The transmission scavenge pump section pulls oil from the bottom of the transmission case through a magnetic screen and sends the oil through the transmission oil cooler to the transmission tank. The magnetic screen should always be checked for debris if a problem with the transmission is suspected.
The transmission charging pump section pulls oil from the transmission hydraulic tank.
Charging oil flows from the pump through two transmission charging filters to the transmission main relief valve and seven modulating valves.
The main relief valve regulates the supply pressure inside the transmission hydraulic system.
Oil unseats the check ball and forces the spool to the right if the transmission system pressure becomes greater than the spring force on the right of the spool. Excess oil will flow to the lubrication circuit and the lube relief valve. The lubrication circuit oil and oil from the lube relief valve flows to the transmission sump. The relief valve is adjustable by turning the adjusting screw on the right end of the valve.
The clutch modulating valves control the engagement of the transmission clutches. The solenoids are controlled by a pulse width modulated (PWM) signal from the Transmission/Chassis ECM. Supply oil flows into the clutch modulating valves and through a passage in the center of the spool. Oil then flows to the tank if the solenoid is not energized.
Oil flow is blocked by a ball and seat if the solenoid is energized. The spool will shift down and the clutch will begin to fill. The signal from the Transmission/Chassis ECM determines how long it takes to fill each clutch.
The transmission lubrication relief valve limits the transmission lubrication oil pressure.
The transmission charging pump section (3) pulls oil from the bottom of the transmission hydraulic tank through a magnetic screen and sends the oil through the transmission filters to the transmission hydraulic controls.
Oil from the transmission charging pump section is sent to the transmission charge oil filters (1) located on the cross member on the right side of the machine.
The rear filter housing has an S•O•S tap (2) and a charge pressure tap (3). The rear filter housing also has a bypass switch (4) which provides an input signal to the monitoring system, via the Transmission/Chassis ECM, to inform the operator if the filter is restricted.
The ECPC transmission hydraulic controls can be accessed by removing a cover plate (5) on top of the transmission. The transmission input speed sensor (6) is located on top of the transfer gear housing. The transmission input speed sensor sends an input to the Transmission/Chassis ECM which checks the speed of the drive shaft to the speed of the engine.
The transmission has pressure taps located on the outside of the transmission which aids in preventing contamination from entering the transmission as well as saving time when checking the pressures on the 777F transmission.
Shown in the lower right illustration are the transmission control valve pressure taps. The lube oil pressure tap (7) and the transmission hydraulic system pressure tap (8) are located toward the rear of the transmission. Oil pressure for the seven clutches can be checked at the remaining seven taps (9) on the transmission.
The transmission modulating valves control the oil to corresponding transmission clutches. The
solenoid valves are:
- Clutch No. 1 Solenoid valve (1)
- Clutch No. 2 Solenoid valve (2)
- Clutch No. 3 Solenoid valve (3)
- Clutch No. 4 Solenoid valve (4)
- Clutch No. 5 Solenoid valve (5)
- Clutch No. 6 Solenoid valve (6)
- Clutch No. 7 Solenoid valve (7)
The main relief valve (8) controls the transmission hydraulic pressure, and the lubrication relief valve (not visible) controls the lubrication pressure. The lubrication relief valve is located below the main relief valve.
Also located on the transmission hydraulic control valve is the transmission hydraulic oil temperature sensor (9). The temperature sensor sends a signal to the Transmission/Chassis ECM indicating transmission oil temperature.
The table in this illustration lists the solenoids that are energized and clutches that are engaged for each transmission speed. This table can be useful for transmission diagnosis.
In this illustration, the transmission modulating valve is shown with no current signal applied to the solenoid. The Transmission/Chassis ECM controls the rate of oil flow through the transmission modulating valves to the clutches by changing the signal current strength to the solenoid. With no current signal applied to the solenoid, the transmission modulating valve is DE-ENERGIZED and oil flow to the clutch is blocked.
Pump oil flows into the valve body around the valve spool and into a drilled passage in the center of the valve spool. The oil flows through the drilled passage and orifice to the left side of the valve spool to a drain orifice. Since there is no force acting on the pin assembly to hold the ball against the drain orifice, the oil flows through the spool and the drain orifice past the ball to the tank.
The spring located on the right side of the spool in this view holds the valve spool to the left.
