7th gear disengages while coasting = Trans Error (Brand new owner!!) - BMW M5 Forum and M6 Forums
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post #1 of 9 Old 12th September 2018, 08:44 PM Thread Starter
jtolittle
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7th gear disengages while coasting = Trans Error (Brand new owner!!)

Hello everyone,

First let me introduce myself. My name is JT and I just purchased a 2006 M5 with 133k miles. I have been wanting one for some time and finally pulled the trigger. I read about the "reliability" of the S85 and SMG so I have some idea of what to expect. I love working on cars, so I don't mind doing the repairs, however I am not very excited about spending $1000 for every part... Oh well! It doesn't matter for now, it's not my daily. And looking at the car in the driveway puts a smile on my face, driving it makes me giggle! It's all I need.
I'm coming from the Sprinter source forum, where I find invalubale information and support for my van, and it seems like the community is just as engaged on this forum. I am excited to be part of it.

So here we go, one week of ownership and the first troubles start!
I took the car on a 400 miles trip to put it through its paces and see what would break. The trans is not happy while coasting in 7th gear. I found a very similar issue from another member but unfortunately, there were no suggestions on how to fix it. His thread is here: https://www.m5board.com/vbulletin/e6...hile-gear.html

When I'm driving the car in 7th gear (manual or drive) and I let my foot off the gas, after a few seconds, it will loose RPM almost going in neutral . The RPM will shift up and down 1-2k and eventually the 7th is out. I can only shift up to 6th. The redline is down to 6.5k. The cog wheel shows up. If i keep very minimal pressure on th egas to the point that I can slow down the car while coasting then the issue doesn't happen. It's only with 0 pressure on gas pedal.
I have played with it a few times and pulled code P2711 twice (Unexpected Mechanical Gear Disengagement) and another one another time but don't remember it. It had to do with TCM. If I stop the car, reset the OBD and go again, I'm back to normal. No Service Engine light at any point.

I suspect an SMG pump but I definitely don't know enough about the car/trans to be certain. Do each gear have their own solenoid for the hydraulics? Maybe the 7th could be clogged/worn..?

As far as I know, here is the maintenance done on the car:
THROTTLE ACTUATORS 100K MILES
CLUTCH AND FLYWHEEL 98K MILES
ROD BEARINGS 83K MILES
OIL CHANGE DONE EVERY 5K MILES WITH ONLY MOTUL OR LIQUI-MOLY
TRANSMISSION FLUSH 130K
SPARK PLUGS AND COILPACKS 95K

The flush at 130k might indicate that the previous owner was trying to fix that problem but I didn't know about it before purchasing the car, and the issue didn't come up while test driving.

I don't have ISTA yet, don't really know what it is but I figure I'll probably have to get into it soon!

Any recommendations on where should I start?

Thank you very much for your support.
JT
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post #2 of 9 Old 12th September 2018, 08:49 PM
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Quote:
Originally Posted by jtolittle View Post
Hello everyone,

First let me introduce myself. My name is JT and I just purchased a 2006 M5 with 133k miles. I have been wanting one for some time and finally pulled the trigger. I read about the "reliability" of the S85 and SMG so I have some idea of what to expect. I love working on cars, so I don't mind doing the repairs, however I am not very excited about spending $1000 for every part... Oh well! It doesn't matter for now, it's not my daily. And looking at the car in the driveway puts a smile on my face, driving it makes me giggle! It's all I need.
I'm coming from the Sprinter source forum, where I find invalubale information and support for my van, and it seems like the community is just as engaged on this forum. I am excited to be part of it.

So here we go, one week of ownership and the first troubles start!
I took the car on a 400 miles trip to put it through its paces and see what would break. The trans is not happy while coasting in 7th gear. I found a very similar issue from another member but unfortunately, there were no suggestions on how to fix it. His thread is here: https://www.m5board.com/vbulletin/e6...hile-gear.html

When I'm driving the car in 7th gear (manual or drive) and I let my foot off the gas, after a few seconds, it will loose RPM almost going in neutral . The RPM will shift up and down 1-2k and eventually the 7th is out. I can only shift up to 6th. The redline is down to 6.5k. The cog wheel shows up. If i keep very minimal pressure on th egas to the point that I can slow down the car while coasting then the issue doesn't happen. It's only with 0 pressure on gas pedal.
I have played with it a few times and pulled code P2711 twice (Unexpected Mechanical Gear Disengagement) and another one another time but don't remember it. It had to do with TCM. If I stop the car, reset the OBD and go again, I'm back to normal. No Service Engine light at any point.

I suspect an SMG pump but I definitely don't know enough about the car/trans to be certain. Do each gear have their own solenoid for the hydraulics? Maybe the 7th could be clogged/worn..?

As far as I know, here is the maintenance done on the car:
THROTTLE ACTUATORS 100K MILES
CLUTCH AND FLYWHEEL 98K MILES
ROD BEARINGS 83K MILES
OIL CHANGE DONE EVERY 5K MILES WITH ONLY MOTUL OR LIQUI-MOLY
TRANSMISSION FLUSH 130K
SPARK PLUGS AND COILPACKS 95K

The flush at 130k might indicate that the previous owner was trying to fix that problem but I didn't know about it before purchasing the car, and the issue didn't come up while test driving.

I don't have ISTA yet, don't really know what it is but I figure I'll probably have to get into it soon!

Any recommendations on where should I start?

