C3 Sensodrive

[ChrisGrey]

We have a C3 (04 plate) which has something called SensoDrive. As I understand it this allows you to press a button to drive in full automatic mode or to use paddles to change gear manually. However when the car is brought to a halt at a junction on an incline (even a slight incline) we have to do what would be a hill start routine (as per a manual gearbox). Stop the car, put on the handbrake, press the accellerator and then release the handbrake. If we don't do this the car rolls backwards.

 

We took the car into the dealers (still under warranty) and they said that this is to be expected with SensoDrive because it isn't really an automatic. If we had purchased an automatic it wouldn't happen.

 

I am very confused by their statement because the button we press SAYS automatic on it and the indicator on the dashboard SEEMS to say that we are in a forward gear. But still it rolls backwards.

 

Anyone have any experience of this (basically I'm looking for validation of either my gut feeling or the garages statement).

 

Thanks in advance.

 

Chris Grey

[wozza]

Yeah like most semi auto boxes you have to give it a bit of gas to stop it rolling backwards it wont be like a full automatic and constantly want to creep forward you have to drive it like a manual but it'll change gear for you and have no clutch pedal thats it.

 

Kinda half way between the 2 systems better in some respects worse in others.

[lupin]

Chris,

Your dealer is quite right. The Sensodrive gearbox is automatic (insofar as it changes gear automatically when required) but works in a different way from the great majority of automatic gearboxes out there. The Sensodrive system is really just a standard manual gearbox with electrically activated clutch and gearshift mechanisms. The car's computer determines when it's time to change gear, cuts the engine momentarily, de-clutches, shifts gear , re-engages the clutch, and restores power - just like a driver would do in a manual car. A "conventional" automatic transmission (as found on most automatic cars) works in a completely different way. There is no clutch mechanism as such - instead, the viscosity of the automatic transmission fluid is used to create drag between two very closely opposed vaned discs (a system known as a torque convertor). The faster the primary disc which is connected to the engine drive-shaft spins - the greater the drag exerted on the secondary disc which drives the gearbox. So, as you increase the engine speed on a stationary car the gearbox begins to turn more strongly until the car eventually starts to move off. All the cogs in the gearbox are permenantly engaged with each other, and gearshifts are effected at the appropriate speeds by restricting the rotation of specific gearshafts, thereby transferring power to others - too complicated to explain here!

The upshot is, that with the standard automatic gearbox the torque convertor exerts a constant slight turning force on the wheels, even when your foot is off the accelerator, resulting in a tendency for the car to creep gently forward - a usefully characteristic for driving in nose-to-tail traffic jams where the car will keep slowly moving forward unless the brake is applied. This same effect also prevents the car from rolling backwards on hills. Clearly no such effect occurs with the "Sensodrive" system, however, because it uses a standard (albeit electrically operated) clutch mechanism rather than a torque-convertor - so no creeping forward in traffic, and no holding its position on hills.

I have no idea why Citroen chose to design this system, when so many excellent conventional automatic gearboxes, suitable for small cars,already exist. Several car manufacturers employed this sort of system back in the 1950s when automatic gearboxes were much more primitive, but all of them were dropped as better systems became available. I just can't see Citroen's reasoning here. I would guess that possibly it may be more fuel efficient than a regular automatic transmission. Maybe they had hoped to make something much better than they finally were able to achieve! Certainly, I have yet to read a good word about the Sensodrive system, and in my opinion its performance has some real safety issues. The feature that most drivers find alarming is the hesitation in engaging the clutch which sometimes occurs when you slow down almost to a stop and then try to accelerate away quickly again - e.g. on approaching a roundabout. It's a weird system that takes a little getting used to. The semi-automatic option, which can be engaged by the button next to the gearlever, allows you to make the decision when to change gear rather than leaving it up to the computer, but the gearchange is still performed electrically as with full auto mode. The gearlever and steering-mounted paddle-shifts are really nothing more than electrical switches.

 

The manual advises against trying to hold the car's position on a hill using the accelerator since, as with a manual gearbox, this will soon wear out the clutch.

[kfk]

I am very confused by their statement because the button we press SAYS automatic on it and the indicator on the dashboard SEEMS to say that we are in a forward gear. But still it rolls backwards.

 

This is the same as when driving a manual car............The gear lever is pushed forward and indicates you are in first gear.....but because you have your foot on the clutch the car can roll back.

 

The senso drive in basic terms has an electronic clutch pedal. The gear indicator merely points out that when you accelerate you will move in the gear indicated....the point here is you have to depress the accelerator to indicate you wish to move off......this will cause the clutch to come to biting point amd then you move in your desired direction of travel.

 

Its an aquired taste is a sensodrive.