The valve spool opens the passage between the clutch passage and the tank passage and blocks the passage between the clutch passage and the pump supply port. Oil flow to the clutch is blocked. Oil from the clutch drains to the tank preventing clutch engagement.
The start of clutch engagement begins when the current signal to the solenoid creates a magnetic field around the pin. The magnetic force moves the pin against the ball in proportion to the strength of the current signal from the Transmission/Chassis ECM.
The position of the ball against the orifice begins to block the drain passage of the oil flow from the left side of the valve spool to the tank. This partial restriction causes the pressure at the left end of the valve spool to increase. The oil pressure moves the valve spool to the right against the spring. As the pressure on the right side of the valve spool overrides the force of the spring, the valve spool shifts to the right.
The valve spool movement starts to open a passage on the right end of the valve spool for pump supply oil to fill the clutch. Oil also begins to fill the spring chamber on on the right end of the spool.
In the initial clutch filling stage, the Transmission/Chassis ECM commands a high current pulse to quickly move the valve spool to start filling the clutch. During this short period of time, the clutch piston moves to remove the clearances between the clutch discs and plates to minimize the amount of time required to fill the clutch. The ECM then reduces the current signal which reduces the pressure setting of the proportional solenoid valve. The change in current signal reduces the flow of oil to the clutch. The point where the clutch plates and discs start to touch is called TOUCH-UP.
Once TOUCH-UP is obtained, the Transmission/Chassis ECM begins a controlled increase of the current signal to start the MODULATION cycle. The increase in the current signal causes the ball and pin to further restrict oil through the drain orifice to tank causing a controlled movement of the spool to the right. The spool movement allows the pressure in the clutch to increase.
During the MODULATION cycle, the valve spool working with the variable commanded current signal from the Transmission/Chassis ECM acts as a variable pressure reducing valve. The sequence of partial engagement is called desired slippage. The desired slippage is controlled by the application program stored in the Transmission/Chassis ECM.
In this illustration, the modulating valve is shown with a maximum current signal commanded to the solenoid. When the modulation cycle stops, the Transmission/Chassis ECM sends the maximum specified current signal to fully engage the clutch.
The constant current signal pushes the pin firmly against the ball in the solenoid valve. The pin force against the ball blocks more oil from flowing through the drain orifice. This restriction causes an increase in pressure on the left side of the valve spool. The valve spool moves to the right to allow pump flow to fully engage the clutch.
In a short period of time, maximum pressure is felt at both ends of the proportional solenoid valve spool. This pressure along with the spring force on the right end of the spool cause the valve spool to move to the left until the forces on the right end and the left end of the valve spool are balanced.
The valve spool movement to the left (balanced) position reduces the flow of oil to the engaged clutch. The Transmission/Chassis ECM sends a constant maximum specified current signal to the solenoid to maintain the desired clutch pressure.
The different maximum specified pressures for each clutch is caused by different maximum current signals being sent by the Transmission/Chassis ECM to each individual modulating valve. The different maximum signal causes a difference in the force pushing the pin against the ball to block leakage through the drain orifice in each solenoid valve. The different rate of leakage through the spool drain orifice provides different balance positions for the proportional solenoid valve spool. Changing the valve spool position changes the flow of oil to the clutch and the resulting maximum clutch pressure.
The operation of the proportional solenoid to control the engaging and releasing of clutches is not a simple on and off cycle. The Transmission/Chassis ECM varies the strength of the current signal through a programmed cycle to control movement of the valve spool. The clutch pressures can be changed using Caterpillar Electronic Technician (ET) during the calibration procedure.
Oil enters the relief valve at the supply port. The pressure of the oil unseats the ball and moves the spool toward the right. Oil flows past the spool and to the tank to regulate transmission oil pressure.
The adjustment screw alters the preload on the spring to adjust the relief pressure.
See You Soon!!!
MARYGAR
Any idea why the 777F transmission is more problematic than the older D, C & B models?
ReplyDeleteThese hydraulic controls have sensors that come as a very feasible feature. With the help of sensors one can get the notification about the pressure levels of the fluids. The pressures levels are important and needs to be under control. The high pressure levels can be a little harmful, so a check on it is essential. This job is done by the valves as most of them are automatic today. There is point set before the pressure is turned on, so when the point is crossed then the sensor gives a signal related to this information.
ReplyDelete