Thread: TDI - Buy Auto or Manual?

My 300tdi has cruise control, after market though by the looks.

Originally Posted by manic

OK I know nowt about how autos work. I thought lock up mentioned in this thread was when it engages 4th gear and the kickdown just pulls the box down one gear if you floor it.

Can some one school me on how the box moves through the gears, locks , kicks down and overdrive's?

In basic terms (and I am open to clarifications) your auto box changes up through the 4 gears at certain speeds and throttle positions. All this time the torque convertor is transferring drive from the engine to the box with an amount (don't know the figures) of loss. Eg. not all the drive provided by the engine is being applied to the box.

At about 75 Kph, the convertor locks up and what ever drive is delivered from the engine is applied to the box. I am not sure if it is 100% but someone will here will chime in with more knowhow.

I have 2 TDi autos and they both have the habit of changing into top gear and locking the torque convertor up at the same time. This has the effect of dropping the engine revs out of the power band and if you are towing (which I do alot), it can be a pain in the bum.

Originally Posted by manic

OK I know nowt about how autos work. I thought lock up mentioned in this thread was when it engages 4th gear and the kickdown just pulls the box down one gear if you floor it.

Can some one school me on how the box moves through the gears, locks , kicks down and overdrive's?

There are two main control systems in your trans that select the gear you're in. One is the governor valve on the output shaft of the box. The other is the kickdown cable connected to the accelerator. As the output shaft speeds up the governor changes the control pressure causing the various clutches to be engaged in sequence to change up the gears from 1 to 4. The kickdown cable partially reverses this trend so that the gearbox changes up gears at a higher output shaft speed. Pulling the gear lever back from "D" simply limits the number of gears available to the number shown. That means if you select "3" you will only have 1 to 3 available and if your speed is high enough to normally select "4" it will change back to "3". Normally a trans will have a speed limited lockout to prevent "1" engaging above a certain output shaft speed, preventing engine overspeed.

Lockup is a function of a clutch inside the torque converter. When not locked up the converter slips and allows gear changes without a massive bang. On your trans, lockup is enabled at 80km/h in high making the torque converter act as a solid connection like a manual trans. No slip means lower engine revs compared to unlocked operation, giving slightly better fuel consumption. Pressing the accelerator changes the control pressure to change gears back to "3" after unlocking the clutch inside the converter.

Overdrive is just the gear ratio of top (4th) gear so the engine is spinning slower than the output shaft. That's of course before drive goes through the transfer case.

Much clearer now thanks

The other item that is significant for the old 4HP22 and surprised me is that is has a unique lockup clutch for 1st gear that is engaged only when in 1st gear and 1 is selected on the shifter and on overrun. It provides good engine braking for steep descents esp in low range and is not much different from a manual, (though still minor slippage through the TC). The other gears in overrun are not much better than a neutral.