CX28 On-Highway Transmission Caterpillar

Clutch Assignments

Illustration 1	g02790314
(1) Clutch 2 (2) Clutch 3 (3) Clutch 4 (4) Clutch 5 (5) Clutch 1 (6) Input shaft (7) Transmission planetary input speed sensor port

The planetary transmission has six forward speeds and one reverse speed. REVERSE, NEUTRAL, and FIRST use only torque converter drive. At lower ground speeds, SECOND speed uses torque converter drive. As the ground speed increases in SECOND speed, the lockup clutch of the torque converter engages. Increased speed provides SECOND speed with direct drive. The torque converter is always in direct drive for speeds THIRD through SIXTH. This provides smooth, automatic upshifting from THIRD through SIXTH and smooth, automatic downshifting from SIXTH through THIRD. Table 1 shows the clutches that are engaged for each speed.

The planetary transmission consists of two rotating clutches, three stationary clutches, and three planetary units. The combination of clutches provide six forward speeds and one reverse speed. No. 1 clutch (5) and No. 2 clutch (1) are the rotating clutches.

The planetary transmission is bolted to the torque converter housing and the torque converter. Input shaft (6) for the planetary transmission is splined to the torque converter turbine. The stator in the torque converter is splined to the front support sleeve.

Transmission Electro-Hydraulic System

Illustration 2	g01638974
(13) Test port (clutch 5 pressure) (14) Test port (clutch 4 pressure) (15) Test port (clutch 3 pressure) (16) Test port (clutch 2 pressure) (17) Test port (clutch 1 pressure) (75) Pressurized oil (76) ECPC solenoid (77) Engaged clutch (78) Disengaged clutch (79) Modulating valve

Oil is supplied by the transmission pump through a filter. Oil (75) is available to each modulating valve (79) .

Control of the transmission planetary is achieved by hydraulically actuated clutches that hold selected components of the planetary group stationary. This allows transmission of power in the selected gear (speed ratio) through the transmission.

Shifting of the transmission is controlled by the electronic control module (ECM). The ECM acts in response to various inputs.

Inputs to the ECM module include the following information.

• Selected Gear

• Engine Speed

• Ground Speed

• Gear Limits

The control of the transmission also accounts for factors such as the temperature of the transmission oil.

Shifting of the transmission gears involves the release of one clutch and the subsequent engagement of the next clutch.

Each clutch is engaged by hydraulic oil pressure that is supplied by the modulating valves. The modulating valves are controlled by pulse width modulated signals (PWM). The signals are sent by the ECM to individual solenoids (76) on each modulating valve (79) .

For details of the operation of the modulating valves, refer to Systems Operation, Testing and Adjusting, "Solenoid Valve (Clutch)".

The pressure of the oil in the clutches is approximately proportional to the electrical current that is supplied to solenoid (76) that controls the clutch.

Illustration 3	g00766511

A high current input is made initially. This overcomes the inertia of the valve and the clutch components and this initiates filling of the oil passage leading to the clutch piston. The current is reduced as the clutch piston is filled with oil and the plates move toward engagement. The current is held at a minimum value in order to allow the pressure of the oil in the clutch to increase slowly. The clearances between the plates and the disks are taken up smoothly. This period is called touchup. Once the plates and the disks are in contact, the current is increased gradually in order to ensure that maximum pressure is applied to the clutch piston in a smooth manner.

Electronic control of this process provides improved quality of transmission shifts.

A combination of clutches engages each of the six forward gears and reverse gear.

Illustration 4	g01639173
(5) Test port (torque converter lock up clutch) (19) Test port (torque converter inlet pressure) (20) Relief valve (torque converter inlet pressure) (34) Torque converter outlet temperature sensor (80) Oil filter (81) Lock up clutch valve (82) Torque converter lock up clutch (83) Torque converter (84) Torque converter outlet relief valve (85) To transmission oil cooler

Power is supplied from the engine, through torque converter (83), and then to the planetary transmission. Torque converter (83) allows the speeds of the engine and the transmission to be matched smoothly. Once the speed of the engine and the transmission is matched, torque converter lockup clutch (82) engages in order to allow direct drive between the engine and the transmission.

Oil is supplied to the torque converter by relief valve (20) .

Oil from the torque converter flows through torque converter outlet relief valve (84) to the transmission oil cooler. The torque converter outlet relief valve maintains a required pressure in the torque converter.

Transmission shifts are dictated by operating conditions. Transmission shifting is automatic or manual depending on mode. The appropriate gear is selected for the speed of the engine and the speed of the machine. Operator inputs can alter the shifting of the transmission. Examples of operator inputs are the selection of a highest permissible gear or the selection of transmission hold. Features such as overspeed protection and the application of the engine compression brake can also alter the shifting of the transmission.