953C Track-Type Loader Caterpillar

Two-Stage Total Electronic Hystat Control (TEHC) Valves

Five two-stage TEHC valves are mounted in the ECM manifold. Four of the valves are used to control the speed and the direction of the machine. One of the valves is used to control the flow of oil to the other four valves. This valve is the override valve. When the machine is in PARK or in NEUTRAL, a signal is not sent to the override valve. When the speed/direction control lever is moved in the FORWARD direction or in the REVERSE direction, the ECM sends maximum current to the solenoid of the override valve. This causes the valve to open fully.

The other four valves are used to control the flow and direction of the output of the drive pumps. The signal pressure that is output from the valves is sent to the pump servo valves. The signal pressure is also sent to the motors. The ECM controls the signal pressure that is output from the valves by changing the signal current that is sent to the solenoids. There is a FORWARD valve and a REVERSE valve for the left side of the machine. Also, there is a FORWARD valve and a REVERSE valve for the right side of the machine.

Illustration 1	g00774160
(1) Spring (2) Cross-drilled Holes (3) Drilled Passage (4) Valve Spool (5) Cartridge (edge filter) (6) Threads (7) Orifice (8) Drain Orifice (9) Pin (10) Solenoid (11) Ball (12) Slot in Threads (13) ECM Manifold (A) Flow from the Override Valve (B) Flow to the Pump Servo Valve (C) Flow to the Tank through the Case

The valves are de-energized until signal current is sent from the ECM. When the machine is moved out of NEUTRAL, the override valve allows oil to flow through the ECM manifold to the control valves. The oil flows through hole (2) that is cross-drilled in spool (4) and into passage (3) that is drilled in the center of spool (4). The oil flows from the center of spool (4) through cartridge (edge filter) (5) and through orifice (7) into the chamber at the right end of spool (4). Pin (9) is not being forced against ball (11) by solenoid (10). Ball (11) is not restricting drain orifice (8). The oil flows through drain orifice (8) and through slot (12) in threads (6) of the valve. The oil is directed through passage (C) to the tank through a pump case or through a motor case.

Illustration 2	g00774161
(1) Spring (4) Valve Spool (8) Drain Orifice (9) Pin (10 ) Solenoid (11) Ball (14) Spring Cavity (15) Armature (A) Flow from the Override Valve (B) Flow to the Pump Servo Valve (C) Flow to the Tank through the Case

When the speed/direction control lever is moved further, the ECM sends more current to solenoid (10). The current is proportional to the position of the speed/direction control lever. The signal pressure that is sent to the servo valve through passage (B) is proportional to the signal current. The servo valve causes the swashplate in the pump to move from the neutral position.

The signal current creates a magnetic force that moves armature (15). This forces pin (9) against ball (11). The force of pin (9) against ball (11) is proportional to the signal current that is sent from the ECM. Ball (11) begins to block the flow of the oil through drain orifice (8). This restriction causes the pressure at the right end of spool (4) to increase. The pressure moves spool (4) to the left against the force of spring (1) .

The movement of spool (4) closes the passage between passage (B) to the servo valve and passage (C) to the tank. The movement of spool (4) opens the passage between passage (A) from the override valve and passage (B) to the servo valve. While the pressure increases in passage (B) to the servo valve, the pressure increases in cavity (14). This pressure and the force of spring (1) balance the pressure at the right end of spool (4). This balance determines the position of spool (4).

Illustration 3	g00774162
(1) Spring (4) Valve Spool (8) Drain Orifice (9) Pin (10 ) Solenoid (11) Ball (A) Flow from the Override Valve (B) Flow to the Pump Servo Valve (C) Flow to the Tank through the Case

When the operator has selected maximum speed with the speed/direction control lever, the ECM sends maximum current to solenoid (10). Pin (9) exerts more force on ball (11). Ball (11) causes further restriction of the flow through drain orifice (8) to the tank. This causes pressure to increase on the right side of spool (4). Spool (4) moves further to the left. More of flow (A) from the override valve is sent to passage (B) to the pump servo valve. This causes movement of the servo valve in order to further move the swashplate. The maximum pressure to the servo valve is reached at the left end of valve spool (4). This pressure and the force of spring (1) cause spool (4) to move to the right. Spool (4) will stop moving when the force on left side of spool (4) equals the force on the right side of spool (4) .