Thank you very much for your support.
JT
This sounds like leakage from the shift rod on the upper actuator block. Pressure is locked off on the shift rods when a shift request is not present, so not many ways this can happen. The SMG pump wouldn't be the cause since it's not running and pressure isn't being applied when steady in a gear.

Any way to read error codes?
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post #3 of 9 Old 12th September 2018, 09:02 PM Thread Starter
jtolittle
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Thank you for your explanantion, it makes sense. The issue only happens in 7th gear though. Now that I am aware of it, whenever I want to coast, I quickly downshift to 6th and let my foot of the gas. Could it only leak when in 7th?

By reading the codes, you mean pulling them with a laptop and a specific software? Is that ISTA? Is it free? sorry first BMW :/ I've seen people posting their code on this forum and they don't look like OBD P---- codes. I'll look into it.
Thanks
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post #4 of 9 Old 12th September 2018, 09:10 PM
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You can get ISTA for free if you spend some time online searching , when i bought my cable they send me ISTA download as well. Just look on ebay. It is very cheap
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post #5 of 9 Old 12th September 2018, 09:16 PM
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Quote:
Originally Posted by jcolley View Post
This sounds like leakage from the shift rod on the upper actuator block. Pressure is locked off on the shift rods when a shift request is not present, so not many ways this can happen. The SMG pump wouldn't be the cause since it's not running and pressure isn't being applied when steady in a gear.

Any way to read error codes?
Ok, but why would leakage occur only when the gas pedal is lifted off? Even if the shift rod leaks the gear shouldn't drop only when coasting. Sounds more like trans mechanical issue to me. Perhaps a combination of shift rod leaking (allowing shift rod to move) and trans issue where it pushes gear out when coasting?

I've seen similar behavior on sportbikes with worn dog gears.

Would this leaking be within the shift rods assembly? Or would this leak into the trans itself? If leaking into the gearbox one could check fluid level periodically to confirm fluid loss.
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post #6 of 9 Old 12th September 2018, 10:05 PM
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Quote:
Originally Posted by jtolittle View Post
Thank you for your explanantion, it makes sense. The issue only happens in 7th gear though. Now that I am aware of it, whenever I want to coast, I quickly downshift to 6th and let my foot of the gas. Could it only leak when in 7th?

By reading the codes, you mean pulling them with a laptop and a specific software? Is that ISTA? Is it free? sorry first BMW :/ I've seen people posting their code on this forum and they don't look like OBD P---- codes. I'll look into it.
Thanks
Get something like BC Cables where you get the cable and the software, pretty sure there are other vendors doing the same as well.
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post #7 of 9 Old 12th September 2018, 11:39 PM
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Quote:
Originally Posted by flacoramos View Post
Ok, but why would leakage occur only when the gas pedal is lifted off? Even if the shift rod leaks the gear shouldn't drop only when coasting. Sounds more like trans mechanical issue to me. Perhaps a combination of shift rod leaking (allowing shift rod to move) and trans issue where it pushes gear out when coasting?

I've seen similar behavior on sportbikes with worn dog gears.

Would this leaking be within the shift rods assembly? Or would this leak into the trans itself? If leaking into the gearbox one could check fluid level periodically to confirm fluid loss.
I'm thinking it's kind of like the pull on a 6mt you feel on the gear lever under acceleration and engine braking. There have to be some forces acting on the synchros, etc, but I'm not very well versed that deep in the internals yet. Still, in my head, I can't help but think no way you're moving that shift carriage unless:

a) the pressure is not locked in. Internal leakage on the shift solenoid valve (leak by back to vent path) or the piston itself into the gearbox (probably not a great thing)
b) the shift piston carriage is cracked broken and the piston doesn't firmly engage any more.
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post #8 of 9 Old 13th September 2018, 03:48 PM
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If it’s a mechanical failure, why would the computer throw a code I would think it wouldn’t know that a solenoid was bleed off pressure? Could wiring fatigue be to blame. Resistance to the solenoid to high to fully engage or keep ingadged? Or something like that. At 133k, unless the trans has been replaced that harness should be toast.
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post #9 of 9 Old 13th September 2018, 05:04 PM
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Quote:
Originally Posted by jcolley View Post
I'm thinking it's kind of like the pull on a 6mt you feel on the gear lever under acceleration and engine braking. There have to be some forces acting on the synchros, etc, but I'm not very well versed that deep in the internals yet. Still, in my head, I can't help but think no way you're moving that shift carriage unless:

a) the pressure is not locked in. Internal leakage on the shift solenoid valve (leak by back to vent path) or the piston itself into the gearbox (probably not a great thing)
b) the shift piston carriage is cracked broken and the piston doesn't firmly engage any more.
Exactly, the synchro assembly keeps the gear from disengaging. The small angle on both back faces of the teeth create a force so that under torque (both on and off throttle) the slider pushes towards the gear-engage direction. Otherwise you would need constant pressure on the shift forks - which would wear them out (worn forks are easy to find on electronic transfer cases for this very reason btw). Same concept as dogs on motorcycle gears I was referring to.

Looking at the smg hydraulic diagram the unpowered condition for the 6/7 solenoid is blocked inlet port and open drain port. This should stop the shift fork from moving even if synchros aren't doing their job. This suggests some leakage (which may be actually designed on the system - shift fork wear if not). But leaking cannot create any force to disengage the gear. Hence 7th gear synchros have a problem.

There could also be something wrong with the gear position sensor reporting the shift rod moving when it hasn't. But the OP is seeing rpm drop, which means gear is truly disengaging and not only the sensor reporting rod movement.
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