DCT Downshift 1

996c4smd · 11-07-2020, 11:38 AM

So I recently commented on a thread where someone had experienced the same problem I'm getting.

Cruising on the motorway in 7th, depress the accelerator all the way to the end (but not pressing the kickdown button), the car switches to 5th but there is a delay of 1 - 2 seconds before it actually selects 5th and starts accelerating. Happens from 6 -> 4 too.

Anyway, today, with the help of a friend, we've managed to figure out exactly when it happens and getting closer to why. With ISTA, we can see when the clutches are engaged & disengaged (open).

What we have found is that if you're cruising and let of the accelerator for around 3 seconds, the clutches automatically disengage. At this point, if you accelerate lightly so that no gear change happens, the clutch re-engages quite quickly and you're able to accelerate. Instead, if you accelerate harder so that a downshift happens (for example, to 5th like in the video), the clutch takes longer to re-engage, and that is the 1 - 2 second delay I'm experiencing.

Digging deeper, I then looked at the nominal (requested) clutch pressures and actual clutch pressures. Interestingly, when I want to accelerate and it has downshifted, the nominal clutch pressure is 0.60 bar for those 1 - 2 seconds. After that, it then requests a higher clutch pressure in increments until it hits 5+ bar. The actual clutch pressure follows suit.

So, we ended up with 2 unanswered questions:

1. Should the clutches disengage when coasting for more than 3 seconds? You can just about feel the disengagement too. Is this some sort of feature to help reduce wear on the clutches?

2. When it has been decided that a kickdown to 5th is wanted, why isn't the car immediately requesting clutch pressure?

The car has been into BMW for a full software update a couple of weeks ago, but this has not helped. I have confirmed the DCT (DKG) software has been updated by looking at version numbers.

It goes without saying, if I'm cruising but keep my foot at a static position on the throttle (so I'm not accelerating), the clutch is always engaged and if I then kickdown to 5th, it's perfect, no issues.

If someone can try this too, that would be great. It's really easy: cruise at 70-80mph, and then let go of the accelerator for about 5s (to be sure). Then, depress the accelerator all the way to the end (but ensuring not to press the kickdown button), the display should change from 7 to 5, and it would be interesting to see if the delay is also there.

996c4smd · 11-07-2020, 02:15 PM

Doing some more research, this could be due to a software update and a "feature" known as "Sail/cruise" mode where the car disengages the clutches. I know my car had a gearbox update in 2017, but I don't know if this feature part of the M5 software.

A guy had similar symptoms here: https://www.m5board.com/threads/dct-...oblems.521089/

11-07-2020, 11:38 AM	#1
996c4smd New Member 0 Rep 6 Posts Drives: 2013 M5 Join Date: Oct 2020 Location: UK iTrader: (0)	DCT Downshift 1 - 2 second hesitation when cruising So I recently commented on a thread where someone had experienced the same problem I'm getting. Cruising on the motorway in 7th, depress the accelerator all the way to the end (but not pressing the kickdown button), the car switches to 5th but there is a delay of 1 - 2 seconds before it actually selects 5th and starts accelerating. Happens from 6 -> 4 too. Anyway, today, with the help of a friend, we've managed to figure out exactly when it happens and getting closer to why. With ISTA, we can see when the clutches are engaged & disengaged (open). What we have found is that if you're cruising and let of the accelerator for around 3 seconds, the clutches automatically disengage. At this point, if you accelerate lightly so that no gear change happens, the clutch re-engages quite quickly and you're able to accelerate. Instead, if you accelerate harder so that a downshift happens (for example, to 5th like in the video), the clutch takes longer to re-engage, and that is the 1 - 2 second delay I'm experiencing. Digging deeper, I then looked at the nominal (requested) clutch pressures and actual clutch pressures. Interestingly, when I want to accelerate and it has downshifted, the nominal clutch pressure is 0.60 bar for those 1 - 2 seconds. After that, it then requests a higher clutch pressure in increments until it hits 5+ bar. The actual clutch pressure follows suit. So, we ended up with 2 unanswered questions: 1. Should the clutches disengage when coasting for more than 3 seconds? You can just about feel the disengagement too. Is this some sort of feature to help reduce wear on the clutches? 2. When it has been decided that a kickdown to 5th is wanted, why isn't the car immediately requesting clutch pressure? The car has been into BMW for a full software update a couple of weeks ago, but this has not helped. I have confirmed the DCT (DKG) software has been updated by looking at version numbers. It goes without saying, if I'm cruising but keep my foot at a static position on the throttle (so I'm not accelerating), the clutch is always engaged and if I then kickdown to 5th, it's perfect, no issues. If someone can try this too, that would be great. It's really easy: cruise at 70-80mph, and then let go of the accelerator for about 5s (to be sure). Then, depress the accelerator all the way to the end (but ensuring not to press the kickdown button), the display should change from 7 to 5, and it would be interesting to see if the delay is also there. Last edited by 996c4smd; 11-07-2020 at 02:42 PM..
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11-07-2020, 02:15 PM	#2
996c4smd New Member 0 Rep 6 Posts Drives: 2013 M5 Join Date: Oct 2020 Location: UK iTrader: (0)	Doing some more research, this could be due to a software update and a "feature" known as "Sail/cruise" mode where the car disengages the clutches. I know my car had a gearbox update in 2017, but I don't know if this feature part of the M5 software. A guy had similar symptoms here: https://www.m5board.com/threads/dct-...oblems.521089/
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