Coasting downhill
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2011 C300 Sport, P1, MM, Wood, Heated FS, DIY rearview camera.
Coasting downhill
I coasted downhill a couple of times last week. It was in D, just let car coast with no gas applied. The car seems to not going faster and faster after it reaches a certain speed (well I should say it goes faster but at slow increase). I didn't down shift at all. Engine break at D7 should be very minimal. Is it a designed safety feature, or do I need to let dealer check it out? My other two cars will go much faster in the same situation for sure, although I haven't try them on the same down slope.
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Nissan GT-R BE / '12 Ducati-1199 Panigale S / '12 C300-4M Loaded/GLK350-4M Loaded
Engine braking while coasting downhill without throttle is programmed into the system and is completely normal. (It will downshift all by itself.)
Last edited by MBRedux; Jan 1, 2012 at 09:38 PM.
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'09 C350, P2, MM, Pano
I don't know if this is really related but it made me think of it. When I'm in a parking lot or something and have the car in D it doesn't really move along unless I hit the accelerator. In other cars I've owned, the car move at least move forward a bit but with the C it is pretty much at a dead stop unless I hit the gas. Anyone know why this is?
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I don't know if this is really related but it made me think of it. When I'm in a parking lot or something and have the car in D it doesn't really move along unless I hit the accelerator. In other cars I've owned, the car move at least move forward a bit but with the C it is pretty much at a dead stop unless I hit the gas. Anyone know why this is?
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From: Llandudno, Cape Town, South Africa
late 2009 CLK 350 Coupe Elegance, '65 Jaguar S Type wires
Just understand one thing. The Torque Converter is partially locked up on a 722.9 on all occasions other than stationary.
When coasting in Drive the indicator will show D but the car could be a few gears lower dependent on speed. You will only see the actual gear ratio you are in if you manually change down.
So recap - In Drive or "D" the display does not indicate which gear you are in.
There is nothing amiss with your car.
When coasting in Drive the indicator will show D but the car could be a few gears lower dependent on speed. You will only see the actual gear ratio you are in if you manually change down.
So recap - In Drive or "D" the display does not indicate which gear you are in.
There is nothing amiss with your car.
Last edited by Glyn M Ruck; Jan 1, 2012 at 07:35 PM.
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From: Bay Area, CA
2011 C300 Sport, P1, MM, Wood, Heated FS, DIY rearview camera.
When coasting in Drive the indicator will show D but the car could be a few gears lower dependent on speed. You will only see the actual gear ratio you are in if you manually change down.
So recap - In Drive or "D" the display does not indicate which gear you are in.
There is nothing amiss with your car.
So recap - In Drive or "D" the display does not indicate which gear you are in.
There is nothing amiss with your car.
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2004 Mazda6, 1993 RX7
Originally Posted by C300CA
I coasted downhill a couple of times last week. It was in D, just let car coast with no gas applied. The car seems to not going faster and faster after it reaches a certain speed (well I should say it goes faster but at slow increase). I didn't down shift at all. Engine break at D7 should be very minimal..
Originally Posted by Glyn
When coasting in Drive the indicator will show D but the car could be a few gears lower dependent on speed. You will only see the actual gear ratio you are in if you manually change down...
.
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Also, great post on TC function. But I thought I had read in the manual that the trans would downshift and lock the TC if oil temp got too high, contrary to the rules posted, but logical if you want to reduce oil heating due to shear/slip in the TC ??
.
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From: Llandudno, Cape Town, South Africa
late 2009 CLK 350 Coupe Elegance, '65 Jaguar S Type wires
So, the car selects the appropriate lower gear to down shift to? That is very smart, smarter than I am. I tried to find something about this in the operation manual but there is nothing. Instead, manual says the opporsit: even if the transmissionn is set at lower gear, when RPM goes too high, car will upshift to higher gears.
I climb out of the hole I live in every day & then coast down the other side of the mountain at 60Km/h limit. By the Kevin method the car has selected 4th or 5th gear for that slope - sometimes 5th because I do not obey the speed limit
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From: Llandudno, Cape Town, South Africa
late 2009 CLK 350 Coupe Elegance, '65 Jaguar S Type wires
I thought for sure that when moderatly accelerating in a gear, or pushing a manually selected gear, the rpms climbed proportional to the speed, like a manual, and like the atx TC was locked?
Also, great post on TC function. But I thought I had read in the manual that the trans would downshift and lock the TC if oil temp got too high, contrary to the rules posted, but logical if you want to reduce oil heating due to shear/slip in the TC ??
