190303D Flashcards
Powershift and Automatic Transmission Control and Shifting
what is the simplest form of transmission shift systems?
powershift shift control systems
why do units with torque converters or torque dividers generally require fewer gear ranges than those with hyd clutches?
engine torque is multiplied through those units before its supplied to tranny
the hydraulic system to control shifting in a power shift can be divided into 3 circuits?
- clutch application
- torque converter charging circuit
- tranny lube circuit
out of the 3 hydraulic circuits, how is pressure compared?
clutch application gets more pressure, then torque converter and lastly lube circuit
how is oil sent to clutch application circuit?
from charging pump driven off the tranny
what does a shift selector valve do?
directs oil flow to proper clutch pistons, ensures other clutch pressure is released back to tank
what is the oil state considered in the clutch pistons when the clutch pressure regulator valve opens?
static oil. except for leakage oil for lubrication
where does the oil go after the clutch pressure regulator valve opens?
to the torque converter
what prevents damage to the torque converter by limiting pressure?
torque converter relief valve
what valve is a clutch pressure regulator valve like?
a sequence valve. keeps pressure in first circuit
where does the oil flow after the torque converter?
to the filter and cooler, before going back to tranny lube circuit.
how are all power shift and auto tranny gearing components lubricated?
pressure lubricated, as the components can’t be submerged in oil
what is max lubrication pressure controlled by?
lubrication relief valve
what must you do when towing machinery with power shift or auto trannys?
bc of pressure lubrication, if engine isnt turning there’s no pressure lube= extensive damage
how can you avoid tranny damage while towing?
elevate drive wheels
remove drive axles
remove driveline to prevent tranny rotation
how do you check tranny oil level?
with engine at low idle and oil at operating temp
what does oil level that’s too low cause?
aeration of the charging pump, resulting in tranny clutch slippage, torque converter cavitation, lack of tranny lube oil pressure
what does an oil level that’s too high cause?
tranny component to be submerged in oil= excessive friction and tranny overheating
although transmission oil has similar characteristics to hyd oil, they are specially formulated to provide?
max life of friction material on transmission discs
the oil pump on a power shift transmission is a 4 section external gear type pump. what are the 4 sections?
- transmission scavenging pump
- torque converter scavenging pump
- transmission charging pump
- steering clutch and brake lubrication pump
what is unique about the transmission charging pump and steering clutch and brake lubrication pump in terms of how the oil is supplied?
both of those pumps have oil going through a strainer first before the pump to prevent contamination
what does the priority valve do on a power shift transmission circuit?
pressure control, ensures oil pressure is available for steering clutch and brake function before transmission function
what does a speed selector spool do?
directs oil from tranny charging pump to speed clutches in planetary tranny
what does the modulation relief valve do?
controls max clutch application pressure
what does a load piston do?
controls rate of pressure rise in clutch pistons (clutch modulation) to prevent shift shock and excessive clutch slippage
what does a ratio valve do?
acts like a torque converter relief valve. limits max pressure inside torque converter when engine is started with cold oil
what does the pressure differential valve do?
maintain clutch application pressure to speed clutch. keep the pressure higher than direction clutch. prevents machine from moving
what does the direction selector spool do?
directs oil from charging pump to direction clutches
what does the torque converter outlet relief valve do?
limits max working pressure in torque converter
how does a transmission oil cooler work?
transfers heat energy by flowing coolant through tubes and allowing hot oil to pass around these tubes
why is the auto transmission shift controls more efficient that power shift?
tranny auto shifts speed ranges at optimum time in machine operating cycle
what do auto trannys shift in accordance to?
ground speed and operator demand for power
how does the control system reference machine ground speed?
with a governor circuit
what is a governor?
device that converts rotation speed of tranny output shaft to hyd pressure signal to control shifting
what is spool movement balanced by on a governor?
centrifugal force from flyweights and force created by hydraulic pressure
governor and modulator circuits are provided oil from?
