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. 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.

MBRedux

Engine braking while coasting downhill without throttle is programmed into the system and is completely normal. (It will downshift all by itself.)

Darkstar19

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?

C300CA

Yes, my C is exactly like yours.
Yes, I know engine break with every car. However, my downhill experience is like my C was in gear lower than D7; it was more like in D5 or D4.

acr2001

It's by design. Why exactly designed that way? I'm not sure. In C (or E in the facelifted model) the car always starts out in 2nd gear so there is less torque at idle to get moving. It behaves more like a manual in the respect, and I find it to be very beneficial to use E mode in traffic for that very reason - you're not fighting the transmission with the brakes.

Glyn M Ruck

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.

Glyn M Ruck

C300CA

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.

kevink2

Next time you are coasting down same downhill, in D, manually downshift to D6 and see if rpms change. If not, you are in 6th gear. If rpms climb, you were in 7th gear.

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kevink2

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 ??

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Glyn M Ruck

+1 Kevin.

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

Glyn M Ruck

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.

Glyn M Ruck

What the manual is saying is that the ECU/TCU working in conjuction with one another will not allow the engine to be overevved.

C300CA

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.

acr2001

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.

C300CA

No matter intentionally or not, your explanation makes good sense to me. All in all, a great C we have.

kevink2

Did you try the modified coast down on the same hill, that I suggested in post #9 ? Just curious what gear you actually are in.

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C300CA

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.

kevink2

Sorry about the delayed response ...

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?

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Glyn M Ruck

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)

kevink2

Check the hydraulic circuit diagram on pg 50 of the big pdf file I linked in the new "sticky". The flow that activates the lock-up is to a closed chamber, and not part of the general flow in and out of the TC from the oil pump driven bt the TC output shaft. So I think the pressure to that chamber is not related to flow to the exchanger.

"Cup of coffee" that something was lost in the translation

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Glyn M Ruck

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!

kevink2

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 .

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Glyn M Ruck

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.

kevink2

Yup. From your other posts, for the 722.9 I assumed "locked" means minor controlled slip. Sorry about the "minor slip" phrase I used for those rare conditions when the TC is completely unlocked.

What did you think about my theory, that the overheat-sequence is based on an engine with boosted power?

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