300TDi Torque Converters & Freewheeling

Hello,

300TDi Discovery, auto - two scenarios...

Scenario 1. Low range, first gear, down hill. Feet *OFF* both pedals. Lots of engine braking. Engine revs correspond to wheel revs.

Scenario 2. High range, travelling down road in 3rd or 4th gear, perhaps, maybe going anywhere from 30 to 70mph. Feet *OFF* both pedals. No engine braking (or very little). Engine revs on tickover and not corresponding to wheel revs at all.

I think the above behaviour is normal (i.e. engine braking at low speed, freewheeling at high speed), but how the hell does it work?

At first, I figured the torque converter didn't work in reverse (i.e. the wheels were incapable of driving the engine), but then engine braking down a steep hill wouldn't work.

I then figured the torque converter *MUST* work in reverse (i.e. the wheel *are* capable of driving the engine), but then when I take my foot off the accelerator at 60mph, the engine revs wouldn't drop to tickover. They'd stay at 2,000 and slowly drop at the same rate as the vehicle slows down.

So, the only conclusions I can come to is that either the torque converter only works in reverse *sometimes* (i.e. at low speed or only in 1st/2nd gear)

OR

The torque converter works in reverse permanently, but something else disengages the engine from the wheels when travelling fast with the accelerator pedal *NOT* pressed (or in 3rd/4th gear).

So... any ideas?

(None of this is a problem by the way, just really interested to know how it all works!)

I think (and I'll stand corrected if wrong!) that once you've got the torque converter "locked in" ie over 50 mph, then when you take your feet off, it will give you best engine braking, if on the other hand you've not got lock-up on the torque converter, then you'll only get a % age of engine braking, as the torque converter isn't locked up either way.

so when travelling above 50 at constant speed (with foot at constant place on accelerator), if I push the accelerator down further the revs increase before the speed increases.

This suggests that my torque converter is not locked. Do they actually lock at all on these?

If it is locked, then when I take my feet off the pedals, the car should slow down nicely and the revs should stay high instead of immediately dropping to tickover.

If the torque converter is *not* locked, then obviously the engine braking will be a lot less, but the revs should still stay high instead of immediately dropping to tickover, like they do.

When i roll down a steep hill, getting faster as I go, in low range 1st gear, the revs increase as I accelerate and slow down as i come to stop at the bottom. This makes sense because engine braking is occuring.

When i coast along a motorway (with the gear lever in "D"), however (which is the same as "coasting" down a hill in 1st gear), the revs are at tickover. Not at 2,000 where they would be if I were attempting to maintain constant speed.

Does that make sense?

I HAVE THE ANSWER!!!! ...I think!

Right... i've been experimenting on my journey home and think I have the answer!

Put simply, above 50 mph, the torque converter locks and remains locked until you go below 50 mph.

While it is locked, the engine revs correspond directly to the speed of the wheels regardless of whether you are accelerating, maintaining speed or "coasting".

When "coasting" or "freewheeling" above 50, the torque converter is locked and engine braking is at it's greatest (i.e. you slow down relatively quickly).

As your speed then drops below 50 mph, the torque converter unlocks and the engine revs drop immediately from 2,000 or so down to 1,400. Tickover is around 700 so this is double.

This shows that engine braking is still occuring (otherwise the revs would be at tickover), but because the torque converter is no longer locked the engine braking is negligable (almost non existant) because you are in top gear (4th) at relatively low revs with one hell of a lot of momentum! As a result, it feels like you are freewheeling when compared to doing the same while above 50 mph.

So...

Engine braking exists at all speeds and in all gears. Above 50 mph the torque converter is locked and as a result, engine braking is relatively noticable.

Below 50 mph, the torque converter is unlocked and so engine braking is not as noticable (unless you forcefully change down a gear or two).

Engine braking while in 1st gear down a steep hill is no different. The torque converter is not locked (because you are below 50 mph), but because you are (hopefully) in low range, your gearbox is effectively in the 0.3rd high range gear (roughly). The result of this is that your engine has to spin relatively fast to achieve a relatively low speed at the wheels and as a result, engine braking is quite high.

It would be much higher if the torque converter were locked, I imagine, but then your engine would stall as soon as you stopped!

Does all that both make sense and sound correct?

If so, that explains the rumour I heard, that automatics don't have as much engine braking as a manual!

you need to stop smoking that ****,it's not good for you brother.

its very simple

you as the driver at low speed have told the gear box not to go above 1st gear and as the car is now locked in first gear and low range u slow's ya down just like ya told it to do

In high range in drive you is letting the auto box deside 4 ya so it does what it thinks is best and yes above 50 the torque convertor does lock up for best fuel economey (ho hum)

so to answer the op yes it does both but only what u tell it

to prove this next time u is off road go down that same hill in low range but select drive I bet ya it runs away with ya and your arse goes boo big time lololol

The torque converter doesn't always lockup at 54mph, if you are accelerating beyond that it will only then lockup when your speed has leveled out what ever speed that may be.
To improve your de-acceleration you can always move down to 3rd.