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| Illustration 1 | g00489255 |
Brake control valve (side view of valve) (1) Solenoids for the parking or secondary brake valve (2) Accumulator piston (3) Parking and secondary brake valves (4) Drain (5) Brake chamber (6) Passage (7) Reducing spool (8) Chamber for pressure feedback (9) Chamber for supply oil (10) Chamber for pilot pressure (11) Spool (12) Drilled hole (13) Passage (14) Shutoff valve (15) Chamber for pilot pressure (16) Chamber for drain (17) Pilot valve (18) Proportional solenoid valve | |
The brake control valve is installed on the frame above the left front side of the bevel gear case. The brake control valve is operated electrically by the electronic control module (ECM). The ECM responds to the operator's movement of the service brake pedal.
The service brake pedal modulates the engagement of both of the brakes in order to stop the machine. The parking brake switch engages both of the brakes in order to prevent the machine from moving.
The brakes are engaged with spring force. Hydraulic pressure is required to disengage the brakes. If hydraulic pressure is lost the brakes will fully engage due to the action of the spring force.
The brake control valve consists of a valve body and manifold. The valve contains a separate section for each brake. The valve also contains parking or secondary valves for the brake sections. There is a pilot valve (17) for each brake. The pilot valves are operated by proportional valve (18) , an accumulator piston (2) , and a reducing spool (7) .
These components operate the same way for each brake to control the brake pressure. In addition, each brake control has shutoff valve (14) . If the pressure from pilot valve (17) drops suddenly, the shutoff valve will gradually drain the brake pressure. This will prevent sudden brake engagement due to an electrical failure. At the same time, the operator will be able to rapidly apply the brakes.
The brakes also have parking and secondary valves (3) operated by two on/off solenoids (1) . The two solenoids (1) are connected to the parking brake switch and the service brake switch (end of travel). The two solenoids for the brake valves are controlled by these two switches in addition to the ECM. The parking brake valve and the secondary brake valve enable the operator to apply the brakes immediately without modulation by the shutoff valve.
Solenoid (18) is controlled by the ECM. This solenoid sets the force of the pressure in pilot valve (17) .
Pilot valve (17) regulates the pilot pressure in chamber (15) , passage (13) , and chamber (10) . The applied force from solenoid (18) will determine the pilot pressure in the passage and the chambers. This force will also determine the pressure setting of reducing spool (7) .
Reducing spool (7) regulates the flow of oil to the brake. The pressure that is maintained is slightly lower than the pilot pressure in chamber (10) which is located at the left end of the spool. This pressure differential is due to a spring, which ensures that the clutch or brake pressure can be reduced to zero.
Accumulator piston (2) reduces fluctuations in pilot pressure. The accumulator piston also provides a volume of oil to the brake valves for operating shutoff valve (14) .
Oil flow to the brake control valve comes from the priority valve. Part of the oil from the priority valve flows to the brake control valve. Part of the oil flows to the modulating valves for the transmission.
Oil to the brake control valve flows into the supply port and through an orifice in chamber (9) . Oil from the supply port also flows through a screened orifice to chamber (15) . The pressure in chamber (15) is regulated by pilot valve (17) . The screened orifice separates the pilot pressure from the supply pressure.
Pressure in chamber (15) increases until the poppet in pilot valve (17) is forced open. The open poppet allows oil from chamber (15) to flow into chamber (16) (drain) to maintain a constant pressure in chamber (15) . The pressure that is required to open the poppet in pilot valve (17) is determined by the force that is applied to the poppet by solenoid (18) .
Oil flows from chamber (15) into chamber (10) (passing through passage (13) in shutoff valve (14) for a brake). Also, the brake pressure in chamber (5) flows through passage (6) and into chamber (8) .
When the pilot pressure in chamber (10) is greater than the brake pressure in chamber (8) plus a small amount determined by the spring at the right end of the spool, spool (7) moves to the right. This movement opens the supply oil from chamber (9) into chamber (5) . The oil then flows through a passage and into the brake.
After the passages for the clutches or the brakes and the chambers are full of oil, the pressure in chamber (8) begins to increase. The increased pressure moves reducing spool (7) to the left. This closes the oil flow from chamber (9) to the brake.
Once the clutch or brake pressure that is set by the pilot pressure is achieved, reducing spool (7) allows oil to go into chamber (5) . The reducing spool allows enough oil into the chamber to keep the pressure constant for the brakes. Also, the reducing spool allows the oil that is necessary to make up for leakage to go into the chamber.
