Limitations on changing gear

[996jimbo]

I have a W202 with a normal (non-tiptronic) auto gearbox.

What would happen if I changed down, say, to first gear at 100mph? Is there a feature to stop me doing this and blowing the engine up and if so, how does it stop me?

I ask because I sometime change down for engine braking and was pondering what would happen if I made a mistake.

Regards,

Jim

 

[mlc]

Whilst not suggesting that you try this at home, I am sure that the gearbox will protect itself from the idiot driving!

My expectation is that the gearlever will allow you to select first, however it will only actually change down as each gear falls within range, that is when you are going slow enough that the next gear down will not cause the engine to over rev. In effect it is doing the exact opposite of what happens when you accelerate way on full throttle, it ensures that you dont over rev.

Why are you using gears for breaking? Assuming that it isnt because you have faulty brakes I suspect you are wasting your time, the limit of rear wheel braking is the same regardless of whether that is engine or brake generated.

Mark.

 

[996jimbo]

It's just handy sometimes and a little more relaxing to change down and gently decelerate rather than braking; my brakes can be a bit fierce. I don't change down if I'm trying to stop quickly. Also conscious of the cost of new front disks.

I know that some manufacturers have a braking type kick down - do Mercs do this (ie change down if you're braking a heavily)?

 

[mlc]

Dont know about the quick change down if your braking hard, sounds too clever by half to me.

When I was learning to drive (30 yrs ago) I was initially taught that since brakes failed so often you should always change down "just in case". Around that time the "new" thinking became brakes are cheap - gearboxes are expensive, so use the brakes. Its worth noting that as my kids have learnt to drive the current teaching is not to change down through the gears but to stay in the higher gear until you need to accelerate again, so for instance they would stop at traffic light in fourth because thats what they were using before they needed to slow down.

I think even with the high cost of brake disks you are still better off to preserve the gearbox.

Mark.

 

[996jimbo]

It's also handy to change down when you've (I've) just done a particularly knobby overtaking manouvre and need to loose a bit of speed but too embarrassed to hit the brakes straight away.

 

[mlc]

yes can see that - but then thats why God invented handbrakes

O sorry forgot, some MB's have the stupid pedal activated "parking brake", a well designed and thought through item if I ever saw one, not.

mark.

 

[pascal]

I would not advise you to use the parking brake to slow the car down, especially after overtaking a car. It is dangerous, and will cause even more undue wear, than the normal brakes.

I too find myself going too quickly after a fast overtaking manuveres on occasions, and am not be 'embarrassed' to use a little braking to get my speed down to my prefered choice. You should still be travelling faster than the car you have overtaken (or why would you want to overtake in the first place).

Not using your brakes, to save the wear on the disks/pads seems madness to me. The rotars (discs) for my car are expensive (330mm) also, but I would never even think of 'saving' them

Regarding Marks comment about us not having a proper parking brake. I would suggest that the SLK has not got a proper MB parking brake due to the restraints on the size of the footwell and cost.

I have being driving MBs (all autos) with proper foot operated parking brakes, for ten years now, and find a hand operated parking brake a nusence on other cars, plus all the central space it takes up.

I drove a CRV diesel (new) today. It had a hand operated parking brake on the side of the centre console, a big improvement. I presume that the parking brakes of the future will be electronically controled switches (as BM)

 

[mlc]

Pascal,

The handbrake comment was of course a joke - even I wouldnt sugget its use as a service brake other than in an emergency. However I cant agree with your comment about the foot operated parking brake. I have just had a C200 CDI on holiday (thank you for the free upgrade Europa). Laying aside the terrible diesel engine, the car was manual. Two things came out of that: 1st, proof that MB still cant build manual gearboxes and 2nd, how do you do a hill start with a foot operated parking brake? I would be interested to know what the official advise is from MB on that (sadly the book was in Spanish so I dont know). Further has anybody taken their driving test in one of these cars, I cant see how its done.

Mark.

 

[davidsl500]

Its foot operated to engage the brake but its released by a hand operated pull handle on the dashboard, so just get the clutch to bite and pull the handle - or buy an auto like sensible people and dont bother with it !!

 

[mlc]

I understand the operation, just cant see how it can be used safely in a manual, and whilst I agree that MB autos are better they do still make manuals and I assume sell plenty of them in the smaller cars.

Sadly whilst I enjoyed the car over the week the combination of poor engine, poor manual gearbox and a couple of stupid features - including the "handbrake" confirmed my earlier believe, a top of the range Mondeo easily beats a bottom of the range C class.

Mark.

 

[blassberg]

I understand the operation, just cant see how it can be used safely in a manual

Mark - what do you think is unsafe?

 

[angus falconer]

I have a W202 with a normal (non-tiptronic) auto gearbox.

What would happen if I changed down, say, to first gear at 100mph? Is there a feature to stop me doing this and blowing the engine up and if so, how does it stop me?

I ask because I sometime change down for engine braking and was pondering what would happen if I made a mistake.

Regards,

Jim

Hi Jim,

The other day I was over-riding the auto box in my non-speedshift C43 and managed to chirp (ie briefly lock up) the back tyres. I was doing 15mph and knocking it down into first as I approached a t-junction.

My conclusion about this generation of Merc auto boxes is that gazillions of deutschmarks were spent on R&D to get the auto upshifts a smooth as a George Galloway speech (whether fully automatic or being overridden by the driver) but nothing was spent on downshifts (especilally when being overridden). Generally the overriden shifts are somewhat slow, clunky, jerky and upredictable. When in fully automatic the engineers seemed to assume that the driver uses the (vast) brakes and the box stays in relatively high gears until the driver issues a clear indication from his right foot that one or more downchanges are needed.

