jetflyboy

I noticed the last few days that as I am coming to a stop, say at a traffic light, the auto. tranny seems to downshift without my help and only at low speeds, 15-25 MPH. I noticed the RPM goes up just a tad to support this theory. Is the car really doing that, or do I need to take it easy on the Espressos?

Mixedfruits

your car does that also when you randomly smash the throttle it acts like your passing a car and down shifts

jetflyboy

Yeah, that one I knew. I just didn't know it did while slowing down. Lern something new almost daily

jctevere

Haha, yes, all cars do this (downshift while braking). Having said that, I find Mercedes to be better then most manufacturers at using downshifts to help slow the car down, while not experiencing any unfavorable jerks. This could probably be why I still had 50%+ brake life at 40k miles on my last w204 versus needing to replace brakes on my Infiniti at 30k miles.

C300CA

My C does it too. I could feel it down shifted without looking at the RPM change. If I know red light will turn green before I reach the cross section I will shift it to N to save bit of gas.

jctevere

Contrary to the belief, this does not save any gas, if anything, it does more harm then good. This is because when your foot is off the accelerator, even though the RPM's are high and falling, the car is not injecting any gas, the RPMs are just the pistons still going up and down. Putting the car in neutral does save drivetrain loss, but then gas starts to be injected into the engine to hold idle RPM. Not to mention, it creats more wear and tear on the tranny, and I know for most manufacturers, when the tranny is in neutral it provides no active lubrication circulation (not sure about Mercedes), and the wheels are still moving (not good).

Bottom line is, learn to coast and use the car's drivetrain and downshift loss to help slow a car down (taking the foot aff the gas sooner when you anticipate a red light, and have the car slow on its own). Also, putting it in neutral, you don't have the transmission downshifts to help slow down the car, thus, you use the brakes more to slow down, and have less brake pad life.

Domm

I had that as well. It has something to do with the transmission learning your particular driving style. I reset my transmission computer by turning the key to "ON", then press the gas pedal all the way to the floor for 5 seconds minimum, then turn off the car (without taking the key out or removing your foot from the pedal) and once the car is off, you can take your foot off the gas and wait for 2 minutes. It fixed this temporarily and I reset it probably every 3 days to maintain it.

jetflyboy

I never really noticed that in my previous BMW 325I, but that is because I always drove it in the shifting mode, I don't know what else to call it. I know the C300 does the same with the shifter when pulled left, but the car's still new to me and I'm still learning it.

I never like to just put it in N and let coast like that.

Are you suggesting that to actually forcing the down shift with the shifter at reasonable speeds is a better way to slow the car down? That's what I always did before.

JimPap

Yes, that's normal.....and easy on the Espresso.

cswang

Don't all automatics downshift as you slow no matter the speed to stay in the best range and to keep the revs from closing on idle? Otherwise you'd be starting from 7th at a light?