Accumulator piston (2) allows a volume of oil to accumulate at pilot pressure. The pressure in chamber (10) causes accumulator piston (2) to move to the left. The movement causes an increase in the volume of oil that is acting on reducing spool (7) . This reduces fluctuations in pilot pressure due to movement of reducing spool (7) . This also provides a volume of oil for operating shutoff valve (14) for brake control.
When the operator calls for increased brake engagement, the ECM lowers the current to solenoid (18) . The reduced current allows pilot pressure in chamber (15) to open the poppet in pilot valve (17) . The open pilot valve relieves the pressure from chambers (15) and (10) . The higher brake pressure in chambers (5) and (8) causes reducing spool (7) to move to the left.
This movement to the left opens chamber (5) to drain (4) . Oil flows out of the clutch or brake to drain (4) until pilot pressure in chamber (10) and clutch pressure in chamber (8) are again balanced, and reducing spool (7) closes the opening to drain (4) .
If an electrical failure causes pilot valve (17) to suddenly lower the pressure in chamber (15) of a brake control valve, shutoff valve (14) reacts in order to hold the pressure in chamber (10) . This happens when the pilot pressure in chamber (15) is lowered faster than the rate that is possible by normal modulation of the service brake pedal by the operator.
The higher pressure in chamber (10) causes spool (11) to move to the left. The spool movement covers drilled hole (12) which reduces the flow out of chamber (10) . This action results in a gradual reduction in pressure in chamber (10) and a gradual application of the brakes.
When the switch (end of travel) for the service brake makes contact, one of the two solenoids (1) on the parking or secondary valves is connected directly to the battery. When both the parking brake switch and the key start switch are in the ON position, the other solenoid (1) is connected directly to the battery.
These solenoids are also connected to the ECM. The ECM performs diagnostics on the solenoids. Also, the ECM provides a redundant supply of voltage to the solenoids for parking or secondary valves (1) . Solenoids for the parking or secondary valves are connected to the service brake switch if the sensor for the pedal is at the maximum position.
When either solenoid (1) is ON, the corresponding secondary valve (3) opens allowing the pilot oil in chamber (10) to go directly to the drain. This causes reducing spool (7) for each brake to immediately relieve all pressure to the brake and the brake is fully engaged.
This is the second method of engaging the brakes in addition to the control of brake solenoids (18) which is provided by the ECM. Check valves isolate pilot chambers (10) for the brakes from each other while a brake is released or a modulated brake operation is performed.
A calibration procedure is used in order to adjust the settings for the brake pressure. Refer to the Service Manual, SENR8367, "Power Train Electronic Control System" for a description of the calibration procedure.
Note: The brakes will need to be recalibrated if any of the following items are replaced or serviced:
- Brake control valve
- Solenoids
- Brake module
- ECM
Service Brake Pedal Operation
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| Illustration 2 | g00490407 |
Brake control valve (1) Solenoid valves for parking brake and for secondary brake (5A) Circuit for oil to right brake (5B) Circuit for oil to left brake (7A) Pressure reducing spool for right brake (7B) Pressure reducing spool for left brake (18) Proportional valves (AA) Supply oil (LL) Drain oil | |
As the operator begins to push the service brake pedal toward the floor, the rotary sensor sends a signal to the electronic control module (ECM). This signal tells the ECM the brake pedal position. The electronic control module (ECM) decreases the current to proportional valves (18) relative to the brake pedal position.
The decreased current causes a decrease in pilot oil pressure which causes reducing spools (7A) and (7B) to move to the left. The movement allows the oil in brake circuit (5A) and in brake circuit (5B) to drain. When the oil is drained the brake oil pressure is lowered.
The current that goes to the solenoid for the proportional valve is dependent upon the position of the brake pedal. The solenoid sets the pilot oil pressure. The pressure in the brake circuit is directly affected by the position of the brake pedal.
When the brake pedal is partially depressed the pilot pressure is reduced. The brakes will be applied lightly. When the brake pedal is fully depressed, the pilot pressure is reduced until the pressure reaches zero kPa (psi) and the oil is drained from the brake circuit. The brakes will be applied fully.
Once the pedal is to the floor the switch (end of travel) for the service brake opens. The switch connects one of the secondary solenoid valves (1) to the battery. The electronic control module (ECM) sends an electrical signal to the other secondary solenoid valve (1) . The energized solenoid valves move reducing spools (7A) and (7B) to the left. The movement routes the oil in brake circuit (5A) and in brake circuit (5B) to the drain. When the pilot oil is drained the brakes will be applied fully.