clutch application circuit
as ground speed of the machine is reduced, governor pressure _______
decreases
an orifice in the line carrying governor pressure to the shift signal prevents downshifts from happening too rapidly if:
output shaft stops suddenly when brakes lock up
governor fails
some auto trannys allow adjustment of tranny shift points. how is this done?
by adjusting shift signal valve spring force. reducing spring force= shifts occur earlier
governor pressure is also used to apply and release the ______ in the torque converter?
lockup clutch
what does the modulator circuit do?
alter shift points of tranny in relation to operator demand for power, indicated by throttle position.
with a fully open throttle, will the modulator circuit shift?
no it will delay the shift
delaying shifts with the modulator circuit can increase machine performance because it:
engine can develop greater hp in each speed range
greater machine momentum before upshifting
why is it beneficial to keep the tranny in a particular speed range when going up a hill?
prevents vehicle from falling below the point of max torque production before downshifting
how is modulator pressure in an auto tranny controlled?
with a modulator valve (regulates pressure in modulator circuit)
how is the modulator valve operated with diesel engines?
by linkage or a cable attached to the throttle
how is the modulator valve operated in a unit with a spark ignition engine?
operated by intake manifold pressure
how does the modulator valve vary the pressure in the modulator circuit?
modulates proportional to position of engine throttle
when throttle is wide open on the modulator valve, is the linkage exerting max or minimum force?
minimum force
modulator pressure is regulated so that highest modulator pressure occurs during ____ throttle operation
closed
lowest modulator pressure occurs at?
wide open throttle
modulator pressure is used to assist _______ pressure to shift the shift signal valves?
governor
when are transmission shift points delayed the most on a modulator circuit?
when throttle is in the wide open position
why are transmission shift points delayed the most during wide open throttle?
because there is no modulator pressure to assist governor pressure to overcome shift signal spring force
what are 4 types of shift dampening devices?
- orifice
- modulation
- trimmer
- accumulator
if a clutch is released quicker than another clutch is engaged, the transmission will become out of gear. what will be the symptoms?
engine rpm flare and momentary freewheeling
if a releasing clutch doesn’t full disengage before the applying clutch engages, what will be the symptom?
ending lugging and slowing of machine speed from internal binding
an orifice controls clutch release rates by:
controlling return line oil from the piston cavity
controlling the rate of clutch application is often called
modulation
how does a trimmer valve work?
by using a valve assembly installed in parallel with the inlet line to the transmission clutch
why would controlled clutch slippage under full throttle and heavy load conditions be undesirable?
it would promote rapid wear of clutch discs and eventually lead to clutch failure
how can you eliminate clutch modulation at full throttle?
connect the spring cavity of the clutch modulation valve to modulator boost pressure
why are electronic transmission shift controls more efficient and flexible than hydraulic shifting?
they can use a microcomputer built into the ecu to perform other functions
what functions can the ecu perform in relation to shifting?
- interface with other vehicle control and monitoring systems
- provide diagnostic info
- monitor tranny pressure + temp
- prevent operator from shifting at a bad time
- control aux functions- neutral start, backup alarm
- allow customizing tranny operating parameters
what are 3 categories electronic transmission shift control system components fall into?
input components
processing components
output components
what are the input components?
- power supply
- shift selector
- speed sensor
4.throttle position sensor - pressure and temp sensors
- machine/engine communication link
what does the transmission ecu do?
- control up and down shifting based on ground speed and throttle position
- control clutch modulation
- provide diagnostic info
- provide warning to operator when a tranny malfunction occurs
What are the output components?
solenoids
transmission warning output devices
backup alarms
what are shift solenoids responsible for?
transmission speed and direction changes, as well as the application and release of torque converter lockup clutches
what is a vehicle interface module used for?
to connect ecu to vehicle electrical system
the function of the clutch pressure regulator valve is to:
limit application pressure applied to transmission clutch pistons
the hydraulic governor in an automatic transmission operates by balancing the centrifugal force of the governor flyweights against?
governor pressure
what component of an electronic automatic shift control system performs the same function as the governor in hyd controlled systems?
a speed sensor
