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| Illustration 1 | g01151999 |
Medium pressure hydraulics system connections | |
The medium pressure function provides the capability to operate attachments that operate at a constant medium hydraulic pressure. Four combinations of medium pressure configurations are possible for a single machine application. These combinations are listed below.
- Medium pressure function
- Medium pressure and joystick steering
- Medium pressure and hydraulic cab riser (HCR) or offset boom
- Medium pressure and HCR or offset boom and joystick steering
The medium pressure attachment configurations are installed or uninstalled in the Cat ET configuration screen.
The operator uses the right hand joystick thumbwheel in order to control the selected medium pressure attachment. For more information on the joystick thumbwheel operation for selected attachments, refer to Operation And Maintenance Manual, SEBU7945.
The ECM must see that the armrest position switch is closed to ground at contact J1-54 (wire E917-WH) before the medium pressure function can be enabled. The ECM must energize the implement lockout relay (implements unlocked) in order to supply +battery power to the medium pressure switch.
The position of the medium pressure switch will determine whether the ECM energizes the medium pressure enable solenoid or the HCR/Offset boom enable solenoid. When either of these solenoids is energized, the associated function is enabled. Then, the ECM will output a proportional output current to the appropriate medium pressure solenoid in order to control the hydraulic oil flow for the selected function. The output current from the ECM is dependent on the right hand joystick thumbwheel position sensor signal that is received at contact J1-50 (wire E474-OR).
Table 1 indicates the medium pressure switch states for a selected function.
| Medium Pressure Switch States For Selected Functions | |||
|---|---|---|---|
| Function | Sw 1 | Sw 2 | Sw 3 |
| Medium Press Enabled | Closed (1) | Open (2) | Open |
| Joystick Steer | Open | Open | Closed |
| HCR/Offset Boom | Open | Closed | Open |
| ( 1 ) | Closed to + battery from implement lockout relay. |
| ( 2 ) | Open to + battery from implement lockout relay. |
Right Joystick Thumbwheel Position Sensor Operation
The pulse width modulated (PWM) signal that is sent to the ECM from the position sensor is calibrated automatically by the ECM every time that the key switch is turned to the ON position. An allowable "zero position" for the thumbwheel is set as a default in the ECM based on where the thumbwheel is expected to be at rest, when there is not pressure of any kind on the thumbwheel. When the left section of the thumbwheel is completely depressed, the ECM will interpret the signal received as a maximum left position command or -100%. When the right section of the thumbwheel is completely depressed, the ECM will interpret the signal received as a maximum right position command or +100%. The ECM will interpret the first 3% of thumbwheel movement in either the left or the right direction as a 0% command. The ECM will interpret 97% of thumbwheel movement in either direction as a 100% command, consequently, the ECM sees a 97% thumbwheel movement in either direction as either a -100% or +100% command. The PWM duty cycle value for these three positions are shown in Table 2.
| Thumbwheel Position Command | PWM Duty Cycle (%) |
|---|---|
| Initial Zero Position | 50 |
| Allowable Zero Range (ZP) | 48 to 52 |
| -100% (Full Left) | 33 |
| +100% Full Right) | 67 |
When the key switch is turned to the ON position, the ECM reads the duty cycle from the sensor. If the duty cycle is within the allowable zero range, then that duty cycle value will become the zero position for the duration of the key switch ON cycle. If the duty cycle is not within the allowable zero position range, then the ECM will assume that the thumbwheel is pressed and will use the last known zero position duty cycle for the current zero position duty cycle value.
During machine operation, as the thumbwheel is depressed further in either direction, the duty cycle signal to the ECM will result in an increased current output to the medium pressure solenoids. This will allow the ECM to increase or decrease the hydraulic flow rate to the tool.
For all medium pressure functions that use the right hand thumbwheel for operation, when the thumbwheel is pressed on the right hand side, the ECM will output a proportional signal to medium pressure solenoid 2. When the thumbwheel is pressed on the left hand side, the ECM will output a proportional signal to medium pressure solenoid 1.
Medium Pressure Enable
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| Illustration 2 | g01152481 |
Medium pressure switch medium pressure enable selection | |
When the medium pressure switch is in the medium pressure enable position, switch 1 contacts are closed to +battery which will energize the medium pressure enable solenoid. The ECM will receive this input at contact J2-31 (wire L998-OR). The ECM will activate one of the medium pressure solenoids with an output current that is dependent on the right hand thumbwheel position sensor signal that is received at contact J1-50 (wire E474-OR) in order to operate the tool.
Joystick Steering
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| Illustration 3 | g01152486 |
Medium pressure switch joystick steering selection | |
Note: In order for the ECM to activate the joystick steering function, the selected machine speed must be low gear.
When the medium pressure switch is in the joystick steering position, switch 3 contacts are closed to +battery which will energize the joystick steering protection solenoid. The ECM will receive this input at contact J2-30 (wire 327-PK). The ECM will activate one of the medium pressure solenoids with an output current that is dependent on the right hand thumbwheel position sensor signal that is received at contact J1-50 (wire E474-OR) in order to operate the steering.
When the joystick steering function is active, depressing the trigger switch on the left hand joystick will momentarily enable the medium pressure function and deactivate the joystick steering function. The joystick steering function will reactivate when the trigger switch is released. When the joystick steering is activated, wire 327-PK that is connected to one side of the joystick steering relay coil is charged at + battery power. When the trigger switch on the left hand joystick is pressed, wire 191-WH which is connected to the other side of the joystick steering relay coil is connected to ground through the trigger switch. This energizes the relay which will energize the medium pressure enable solenoid through wire L998-OR. The ECM will disable the joystick steering function while the trigger switch is depressed.
Hydraulic Cab Riser Or Offset Boom
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| Illustration 4 | g01152504 |
Medium pressure switch HCR selection | |
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| Illustration 5 | g01152496 |
Medium pressure switch offset boom selection | |
Note: The hydraulic cab riser function and the offset boom function can not be installed on the machine at the same time.
When the medium pressure switch is in the hydraulic cab riser position (if installed) or the offset boom position (if installed), switch 2 contacts are closed to +battery which will energize the HCR / offset boom solenoid. The ECM will receive this input at contact J2-38 (wire F864-WH).
If the HCR function is installed, the ECM must detect the HCR door switch as being closed to ground at contact J1-44 (wire E997-BR) in order to enable the function. The ECM will activate one of the medium pressure solenoids with an output current that is dependent on the right hand thumbwheel position sensor signal that is received at contact J1-50 (wire E474-OR) in order to operate the HCR function.
If the offset boom function is installed, the ECM must detect the input from the medium pressure switch only.