.
Also, great post on TC function. But I thought I had read in the manual that the trans would downshift and lock the TC if oil temp got too high, contrary to the rules posted, but logical if you want to reduce oil heating due to shear/slip in the TC ??
.
When oil temp rises they unlock the converter & change down for greater oil flow thus dissipating heat via the transmission oil cooler.
Last edited by Glyn M Ruck; Jan 2, 2012 at 05:19 AM.
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From: Llandudno, Cape Town, South Africa
late 2009 CLK 350 Coupe Elegance, '65 Jaguar S Type wires
So, the car selects the appropriate lower gear to down shift to? That is very smart, smarter than I am. I tried to find something about this in the operation manual but there is nothing. Instead, manual says the opporsit: even if the transmissionn is set at lower gear, when RPM goes too high, car will upshift to higher gears.
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Another observation: I noticed a few times that if I accelerate in a hurry and RPM goes high to around 5 ~ 6000, it takes noticeably longer to come down after I let go of the gas. It stays at that high RPM for like 1 to 2 seconds. Is that typical? At lower RPM, it comes down instantly after I take off my foot.
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Another observation: I noticed a few times that if I accelerate in a hurry and RPM goes high to around 5 ~ 6000, it takes noticeably longer to come down after I let go of the gas. It stays at that high RPM for like 1 to 2 seconds. Is that typical? At lower RPM, it comes down instantly after I take off my foot.
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This too is by design. It was explained to me that this behavior engages only during very aggressive driving, because it helps keep you in the power band when you release the throttle, when you hit a curve for example. This way as soon as you jam on the gas pedal again there is instant power without shifting. I have no idea if this really was the intention, but it does work great for that.
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Not yet. That hill is far away, nearby a not so close friend's house where I will probably not go again shortly. I will find a similar hill, stretched enough for me to try your method.
Last edited by C300CA; Jan 3, 2012 at 11:48 AM. Reason: spelling
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Kevin. The converter is never hard locked for NVH reasons. When hard locked it transmits vibration. So it's maintained in controlled slip for efficiency.
When oil temp rises they unlock the converter & change down for greater oil flow thus dissipating heat via the transmission oil cooler.
When oil temp rises they unlock the converter & change down for greater oil flow thus dissipating heat via the transmission oil cooler.
I agree just a down shift will help with increased oil flow rate. It's the TC unlock that bugs me.
The reason I thought they would downshift and Lock-Up the TC is based on an experience I had with another car with 5 spd auto with TC lock in 4th and top 5th, and the related calc's.
I routinely went up a 1/2 mile hill with constant grade. I noticed I could go up at 40 mph and 2000 rpm, in 4th with TC locked, or in 5th unlocked, with more load (lower manifold vac, more pedal depression).... How much hp was that TC expelling in heat, when unlocked in 5th and slipping?
---> ratio of 4th over 5th gear = .935/.685 = 1.365
HP needed to climb hill, based on 4th with TC locked:
- T4 -> Torque @ output of TC in 4th gear. 5252 = constant
HP_climb = T4 x 2000rpm/5252
HP_climb = .38 x T4
-----------------------------------------
- T5 -> Torque @ output of slipping TC in 5th gear
T5 = 1.365 x T4 ( ie: more torque needed in 5th to climb hill)
RPM change across slipping TC in 5th:
RPM_slip = ( [1.365 x 2000] - 2000 )
RPM_slip = 730 rpm
HP from TC slip, creating heat in oil:
HP_slip = ( 1.365 x T4 ) x ( 730rpm) / 5252
HP_slip .19 x T4
Note that the hp wasted in heating the TC oil in 5th is 50% of the hp needed to climb the hill !
This example is exaggerated due the big rpm drop from 4th to 5th, and the amount of slip apparently allowed in the non MB car. I don't have a similar example for the C300, since the TC always seems to be locked-up (trivial slipping per MB).
Any way to double check the service manual to verify the TC is unlocked when oil overheats?
.
Last edited by kevink2; Jan 22, 2012 at 05:02 PM.
