JButtonMCL

I'm so annoyed with my 2016 W205 C300 4Matic! My Aux battery died shortly after warranty expired and I went to a third-party shop to get it replaced (known for working with European cars), this is where things went downhill. When I first purchased the car in 2016, the car would always start in D2 and shifts were buttery smooth, and it sounds like most C300's start in D2 but after replacing the Aux Battery, the car began starting in D1.

The shifts became very jerky 1 -> 2 -> 3 -> 4 and 5 -> 4 -> 3. It's a night and day difference between the first 4-years and the last few months, when its cold the shifts jerk you forward, and it short-shifts from D2 -> D3 instantly too. It's really jerky when its cold, and less jerky when its warm but still jerky, you can easily tell with each gear change. There isn't any CEL for my OBD2 scanner to bring up any codes. I've 'resetted' my transmission manually twice and hasn't changed much (if any).

Does anyone know what might be wrong and why its so jerky, and why is it starting in D1 now as opposed to D2 (like it has in the first 4-years)? What should I do?

Thanks everyone!

Cosworth2000

It's weird that it's in C and is starting in D1. It should make sure it's not in S but in C. The other thing to take note is to unplug the battery, make sure your doors are unlocked but before unplugging the battery, also make sure the remote is more than 2 meters away from the car and let it sit for 15 minutes. This should reset all the modules in the car for the next steps.
Make sure the key remains far from the car, then plug the battery back in, then go and get the key, sit in the car but do not start it, start it in ACC and wait 30 seconds. Turn it back off and then start the car and take it for a drive on the road and then the highway. You may have to reset the windows if this may also reset the window modules.

Phat_J

Have you tried a transmission reset?
https://www.mercedesmedic.com/reset-...-instructions/
Edit: never mind, I see you tried that. I'll just leave the link for anyone else who needs it

JButtonMCL

Do you have instructions on how to disconnect a battery on these cars?

Cosworth2000

You just follow the steps to remove and change the battery instead you don't remove the battery from the location.

CaliBenzDriver

The tranny is becoming increasingly jerky even at operating temperatures.


The excellent tranny poor performance is caused by a combination of multiple factors:
1- CAN delays directly impacts the ECU-TCU work
2- MAP is affected by blow-by pressure.


+1 for "car Reboot" therapy:
This easily restarts soft-crashed sleepless modules like SAM computer.

​​​​​​-- This tranny normally shifts ultra fast and seamlessly like it's not even there. Realize that currently everything tranny does is slightly late.

-- When shifts are coordinated, the engine throttle dips for shifts to match RPM.


> Crankcase blow-by impacts MAP:
-- This inputs is normally corelated to throttle demand to kick-down shifts.

-- When MAP input is significant the tranny shifts super easily to deliver increased output.
You know how the ECU likes to shift gears quickly for low RPM... well this is the other way to get higher RPM on demand. Even a slight demand causes kickdown to have engine near 1500 /2000.RPM normal driving.
(We not talking pedal to the metal kick-down WOT).

-- Destructive crankcase pressure is caused by piston rings being stuck by burned oil. Too much combustion pressure is leaked through causing "oil in intake" swamp and abnormal MAP pressures.

-- Better oiling helps free piston rings back to seal gases. Low viscosity 0W-30 is a no-no...
a 10 or 15W- 40 help along normal oil pressure delivery.

- Somebody may know how to clean carbonated rings without disassembling the whole engine...?This is in part what's needed here.