Automatic gearbox behaviour.

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EDC5
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Automatic gearbox behaviour.

Post by EDC5 »

After stumbling upon this quote I thought I'd ask some questions relating to the behaviour of automatic gearboxes:
milleplod wrote:
15 Jan 2017, 10:27
My C5 autobox is exactly the same in that it sometimes seems to need a second or two to work out exactly what its supposed to be doing. I can predict when its going to happen though....at a particular local junction, for example, where I'll be approaching on a closed throttle, slight downhill incline....I take a view, its clear....foot on accelerator....and wait for the box to have a good old think about things! Of course, its only a second or two, but feels like an eternity!

The interesting thing is that our previous automatic cars all did exactly the same sort of thing at the very same junction
Pete

This delay can be very annoying. It mainly happens when the throttle is closed and you are coasting such as when nearing a roundabout.

This is how it went for me when I first got my C5:

Slowing for a roundabout, spot a gap, release brake and apply some power.... nothing.... apply some more power... nothing.... push the pedal nearly all the way because that gap you spotted on the roundabout is shrinking fast and there's not enough room to come to a stop now... then all of a sudden the revs jump to 3000 rpm and it's off! makes you look like an idiot with passengers in the car :oops:

I've since learned to 'tickle' the throttle as you're coming up to a roundabout as that seems to give it a head start when you do call for some power.

I'm still curious as to the cause of the delay. To my mind the delay appears to be electronic, rather than any quirkyness to do with the torque converter.
The engine just doesn't rev for that one or two second between putting your foot down and the car responding. I'm wondering if the gearbox ECU can veto the demand for power momentarily as it declutches / reclutches or does some other operation that would be interrupted by a sudden increase in power?

I'd be interested to hear from others who have driven an Auto (new or old) about this behaviour and whether it is not present on cars without fly-by-wire throttles.

Gibbo2286
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Re: Automatic gearbox behaviour.

Post by Gibbo2286 »

In the user handbook there is this:

"During braking the gearbox may kick down automatically in order to offer an effective engine brake.
If you rapidly lift your foot off the accelerator, the gearbox will guard your safety by not changing up again."

I think this is what causes that delay, they're trying to out think driver behaviour.

I have one particularly steep hill that I descend daily and I always feel that the box holds on to the lower gear for longer than I would with a manual box and at the same time blocks from quickly accelerating away.

Nevertheless I find the car a joy to drive.

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EDC5
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Re: Automatic gearbox behaviour.

Post by EDC5 »

Gibbo2286 wrote:
28 Nov 2018, 12:53
In the user handbook there is this:

"During braking the gearbox may kick down automatically in order to offer an effective engine brake.
If you rapidly lift your foot off the accelerator, the gearbox will guard your safety by not changing up again."

I think this is what causes that delay, they're trying to out think driver behaviour.

I have one particularly steep hill that I descend daily and I always feel that the box holds on to the lower gear for longer than I would with a manual box and at the same time blocks from quickly accelerating away.

Nevertheless I find the car a joy to drive.


That's an interesting excerpt from the handbook, and seems to explain the change down behaviour when braking and approaching a hypothetical roundabout.

This is where I have trouble though, for the engine brake to be effective then the lock up clutch must be engaged; as the torque converter will not effectively couple the engine and transmission in reverse, and therefore, no engine braking will occur. So, as you are decelerating the lock up clutch is engaged and the gearbox will drop down the gears in order to help.

Why then, when you put your foot down is there no throttle response? The gearbox is already in an appropriate gear for the speed and the lock up clutch is engaged, these are perfect conditions for a fast response. Instead, it's as if the engine is prevented from accelerating until the gearbox has declutched, as evidenced by the revs shooting up when the gearbox does allow engine power. Why then does the gearbox want to prevent any torque input when it is releasing the lock up clutch and why does it want to release the lock up clutch anyway?