This of course is different story in the speedshift boxes that I have tried in the later C-classes where you can play tunes up and down the box. I asssume they will have a degree of electronic intelligence to manage the down shifts. But I have only ever driven mate's cars so haven't wanted to test my theory in extremis.

 

[Wezzel]

I do tend to use the auto box manually when decending long steep hills. In the past I have not done this whilst driving in the Lake District and caused the brakes to overheat and fade (horrible smell of burning). I know autos have bigger brakes but there is nothing wrong in using a lower gear when decending hills.

Incidentally back in the days of riding motorcycles It was always drummed into me to "be in the right gear, at the right speed, at the right time". If you abide by this rule you are always have the option to accelerate or brake at a moments notice. If you are coasting towards a junction at 20mph in fourth gear (manual box) and suddenly need to accelerate you end up scrabbling for the correct gear and usually end up choosing the wrong one.

Just my two penneth

 

[paulcallender]

The only situation I can think you'd want to downchange manually, is if you're driving really quickly and need to set the car up in a lower gear, eg if a corner is coming up and you need the low gear/high rpm to accelerate out of it. But, the problem is that the downchange causes a change in the engine braking effect, significant enough to lose rear end grip, if you're already near the limit of adhesion (as you may be, if you're caning it round corners).

Try coming up to a slow/tight corner and knock the car down a gear manually. It will 'jerk' into the gear after a little delay, and you may have just started your turn in. This change, combined with the weight transfer to the front of the car, will probably induce oversteer, which will make your turn slower overall. Especially if you hit a tree!

So, the correct way to drive it would be to sacrifice some speed into the corner, change down earlier than the last minute, to ensure you're in the lower gear out of it. Its a subtle compromise, which may be slower if your particular car is unable to fully take advantage of the better acceleration due to the lower gear.

In summary, for corners, slow in, fast out!

In answer to the original question, yes, the auto box should delay the downchange to avoid over-revving the engine, but since this logic is controlled by hydraulics, there may be some error and you might over-rev. Try it out, let us know if you end up spraying thousands of Mercedes engine components all over the road! You'll be doing the rest of us a service!

 

[stwat]

To avoid the nasty "jerk" when you downshift, just keep your foot in the same position on the accelerator pedal so that the revs build up just before the change down. I do this all the time and it downshifts very smoothly. Its basicaly just like blipping the gas pedal when you changedown in a manual to match the engine revs with the gear ratio you want choose... ;-)

Oh, and BTW, to hill start with the footbrake/parking brake, you just find the biteing point of the clutch and then pull the hand release. Just like you would with a normal hanbrake!!! Ive driven quite a few foot operated parking brake mercs, and i just cant see why so many people have problems with it!!!!!????????????!!!!!! The only differance is that you apply the brake with your foot. To release it is just the same as any other car! Find the biteing point, and release!!! Its as simple as that.

Stu

 

[blassberg]

Oh, and BTW, to hill start with the footbrake/parking brake......................... Find the biting point, and release!!! Its as simple as that.

Yup, exactly that.

 

[Myros]

the only grief I have with the foot/parking brake

is that it makes it impossible to comply with the letter of the law about applying the parking brake during temporary stops, without upsetting the flow of the drive. You can't just flash it on and off during a pause as you would with a handbrake. So either use it,get all out of kilter and spoil the flow, or don't use it and risk roll back or bounce forward. A significant problem for we manual gearbox types.

 

[angus falconer]

is that it makes it impossible to comply with the letter of the law about applying the parking brake during temporary stops, without upsetting the flow of the drive. You can't just flash it on and off during a pause as you would with a handbrake. So either use it,get all out of kilter and spoil the flow, or don't use it and risk roll back or bounce forward. A significant problem for we manual gearbox types.

....or you don't use the parking brake after a series of high speed braking episodes and when you come to a halt you just keep your foot on the brake and warp your C43 disks due to heat build up. Ahem.

 

[996jimbo]

....or you don't use the parking brake after a series of high speed braking episodes and when you come to a halt you just keep your foot on the brake and warp your C43 disks due to heat build up. Ahem.

Oh don't bring this up. I remember when I first got an auto someone telling me to try and knock it into neutral and use the handbrake when stopped to avoid just this. Made me paranoid about warped disks and now you've reminded me again!

I agree that a foot operated parking brake is a ridiculous thing. I like to be able to set the brake with my hand and release it again with my hand. My left foot is for a clutch or just waggling around, it's not for braking. Unless I fancy myself as a rally driver (in a manual car) and try braking with it in which case sooner or later I end up with my nose flattened against the windscreen.

 

[angus falconer]

I fitted new discs late autumn year and was hacked off to find I had warped a front one by February this year in 2500 miles.

Then one day on the way to work I got "pole position" (ie front of the grid) at the lights three times in a row. As did a Seat Cupra R. It was a dual carriageway. Oh well the rest is predictable.

Anyway after the third braking manoevre I came to a halt and looking through my rear view mirror (of course) at the Seat I saw loads of smoke. I though hahaha the Seat's on fire. Then all smugness vanished when I realised that it was my rear brakes.

So, I smoked the Seat (metaphorically and iterally - their cabin filled with and there was much holding of noses, opening of windows) but also realised that repeated heavy stops combined with periods sitting still = heat build up meaning warped discs.

So now I do little forward creeps to try to avoid this happening again.

Fingers crossed