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From: Llandudno, Cape Town, South Africa
late 2009 CLK 350 Coupe Elegance, '65 Jaguar S Type wires
I understand your logic. However, as I understand it, with the Benz TC setup the lock up clutch in slip mode is a large heat contributor & when locked there is no/very little oil flow through the convertor. So they unlock by choice to increase oil flow rather than stopping the heat generating slip but reducing oil flow. A choice dependent on this design. I have not seen published heat generation (energy dissipation) of the TC itself vs %age clutch slip heat generation. I have done some training in Asia for Benz to show that a senior oilco marketing man is prepared to role up his sleeves rather than delegate & while my main subject has always been lubrication I've handled a few Benz training modules as well which is why I have their training material which is in the public domain anyway. I've accepted they know what they are doing with their design. Maybe we need to ask a Benz engineering person these questions & reasons for their choices/design. The issue as explained to me revolves entirely around the fact that locking the TC clutch drastically reduces oil flow to the converter & thus reduces cooling.
Here is the telling comment -
• Oil supply pressure to converter is varied depending on the amount of slippage (desired)
- open = high oil flow
- slipping = reduced oil flow
- (approaching full lock - no oil flow)
Here is the telling comment -
• Oil supply pressure to converter is varied depending on the amount of slippage (desired)
- open = high oil flow
- slipping = reduced oil flow
- (approaching full lock - no oil flow)
Last edited by Glyn M Ruck; Jan 22, 2012 at 05:27 PM.
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I understand your logic. However, as I understand it, with the Benz TC setup the lock up clutch in slip mode is a large heat contributor & when locked there is no/very little oil flow through the convertor. So they unlock by choice to increase oil flow rather than stopping the heat generating slip but reducing oil flow. A choice dependent on this design. I have not seen published heat generation (energy dissipation) of the TC itself vs %age clutch slip heat generation. I have done some training in Asia for Benz to show that a senior oilco marketing man is prepared to role up his sleeves rather than delegate & while my main subject has always been lubrication I've handled a few Benz training modules as well which is why I have their training material which is in the public domain anyway. I've accepted they know what they are doing with their design. Maybe we need to ask a Benz engineering person these questions & reasons for their choices/design. The issue as explained to me revolves entirely around the fact that locking the TC clutch drastically reduces oil flow to the converter & thus reduces cooling.
Here is the telling comment -
• Oil supply pressure to converter is varied depending on the amount of slippage (desired)
- open = high oil flow
- slipping = reduced oil flow
- (approaching full lock - no oil flow)
Here is the telling comment -
• Oil supply pressure to converter is varied depending on the amount of slippage (desired)
- open = high oil flow
- slipping = reduced oil flow
- (approaching full lock - no oil flow)
"Cup of coffee" that something was lost in the translation
.
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From: Llandudno, Cape Town, South Africa
late 2009 CLK 350 Coupe Elegance, '65 Jaguar S Type wires
Yes Kevin. I have that full training module on the 7G in pdf. form. When I briefly looked at the flow diagram it was not 100% plain to me. I will look again with a more critical eye. Let me try & work out what is actually happening in the valve body. I would send you the original but our crummy ISP's in SA clamp us at 5 MEG attachment size & it is 17.4 MEG. - anyway you have access to it.
It would be hilarious if the Benz training program & English WIS of the day were wrong!
It would be hilarious if the Benz training program & English WIS of the day were wrong!
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Thanks for the help. Fig 50 shows a separate oil feed, through duty cycle controlled device, to the Lock-up load device. There is a separate oil input to the TC, and another exit to the oil cooler.
I noticed the lock-up has multiple clutch plates, like high load capacity multi disc clutches for cars.
Note that the only time the TC is unlocked for minor slip is at unusually low rpms in lower gears, per a chart.
It could be that what the high temp condition is all about, is if the applied torque is greater than the lock-up capacity (like an added turbo), slip would be excessive and generate more heat than the open TC. I've found that Manuals (and Finite Element Program results) are usually right .
.
I noticed the lock-up has multiple clutch plates, like high load capacity multi disc clutches for cars.
Note that the only time the TC is unlocked for minor slip is at unusually low rpms in lower gears, per a chart.
It could be that what the high temp condition is all about, is if the applied torque is greater than the lock-up capacity (like an added turbo), slip would be excessive and generate more heat than the open TC. I've found that Manuals (and Finite Element Program results) are usually right .
.
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From: Llandudno, Cape Town, South Africa
late 2009 CLK 350 Coupe Elegance, '65 Jaguar S Type wires
The TC is never fully locked due to vibration transfer. On the 722.6 there were certain conditions where the TC was fully locked & you could feel it.
Presume you have seen this regarding 722.9.
Here are the 722.6 lock conditions.
Presume you have seen this regarding 722.9.
Here are the 722.6 lock conditions.
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What did you think about my theory, that the overheat-sequence is based on an engine with boosted power?
.
Last edited by kevink2; Jan 24, 2012 at 07:17 PM.
