how the decision and the

gearshift lever selected by the operator chooses the

action of shifting is initiated.

desired shifting program. The selections available to the

b. Manual Valve. The manual valve is the

operator of a typical two-speed unit are:

device that selects the desired shift program through the

(a) Neutral (N). In neutral (N), the

position of the gearshift lever. A manual valve is

engine freewheels, providing no driving force to the

basically a multiport spool valve that switches line

vehicle.

pressure to selected passages as it is moved through its

operating positions. A hypothetical manual valve for a

(b) Drive (D). in drive (D), the

two-speed transmission is illustrated in figure 23-14 in all

transmission provides automatic shifting through the low-

of its operating positions.

to high-gear ranges.

c. Governor. The governor modulates line

pressure to produce a signal that is an indication of

transmission is locked in the low range and no automatic

vehicle speed. This signal is used by the valve body to

shifting occurs.

formulate gear-range selections for the transmission.

The governor uses a spool valve that is operated by

(d) Reverse (R). Reverse (R)

centrifugally operated weights. The weights are rotated

reverses the direction of engine torque to drive wheels,

by the output shaft of the transmission because it is

and the vehicle is driven backwards.

solidly linked to the drive wheels and, therefore,

consistently provides a true indication of vehicle speed.

(e) Park (P). Park (P) is the same as

As the weights rotate they are acted on by centrifugal

neutral except that the drive wheels are locked by a

force which tends to pull them outward from the axis of

rotation. The weights are pulled

positive latching device within the transmission.

The three-speed transmission has all of the above

selections plus drive second. This position allows

23-16