autogearbox question

[kharva007]

does an autogearbox contain a clutch disc that can be worn out? and result in lack of pulling power? or a flywheel or sumthing?
also any1 got a diagram of a 2001 2.0 passat auto gearbox please?

i would like to see wot parts are inside a gearbox
any help guys?

[Lex Luthor]

http://en.wikipedia.org/wiki/Automatic_transmission

and links from there.....

[Col]

An auto box has 'brake bands' which I believe are a friction surface and help with the meshing of the gears.

Auto boxes are rocket science I'm afraid.

But most auto box problems are fluid level and fluid condition related.

[guyg]

I agree with BigCol – 1st stop is to check your fluid level; also, when was it last changed? Brakebands and clutches can fail, but as Col says, most problems are caused by low or contaminated fluid.

I don't know about the specifics of the Passat auto box [mine’s a manual] but most of the conventional [not Tiptronic or DSG] auto boxes work on similar principles.

Here goes with a brief outline of how auto boxes work...
[Others please comment if this isn't 100%, it's been a while since I played with any auto boxes]

There's a torque converter on the flywheel; this is essentially two discs with vanes that pass close to each other, the whole thing is filled with transmission fluid. One half is bolted to the flywheel, the other is mechanically coupled to the input shaft of the gearbox.

Power transfer to the driven plate occurs courtesy of the viscosity of the transmission fluid and the proximity and design of the vanes.

When you go to pull away, the design allows for a certain amount of slip, as the differential in speed between drive and driven plates, the combination of viscosity and turbulence caused by the design of the vanes causes the friction between the two to increase dramatically as revs rise and thus more torque is transferred to the gearbox.

At standby in idle, you can normally detect some 'creep' but as the revs rise you will feel more 'pull' as the upper differential speed is reached. This is why they’re nicknamed ‘slush boxes’. This differential speed under load also wastes power and creates a lot of heat, so most modern implementations have oil coolers and a clutch that will lock up the torque converter once the car is underway.

If you tow with an autobox, you should have an oil cooler fitted of your car doesn’t already have one, as towing puts a real load on the torque converter. It is possible to overheat the both the torque converter and oil which can lead to further problems.

Meanwhile, inside the box itself, there are sets of 'planetary' gears [one inside the other] these are arranged in ratios and are all attached to the same output. The outer gear has to be stationary for power to be transferred, so they can all be mechanically engaged at the same time as long as the outer rings are allowed to 'spin'. Ratio selection is carried out by braking [stopping] the outer planetary gear from the chosen ratio set by using hydraulically operated brake bands around the outside of the gearset.

Often, a set of planetary gears is used more than once [e.g 1st and 3rd] by having another set of gears on the input or output that changes the overall ratio [like an overdrive]. These are usually operated by a hydraulic clutch in conjunction with the brake bands to select gears. You normally won’t know anything about this as the box just works it’s way through all the ratios as needed.

In older designs, all the decision making regarding speed, clutches and brake bands [and ultimately which gear ratio] were carried out with lots of hydraulic sensors and valves based on fluid pressure. This was known as the valve body and reliable operation relied on clear, clean fluid.

Although much of the decision making is now electronic via the ECU, there are still sensors and actuators that are hydraulic in operation. Any debris of any kind in the ATF fluid can screw up the logic and cause mayhem - if the wrong combination of brakes or clutches are engaged simultaneously, disaster can follow.

If anything starts to break up the debris can cause further problems.

I once had a box that would loose all power every so often and then mysteriously ‘fix’ itself if allowed to rest for a while. It turned out there was some sludge in the fluid and every so often it would block the pump inlet, pressure would drop and the drive would disappear. A thorough flushing of the box and valve body cured the problem.

Hope this helps - if there are any auto box gurus out there please feel free to correct!

Cheers

Guy

[kharva007]

thank u so much guys especialy the last post, can belive u typed all that for me cheers mate!!!

i owe u a pint!!!

[guyg]

Auto boxes are rocket science I'm afraid..

Rocket science or Black Art - I can never decide which!

kharva007 - It's a pleasure - I've had some great tips and help from this site so I think it's up to all of us to put a little back from time to time. I just hope it's correct!

Cheers

Guy

[kharva007]

just 1 more question?

was sitting in ma car today with ma engine running and car stable for bout 20mins, and i noticed that my average miles to gallon was droping down rapidly on my trip cpu any help.

9.9mpg,
9.8
9.7
9.6
9.5
9.4
9.3
9.2
9.1
9.0
8.9
8.8
8.7
8.6
8.5
8.4
8.3
8.2
8.1
8.0
7.9
and so on untill 4.3mpg then i got ****** and went home

wots going on???

[Col]

Quite normal for it to drop as it is the average is being calculated all the time the engine is running.

[simon4motion]

I would like to know if anyone has fitted an additional oil cooler to a tiptronic gearbox. I contacted Kenlow and they offered a cooler for a VW box, but not if it was tiptronic. I will be using the car for towing and do not wish to cook the box. The car is a 2001 passat 2.8 4motion tiptronic 5spd box. It has the usual pipe running through the end of the radiator as standard auto box cooling but this will not be good enough in hot places when manouvering. Whatever cooler I do get I intend to fit an electric fan to it for use in slow moving traffic or moving around slowly in a field.

[godfried]

Look at this and follow any further links. http://www.taligentx.com/passat/maintenance/atfchange/