Reply With QuoteSo heres where it happens, the brain...
this magical assembly of spool valves, one way valves, regulating valves, galleries and restrictions is what makes everything work....
Im not going to cover the actual assembley of the brain more what influences it and the base logic of how it makes it decisions...
Excluding electronic brains which use electronics in place of hydraulics they basically follow the same sort of setup.
There are several inputs to the brain.. 2 of these are external from the box and are driver inputs.. and there are 3 internal inputs
The 2 external inputs are the throttle position and the range selection.
The throttle sensor is there to let the gearbox know how much power is being applied from the engine and delays the upshift and increases the clamping pressure applied to the actuators for the brakes/clutches.
The range selector is there to let the gear box know what gears its allowed to play with.
the main internal inputs are Main oil pressure, Turbine speed and Vehicle Speed.
The main oil pressure is primarily used to stop anything from happening until its got enough pressure for lubrication and actuation of things... much like the low oil warning light on the engine. It also provides some of the input for the control of the converter lockup.
the turbine speed is soley used to control the converter lockup in conjuction with the throttle input and main oil pressure.
The vehicle speed input is soley for controling upshifts and downshifts.
most of what occours inside the brain is a balancing act. The pressure from one set of inputs works against the pressure from the the others and then does its thing to restore this balance when there is enough imbalance...
so in a nutshell the main blances are....
Turbine speed versus impeller speed via the throttle sensor for the lockup control.
when there is too much speed difference lockup is not allowed... when the turbine speed reaches a certain speed the pressure from its pump provides enough pressure to permit actuation of the lockup clutch. in some boxes there is a seconday input from the main pump via the throttle position sensor that will allow lockup to occour at other than full RPM's
for gear selection the range selection control simply opens and closes ports and one way valves enabeling or disabling actuators as required.
Automatic gear selection is achieved by balancing the output speed input against the throttle pressure input..
this is not quite that simple, each gear has its own "restriction size" and the taller the gear the greater the restriction, the reason why this is will become apparent...
When you mash the loud pedal the gear box gets the go right now message, maximum pressure is applied to the brakes and clutches and the lowest acceptable gear for the current speed is selected, the converter is unlocked and your outa there. This is when the gear box is in the kickdown mode.
when you back off the noise a little the balancing act resumes. As the vehicle speeds up pressure on the speed input rises and eventually overcomes the throttle pressure and the next gear up is selected. When the next gear is selected the restriction size changes and the balance is off set a little not enough to push it back to kick down but enough to let the gearbox know that your still accelerating.
When you ease off to the point of cruising both pressures balance out so nothing happens other than there is a relaxing of pressure holding the brakes and clutches.
when you back right off and let the vehicle roll the pressure from the speed input is a lot higher than the throttle pressure so the box will shift up to the highest gear permissable.... (another part of the reason why autos dont like engine braking downhill). eventually the vehicle speed will drop down enough that the input speed pressure is so low the box will downshift.
And that ladies and gentlemen is the basic 101 behind automatic gearboxes... Its not perfect in all respects but its close enough to let you have a "working knowledge" of them.
the last post will cover the pros and cons of autos.... As I see them....
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