Nerdboss

I bought a used 2021 GLC 300 4matic with 17k miles last week. Usually I leave the vehicle in comfort mode but I decided to see how manual mode would work. I was driving at around 20 mph and decided to downshift from 3rd to 2nd. I clicked the down paddle and nothing happened so I clicked it again, and then I have probably the most severe jolt I've ever felt as the car slammed into a lower gear. What in the world? Is this normal for this car?!? Is it possible to overrev the car by accident? I've never had an automatic car that would downshift farther than the engine could handle.

I'm coming from BMW so I realizerealized X3 will probably have much better transmission tuning and performance, but is the transmission really this slow to shift and this harsh?

CaptainMal

I have a 2021 and drive in manual fairly often. I just don't really like the normal "auto" shift points. This has never happened to me when downshifting. This isn't the fastest transmission in the world, but it has always been super smooth for me, in all modes, even sport/sport+.

Nerdboss

Interesting. Not sure what I did to make it do that, but I couldn't reproduce it when I tried to. Good to know that someone else said it's not a super fast transmission. I'm used to the ZF8 in BMW and the shifts on those units are instant and that's what I'm used to. I put the GLC into sport mode and the shifts were more of where I wanted them to be in that mode so I don't see a need for the manual shifting.

CaptainMal

I know what you mean. My last car was a MK7.5 Golf R with a dual clutch that I drove in manual 100% of the time. That was lightning compared to the GLC300. It was lightning compared to most cars that aren't supercars. I've been driving the GLC for a year now and I'm still getting used to it.

W205C43PFL

BMW that calls themselves the Ultimate Driving Machine with the promise of delivering Sheer Driving Pleasure, it is understandable that their vehicles are meant to be sporty. The air intake on BMWs are also lower too. Mercedes on the other hand used to be "the old man's car" and luxury is the main focus of the brand. Luxury means smooth and seamless shifts with the trade off of slow shifts but the target market doesn't need fast shifts anyways. In comes AMG, dual personality, a flick of the switch you turn a your typical gentleman vehicle to an "aggressive" gentleman. The shifts from the AMG's Speedshift transmissions are rapid and won't leave you disappointed.

That being said, welcome to Mercedes-Benz and congrats on your vehicle.

W205C43PFL

To answer your question on the hard shift you felt, it is because the transmission was probably upshifting (or ready to upshift) and then you wanted it to downshift so it hesitated and when it did downshift at the last minute when your vehicle speed was almost out of the range the optimal speed for the gear, it had no choice but to force a shift because you insisted.

Edit: Another thing is, modern MB transmissions to a certain a degree does adaptations. Since you said you brought the vehicle used. The previous driver's driving style might mismatch with yours and therefore the transmission is confused. Drive it a bit more and it will eventually learn your style of driving.

Nerdboss

Ah yep, I forgot about that. BMW also does adaptations, but i've only driven the car about 250 miles so far, so I wouldn't be surpised if it's still adjusting. I'm not trying to race it or anything, but sometimes I like to have a little fun. It's funny you mentioned the old man thing, I've never viewed Mercedes as old man in the same way that I have Cadillac or Lincoln. I'm in my late 20s and just felt like the GLC is a better built vehicle vs the BMW x3. The solidity and build quality in the GLC is very apparent compared to BMW.

superswiss

Just kinda adding to what was said above. Regular Mercedes-Benz models aren't really meant to be driven in manual mode much. That's partly evidenced by most of them lacking a button to activate permanent manual mode. You have to configure and use Individual mode to keep it in manual mode. Just using the paddles puts it in temporary manual mode, which quickly reverts back to automatic. AMGs on the other hand are meant to be driven in manual mode and have a button to put it in manual mode and leave it there at any time and they also don't automatically upshift at redline, which is commonly referred to as a true manual mode like a manual transmission minus the clutch pedal. The 63 models also have a clutch instead of a torque converter to further underscore the performance nature of the car. Manually shifting a torque converter automatic is rather numb most of the time. The ZF8 is no exception. It might shift faster than the 9G Tronic in regular MBs, but it's still numb.

mikapen

You were probably driving faster than first gear would allow without exceeding Red line. The car will lock out a downshift if it would overrev the engine, until the car slows enough. Then when you do downshift, you're immediately spinning the engine at Red line, and that would probably feel like a jolt.

​​​The downshift is also influenced by how hard you are braking and other factors. Do you have 4matic?

Also, when you leave it in automatic, the transmission learns your driving style and adapts. So if you drive aggressively for a while, it will shift more aggressively.

But no, you can't overrev the car by accident.

Nerdboss

That makes sense. And yeah I have 4matic, does that change transmission behavior?

Nerdboss

Theres an M button in the center console that puts the car into permanent manual mode, and it has paddle shifters, so I figured it was OK to shift manually.

The ZF8 is used in all BMW modBMs, even the high end M models. It's about the fastest transmission I've ever used, including DCTs. I wish Mercedes would have used the ZF8 instead of designing their own 9 speed. Though the 9g tronic isn't bad by any means.

W205C43PFL

Really, I am surprised there is, is your vehicle actually a GLC 43 or GLC 63? are you sure it is a GLC 300? How come there is a manual button? Did the previous owner swap the console?

W205C43PFL

That is such a great point you brought up! I totally forgot about the possibility that OP might had downshifted when the engine was reving too high, thanks for chiming in that was probably precisely why OP's transmission acted the way it did.

Nerdboss

Yes, its a GLC 300 4matic. All of the GLCs I've looked at have the manual button on the center console to enable full manual control. Mine is a 2021, maybe the pre-facelift models didn't have them?

Nerdboss

It was so harsh, but yeah that's probably what was happening. I wasn't accelerating and I wanted to downshift. I haven't had the same issue again. Just learning how the car works. I've been driving BMWs for a very long time life so I'm very accustomed to how they behave and it's weird to drive another brand.

superswiss

Yeah, some regular MB models have the M button. I guess the GLC 300 is one, which is odd, because the C 300 doesn't have it.

The ZF8 is actually rather slow. 200 ms shift times, compared to the 50 ms of a sport DCT and less than 100 ms of AMGs Speedshift MCT. Seems fast, but 200 ms is still human perceptible, but not 100 ms or less. Not talking about additional lag here between clicking the paddle and the transmission actually executing the shift, that's a whole other topic. The ZF8 is a common complaint with the M5 and M8, so much so that BMW M went back and reworked the software for the M3/M4, but the verdict is that it's still numb. MB/AMG still developing their own transmissions and specifically the AMG multi-clutch transmission is why I like the brand actually. Everybody else is abandoning their DCTs etc. in favor of the ZF8 and every car starts feeling the same. I personally dislike anything with a torque converter unless it's just a plain commuter car. The new M cars all using the ZF8 is a major reason I'm not interested in them. I think a performance car needs to have a clutch for proper drivetrain response. AMG still gets this, at least in the 63 models.

However, we are talking about the GLC 300 here, which is not a performance model, so comfort is what it is tuned for. Hopefully, the harsh shift was a one off and it just needs to learn your driving style.

W205C43PFL

Thanks for your reply, I didn't know GLC 300 had a manual button in the centre console.