Thread: The Grey Box of EVIL.. (aka autotransmission)

This is the primer for my upcoming automatic transmissions box posting... a Disclaimer if you will... I've decided to break the posting up a little so its not a single multi page post and so that the basic ideas can be explored and understood as required.

Today after professing my distaste for autos for various reasons (I'll delve briefly into that as I explain how they work) I will cover the basic concept of how they work, paying particular attention to the torque converter. Its not my intent to go so deep as to cover repairing them but give you the generic overview of what parts do what. I also wont cover how the brain makes its decisions as thats the part that we are looking to modify to trick the box into locking the converter.

I know what I know becuase as part of my trade I have to be able to fault find them enough to tell the auto specialist what it was doing before I yanked it out and replaced it.

I choose to start the story now...

I dont like autos. (proffession done)

It all starts with a little drum of magic called a torque converter (T/C) (this is the bit of the auto I do like) If you think of this as a kind of clutch your in the right frame of mind. It goes between the acutal automatic transmission and the crankshaft, they mount on, what for simplicities sake, I shall call a lightened flywheel.

Heres what it does... It uses an impeller (driven by the engine) to make oil move and drive a turbine(hooks up to the front of the transmission). At the heart of it all is a one way clutch to which a stator is mounted. External to the impeller and turbine but still in the drum is an honest to goodness clutch, when engaged it physically links the turbine to the impeller.

A simple demo of this can be done with a pair of desktop fans place them facing each other and turn one on flat out and the other will turn.

Now by lots of laws of physics If I put spin on the input (impeller) and restrain the ouput (turbine) something really cool happens I get more torque out than I put in but at lower RPM's. As the output speeds up the torque multiplication drops off untill I hit 1:1 which is about when I want to lock up the converter. (everything must turn at the same speed) This feature is what makes autos so damn good at towing.


Hanging off of the back of the impeller inside the gearbox is an oil pump. As soon as the engine is turning its pumping. Its purpose in life is 3 fold

1. It fills the torque converter and provides flow through the converter to the oil cooler to prevent you cooking the oil.
2. It provides the hyrdaulic pressure to actuate all the parts in the box, the brain(decides when to change gears) the actuators (changes the gears) and the Torque Converter Lock up Clutch(T/CLC) if fitted
3. provides lube oil pressure to the rest of the box.

(point 2 is the main reason I dont like autos loose oil/pressure for whatever reason you get neutral and for more than one reason)

Mounted off of the turbine side of the T/C on the input shaft is a form of govenor, its purpose in life is to tell the brain when the turbine is spinning at appropriate RPMs for lockup to be effected.

Behind this we have lots of planetry gears, brake bands, bearings, clutch plates and actuaters, for the purpose of this thread it will suffice if you think of this part of the box as a manual gear box with hydraulicly actuated selector rods as opposed to lever and linkages.

Behind this you have another governer Its puropse in life is to tell the brain how fast the vehicle is going and when to change gears. behind that you have the speedo output and the output spline/flange.

Ocassionally there is a second pump fitted to the output shaft to assist in providing pressure at low vehicle and engine speeds.

Above, beside, below all of this is the brain... It reacts to several inputs to decide what to do... In order it reacts to

Oil pressure (is there any then limits how much for each circut)
turbine speed (when to lock the T/CLC)
accelerator position (how long to hold onto each gear/downshift)
Range selection (PNR1,1-2,1-2-3 for a 3 speed)
Vehicle speed (how quickly to upshift, what gear to be in)



Thats about as detailed as it needs to get for now...

Hopefully tomorrow I will get more info on the brain, which we need to trick (the bit that confuses the hell out of me most of the time) which controls the T/CLC

And I will go fully into the workings of the T/C and the primary states of it from stopped to locked (these are my generic terms )

Idling(doing nothing, like a manual when you have your foot on the clutch)
Slipping (torque multiplying, sort of like riding the clutch)
Coupling (hydraulic version of clutch fully enganged)
Locked Up (mechanical clutch engaged)


As a precursor when your driving through the brakes in an auto you are not locking the converter as such but are getting it to at least the slipping point and preferabley closer to the coupling point. Once the converter has started to work, it will work with the driving forces reversed (engine braking) and once it started to drive the engine then the pump will keep the converter filled and working.



For Bonus points.... (im sure I/someone can come up with some kind of a reward/prize for getting it right)

There is enough info in this post to be able to answer these questions..

1. Why cant I push start a normal auto (no second pump)

2. Why doesnt the T/C automatically come up to coupling point if I start a descent at idle (or why do I need to rev the engine to initate engine braking)

I'll have a go at these im only a mild mannered IT person, but have spent enough of my life poor where the only option was to fix my own cars, or walk

There is enough info in this post to be able to answer these questions..

1. Why cant I push start a normal auto (no second pump)

I once read somewhere (my old chrysler days) that you could, but you needed to get the car up to 80km/h. (urban myth?)

Ignoring the above i would take a stab in the dark and say its because the oil pump wouldnt be running... (im thinking on the fly here)... but then there should be enough oil existing in the torque convertor to make the coupling... (still thinking on the fly)... but the oil pump is required to select gears and if gears cant be selected then the oil pump which is on the front of the auto cant turn to pump oil... catch 22 situation

2. Why doesnt the T/C automatically come up to coupling point if I start a descent at idle (or why do I need to rev the engine to initate engine braking

This would be dependant on the stall speed of the torque convertor and the idle speed of the vehicle

But im guessing the reason is the diference of rotation speed on each side of the T/C is not significant enough to create the coupling required.

I dont like autos. (proffession done)

I like autos for the exact same reasons you dont, i dont know how they work so i dont have to fix them