Powershift Control and Shifting Flashcards
Typical clutch application pressures range from
100 - 500 PSI
Typical Lube pressure
30-50 PSI
TC relief valve
Located in parallel to the inlet of the TC, dumps directly bank to sump
Used to limit max pressure in the TC
Typical TC charge presure
50-125 PSI
Lube relief valve
Controls the max lube pressure, usually located after cooler and before oil get to lube components
What are the 4 pumps located in the powershift oil pump group
Transmission scavenging pump
TC/TD scavenging pump
Transmission charging pump
Steering and Brake lube pump
Trans Shift Selector - Load piston
Controls the rate of pressure rise in the clutch pistons
Called shift modulation and is done to prevent shift shock, and clutch slippage
Trans Shift Selector - Pressure differential valve
Controls which clutch will apply first (F/R or 123)
TC outlet relief valve
Limits the maximum working pressure in the TC. Spool is usually identical in size on both sides, so it has a pilot drilling to allow oil to enter opposite side of spring to create pressure diff.
Valve spring cavities
Almost all control spring cavities have a drain back to tank to prevent oil from getting trapped
Trans oil cooler
Long tubes inside for coolant to travel through is called straw type cooler
located following TC
The 3 hydraulic circuits in a basic shift control system are
Clutch application circuit
Converter charging circuit
Trans lube circuit
Oil flow for trans control system operation is supplied by
Charging pump driven at engine speed
In a basic hydraulic shift control system, speed and direction changes are made by the
Shift selector valve
The function of the clutch pressure regulator valve is to
Limit the application pressure applied to the clutch pistons
The circuit that receives oil flow after the clutch application circuit has been satisfied is the
Converter charging circuit
Maximum pressure in the TC is controlled by
TC relief valve
What is the function of a trans scavenge pump (if equipped)
Remove leakage oil from the trans case
Clutch modulation refers to
Controlling the build up of pressure in the clutch piston allowing controlled slippage of the clutch discs
The factors that influence the point at which an automatic transmission shift are
Machine ground speed
Operator demand for power
Modulator circuit
Is used to alter the shift points of the transmission in relation to the operator demand for power
Modulator valve operation
When greater power demand is called for the cable moves tapered end reducing the spring pressure created in valve, this means instead of adding 50 psi (EX) to help governor it adds closer to zero meaning that vehicle speed has to be increased to raise governor pressure to make next upshift
4 types of shift dampening devices
orifice control
modulation
trimmer
accumulator
Orifice control shift dampening
Commonly used to control clutch release rates
placed in line between relay valve and sump to slow the rate at which springs push oil back
Trimmer valve shift dampening
When pressure starts to develop in clutch a piston in valve moves allowing some oil to go to sump, as pressure move through orifice in piston, piston is pushed back up and sump port is blocked again continuing to fill clutch
The function of the governor circuit in auto trans is
Provide a hydraulic signal that is proportional to machine ground speed
Governor flyweights balance against what
Governor pressure
How are shift point normally adjusted in auto trans
By varying the shift signal valve spring force
The function of the modulator circuit is what
Provide a hydraulic signal that is proportional to the position of the engine throttle, this pressure is used to delay shift points when throttle is wide open
4 functions of the ECU in an electronically controlled shift system
Control up and down shifts
Control clutch modulation
Provide diagnostic info
Provide warning to operator when malfunction occurs
What output device is responsible for causing speed and direction changes
Shift solenoids
