TM 5-3805-281-24-1
Sub-System Diagnostics
ENGINE AND PUMP CONTROLLER CIRCUIT THEORY OF OPERATION
When an operating mode is changed, the EPC causes
The engine and pump controller (EPC) A2 monitors
the EC motor to rotate, changing the engine speed.
and controls engine speed and all digging functions,
When the engine speed matches the requirement for
propel functions, hydraulic pump functions, and pilot
the mode selected, the EC motor stops rotating and
functions.
the engine speed remains constant. If another mode is
selected, the process is repeated until the engine
The EPC applies service codes from EPC pins B8,
speed has changed to match the new mode speed
B15, and B16 to diagnostic connector X1 for analysis
requirement.
of hydraulic and electrical systems by the diagnostic
computer.
AUTO A/I IDLE MODE
ENGINE CONTROL CIRCUITS
An RPM dial is provided to vary engine speed to any
speed between slow idle and fast idle if E or HP
ENGINE SPEED CONTROL
modes do not provide a speed adequate to the job.
9015
Each time the engine is started, the EPC automatically
The engine speed control circuits monitor and adjust
15
activates the idle mode to run the engine at the engine
the engine speed to match the operating modes
69
RPM dial setting.
selected. The engine and pump controller (EPC)
monitors the engine speed via signals from the engine
When the auto idle mode is activated by the auto idle
speed sensor (B16) applied to EPC pins B6 and B13.
switch, a signal from the monitor controller is applied
The EPC controls the engine speed by sending
to EPC pin D6 informing the EPC that auto idle is
rotation control signals to the engine control (EC)
selected. With auto idle selected, mode switches E
motor (M2) from pins A12, A24, A25, and A26. The EC
and HP control engine speed during machine
motor is mechanically linked to the engine throttle
operation. However, if a hydraulic function is not used
linkage. As the motor rotates, the engine throttle
for more than 4 seconds, the EPC automatically
linkage is moved, changing the engine speed. The
reduces the engine speed to auto idle.
engine control (EC) sensor (B17) provides a signal to
EPC at pin D15 indicating the amount of EC motor
LEARNING SWITCH (S16)
rotation.
If the engine and pump controller or EC sensor is
When the engine is started, the dig hydraulic work
changed, the EPC learning sequence must be
mode is selected by logic circuit in the monitor
activated the next time engine is started. The learning
controller. Each time work mode select switch is
switch activates the EPC learning sequence by
pushed a different work mode is selected (Dig,
applying a ground to EPC pin B5. When the switch is
Grading, Precision, and Attachment).
activated, the slow idle engine speed is registered in
the EPC memory. After maximum power engine speed
Operating mode selection signals from the monitor
is stored in EPC memory, the EPC calculates the
controller and display are applied to EPC pins B11,
economy mode (E) speed as a percentage of the slow
D3, D4, D5, and D6. The combination of signals
idle speed.
instructs the EPC which mode has been selected.
Continued on next page
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