Can someone explain auto gearbox slip

[dieselderv]

Hi all I have a 2004 v70 d5 auto,if I put my foot down it kicks down fine and moves but not as much as it does when it gets to 3500rpm it seems to get to 3500 rpm then the revs drop back down to under 2800rpm and it takes off like a rocket it's as if it's changing up another half a gear if you no what I mean..it's kind of like putting your foot down in a manual car with a slipping clutch then all of a sudden the clutch grabs and off it goes..does that sound like auto box slippage..does an auto have a friction plate that can be changed..or could it be the turbo vains sticking ..please help the mot is due this month and if it doesn't fail on much I'd like to keep it ,,it's on 193,000 miles just had cambelt tensioners aux belt and gear oil change and it's really tidy...if it's gearbox slippage then I'm afraid I'm gonna have to look for something eles many thanks

[Georgeandkira]

In an automatic transmission pressurized fluid, routed through electronically controlled valves, has to push against pistons in cylinders to actuate change. These pistons press against stacks of sheet metal rings which have fiber bonded to them. This pressure effectively makes them act as a solid, single piece. Gears are at the end of the line and are engaged. Simultaneously, other valves are opened to allow fluid in other cylinders to drain thus allowing other gears to disengage.

Slip can happen when the material on the rings (aka clutch pack) has worn off or when the pistons in the cylinders mentioned above (aka servos) stick, leak or otherwise fail to do their job.

That's slip in a nut shell.

Turbochargers' vanes sticking (either rotationally or in pitch control in units so equipped) can contribute to "turbo lag". I must confess to not knowing enough about turbo construction/operation to speak of the likelihood of these things happening.

If the oil has been maintained properly (correct type + changed regularly), turbos lasting well beyond your mileage are commonplace.
If something mechanical prevents variable vanes from actuating, the resulting boost pressure won't be correct.
Leaking air/vacuum lines can also effect a turbo's efficiency.

I have a great illustrated pamphlet describing your turbocharger but cannot attach it to this post.

[dieselderv]

What an amazing reply thankyou very much...still not sure if it answers my question though ..thankyou I now know how an auto box works it sounds incredible really how it works.who invented the auto box and how on earth did they come up with that idea lol...that sounds like I'm not happy with your reply but I really am thankyou

[petey]

There are lots of friction plates and also bands that wrap around drums within an auto box, and any or all of these can slip in the correct circumstance, usually bad oil leading to reduced pressure on the friction materials.

But there is another friction surface within the transmission that is often overlooked though and that is within the torque convertor (TC). Imagine a bath with two fans really close together. When one fan (connected to the engine) turns, the second fan (connected to the input shaft) gets a load of the entrained fluid and starts to spin as well and that's how a fluid flywheel works. The torque convertoring happens when you have a 'stator' that sits between the two fans which acts to dramatically increase the torque multiplication when the transmission is 'stalled'. So when you're stationary, you get loads and loads of torque, and as the transmission speed gets closer to the engine speed, the torque multiplication drops off.

But!
With the system described above, there is also some slip. 2% may not sound like a lot, but if 2% of your fuel dripped on the floor every time you filled up, you'd be annoyed by that.
So there is also a friction plate between the two fans so that when the ECU decides, it can force, using hydraulic pressure, plates connected to the two fans such that they lock together, effectively removing the opportunity for a speed difference.

In practice, this locking up of the TC will occur when the transmission believes that the input and output speeds of the TC are similar and a gear change isn't needed anytime soon. As soon as a gear change is required, it's important to release this clutch so that the shock of gear change can be absorbed into that 2% slip. Drive a car with a locked TC clutch and you'll see what hard changes are all about....

Dirty or low oil level will produce this effect. If allowed to perpetuate, it will absolutely get worse and damage more things.

(The function of the torque convertor is why you cannot push start an automatic:- it is frequently believed that you can never push the car fast enough to get the fans spinning together, but actually, the TC also has a permanent drive to an oil pump just inside the nose of the transmission. If the engine isn't turning, the TC isn't turning and therefore the oil pump isn't turning. You therefore have no method of producing the oil pressure required to operate the clutches and bands required to 'be in gear'. Therefore you're never going to be able to turn any components within the transmission and therefore quite unable to turn either of the fans in the TC.
Be aware that this oil pump is a multi stage affair, and when it's not turning, you're also not getting any lubrication to the transmission components, and that's why you must not tow an automatic transmission vehicle; the transmission depends on pump lubrication not splash lubrication.)

*And yes, you can push start an early BMC Mini auto with the early AP bevel transmission, but only because it includes a small oil pump specifically for that purpose. It was ditched very quickly as a cost and weight saving and explained away as a fuel efficiency.

** And yes, I was bored at lunchtime....

[Clan]

Originally Posted by petey

There are lots of friction plates and also bands that wrap around drums within an auto box, and any or all of these can slip in the correct circumstance, usually bad oil leading to reduced pressure on the friction materials.

But there is another friction surface within the transmission that is often overlooked though and that is within the torque convertor (TC). Imagine a bath with two fans really close together. When one fan (connected to the engine) turns, the second fan (connected to the input shaft) gets a load of the entrained fluid and starts to spin as well and that's how a fluid flywheel works. The torque convertoring happens when you have a 'stator' that sits between the two fans which acts to dramatically increase the torque multiplication when the transmission is 'stalled'. So when you're stationary, you get loads and loads of torque, and as the transmission speed gets closer to the engine speed, the torque multiplication drops off.