This position maintains maximum available pressure in passage (B) to the servo valve until the signal current from the ECM is reduced. The signal current is reduced under any of the following conditions.

• A steering pedal is pressed.

• The center pedal is pressed.

• The speed/direction control lever is moved in order to slow the machine speed.

• The ECM senses an engine underspeed event.

Drive Motor Control (Maximum Displacement)

Illustration 4	g00728756

Drive Motor Control (Maximum Displacement)

The angle of the sliding valve plate in the drive motor is controlled by reduced signal pressure oil (8D) from either the left or right steering valves.

The sliding valve plate regulates the speed that is output by the drive motor.

The drive motor does not destroke unless the drive pump is at the maximum angle.

The illustration shows actuator (2D) in the maximum displacement position. Charge pressure (7D) is directed into the valve, around pilot spool (10D) and into the spring chamber of actuator (2D) .

Drive Motor Control (Minimum Displacement)

Illustration 5	g00728757

Drive Motor Control (Minimum Displacement)

As signal pressure (8D) increases, pilot spool (10D) pushes on regulation spring (4D). The machine speed increases as a result.

As pilot spool (10D) moves up, charge pressure oil is directed to the top of actuator (2D) through passage (1D). The oil flow through passage (1D) causes actuator (2D) to move down against the force of actuator bias spring (3D). The sliding valve plate is mechanically connected to actuator (2D) with control pin slot (13D) .

Graphs for the Range of Stroking of the Drive Pumps and Drive Motors

Method of Transmission Speed Control (Response to Mechanical Adjustment)

Illustration 6	g00722439
Description of the test for the Range of Stroking of the Drive Pumps and the Drive Motors

The graph shows drive motors (M) and drive pumps (P). As signal pressure (A) increases, machine speed (K) increases.

At 3.5 km/h (2.17 mph) or 800 kPa (116.0 psi), the swashplate in drive pump (P) is at the maximum angle. Drive motor (M) begins to destroke up to the maximum speed of 10.0 km/h (6.21 mph).

Calculate Machine Speed over a Specified Distance

Use the following table in order to obtain the machine ground speed:

Ideal Machine Speed versus Pilot Pressure

Illustration 7	g00722643

The graph shows ideal speed range (K) versus signal pressure (E). As pilot pressure (E) increases, machine speed (K) increases at a smooth rate throughout the full speed range.

Calibration Points

Illustration 8	g00722735

The graph shows the calibration points of the transmission ECM. Each position is calibrated by using the speed/direction control lever.

Each of these positions represent twenty percent increments in the maximum speed setting of the machine. The transmission ECM compensates for tolerances within the components in each drive loop.

Range of Stroking of the Drive Pumps and the Drive Motors (Tolerances)

Illustration 9	g00722458

The graph shows the tolerance for the drive pumps and for the drive motors.

The transmission ECM compensates for the tolerances. Thus, the straight lines on the graph "Ideal Machine Speed vs. Pilot Pressure" are created.

The major advantage of the electronically controlled system is the transmission ECM that compensates for tolerances. The transmission ECM keeps the tracking of the machine straight.

Mismatch of Test for the Range of Stroking of the Drive Motors and the Drive Pumps

Illustration 10	g00722776

The graph shows destroking of the drive motor at the incorrect time.

Two conditions can exist.

Underlap - The drive motor does not start destroking until the drive pump has reached the maximum swashplate angle. Machine speed (K) does not change during a small range of signal pressures (E) .

The graph shows "Underlap". The drive pump has reached maximum angle (G) and the drive motor has not started destroking. The flat horizontal line is the "dead spot". Machine speed (K) does not change.

The result of the "dead spot" is evident during straight line travel. The machine travels straight until the "dead spot" is reached.

One track speeds up, while the other track maintains a constant speed. This occurs until the speed/direction control lever is moved further up the vee and the "dead spot" is passed.

Overlap - The drive motor starts destroking before the drive pump has reached the maximum swashplate angle. The result is a fast increase in machine speed (K) during a small range of signal pressures (E) .