But!
With the system described above, there is also some slip. 2% may not sound like a lot, but if 2% of your fuel dripped on the floor every time you filled up, you'd be annoyed by that.
So there is also a friction plate between the two fans so that when the ECU decides, it can force, using hydraulic pressure, plates connected to the two fans such that they lock together, effectively removing the opportunity for a speed difference.

In practice, this locking up of the TC will occur when the transmission believes that the input and output speeds of the TC are similar and a gear change isn't needed anytime soon. As soon as a gear change is required, it's important to release this clutch so that the shock of gear change can be absorbed into that 2% slip. Drive a car with a locked TC clutch and you'll see what hard changes are all about....

Dirty or low oil level will produce this effect. If allowed to perpetuate, it will absolutely get worse and damage more things.

(The function of the torque convertor is why you cannot push start an automatic:- it is frequently believed that you can never push the car fast enough to get the fans spinning together, but actually, the TC also has a permanent drive to an oil pump just inside the nose of the transmission. If the engine isn't turning, the TC isn't turning and therefore the oil pump isn't turning. You therefore have no method of producing the oil pressure required to operate the clutches and bands required to 'be in gear'. Therefore you're never going to be able to turn any components within the transmission and therefore quite unable to turn either of the fans in the TC.
Be aware that this oil pump is a multi stage affair, and when it's not turning, you're also not getting any lubrication to the transmission components, and that's why you must not tow an automatic transmission vehicle; the transmission depends on pump lubrication not splash lubrication.)

*And yes, you can push start an early BMC Mini auto with the early AP bevel transmission, but only because it includes a small oil pump specifically for that purpose. It was ditched very quickly as a cost and weight saving and explained away as a fuel efficiency.

** And yes, I was bored at lunchtime....

The BW 35 from the 1960's and 70's had a pump driven from the output shaft too , so two pumps which made towing for long distances at reasonable speed possible :-)

[petey]

How do two pumps improve towing?
Towing issues usually (read always) come from overheated oil; the extra work required to get the more inert mass rolling results in a lot of losses at the TC and that's realised as excess heat. When towing, it's important to fit an oil cooler to remove this heat, but I'd not heard of an additional pump. If there's not enough pressure to hold the bands on, then you won't do well no matter what the driving conditions....

Still, every day's a school day.

[Dippydog]

I was always told that you can't tow a car with an auto box without disconnecting prop shaft/drive shafts or lifting the driven wheels off the floor,however I found out a few years ago that my Omega auto can be towed either 40mls at up to 50mph or 50mls at up to 40mph[I can't recall which]this also to applies to other makes models of cars but it would pay to make sure on any given make/model before trying this and better still not to tow one in the first place.

[Clan]

Originally Posted by petey

How do two pumps improve towing?
Towing issues usually (read always) come from overheated oil; the extra work required to get the more inert mass rolling results in a lot of losses at the TC and that's realised as excess heat. When towing, it's important to fit an oil cooler to remove this heat, but I'd not heard of an additional pump. If there's not enough pressure to hold the bands on, then you won't do well no matter what the driving conditions....

Still, every day's a school day.

well the second rear pump will be driven when the car is BEING towed behind a breakdown lorry so will continue to lubricate the transmission under oil pressure , even when the engine is not running . However these were not fitted once the AW 4 speed auto boxes came out in the early 1970's . All volvo auto transmissions have a heat exchanger to get rid of excessive heat when you are towing a caravan … they need to run at over 120C to evaporate moisture from within the box .

[Clan]

Originally Posted by Dippydog

I was always told that you can't tow a car with an auto box without disconnecting prop shaft/drive shafts or lifting the driven wheels off the floor,however I found out a few years ago that my Omega auto can be towed either 40mls at up to 50mph or 50mls at up to 40mph[I can't recall which]this also to applies to other makes models of cars but it would pay to make sure on any given make/model before trying this and better still not to tow one in the first place.

yes it applies to Volvos too , same conditions , it is common sense really there will be oil splashing around all over the place inside the box when towing the car ..

[petey]

Originally Posted by Dippydog

I was always told that you can't tow a car with an auto box without disconnecting prop shaft/drive shafts or lifting the driven wheels off the floor,however I found out a few years ago that my Omega auto can be towed either 40mls at up to 50mph or 50mls at up to 40mph[I can't recall which]this also to applies to other makes models of cars but it would pay to make sure on any given make/model before trying this and better still not to tow one in the first place.

The same applies to truck transmissions, indeed any vehicle where the transmission is lubricated by pressure fed oil rather than splash lubrication. Without the oil pump running, there is no formal lubrication, but in the case of the GM box, presumably they believe that there is a sufficient amount of residual oil left in critical corners to maintain lubrication when being towed. Personally, I would NEVER chance it, absolutely not at speed. Tow it enough to get it off the side of the road and leave it at that.



Re Clan, not only was I bored at lunchtime, I was clearly tired as well. You were referring to the car being towed, rather than it towing something else. My bad...