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Discussion Starter · #1 ·
I own a GLA45 AMG. I decided to test the AWD system since I camp off-road and wanted to drive the vehicle on dirt/rocky/gravel roads.


Tests
I drove it to a new subdivision where the dirt road stopped, and continued as gravel covered. I slowly drove onto the gravel with an extremely slight grade. Once all 4 wheels were on the gravel, the tires started to spin.



Only the front right and rear left tires spun. The other 2 tires did not move at all.
So, only one tire per axle was spinning while the other tire was stationary.
The car was stuck, I could not move forward. I tried Controlled Efficiency, Sport, and Manual modes. Same result. All done in the standard traction control mode.
I was able to back out in reverse.


See the pictures and video. I had the same results occur 2 other times on dirt, and sand. I didn't take video of those since I was alone.
https://drive.google.com/open?id=0B8URuP4vPk8kTi1nTzJTYjREeXM


The other test I performed, I did on the side of the road after it had snowed. I drove the car to put the right 2 tires on the soft shoulder. (grass underneath), but covered with snow. The other 2 tires were on the paved road. In this case the 2 tires in the snow spun freely without moving the car forward, or providing traction. The 2 tires that had the best traction (on the paved surface), did not move. I was alone and could not take video, but I took pictures.
https://drive.google.com/open?id=0B8URuP4vPk8ka1dUci0zY05mc1E


I brought the car to the dealership before I took video or pictures, and explained what was happening. They told me they checked out the car, and everything was working properly.
After sending them the pictures and video, I took the car in to the dealership again. This time they acknowledged the issue. They opened a PTSS in the problem reporting system. It was forwarded to Germany, and then finally to the AMG division. After reviewing my pictures & video, they closed the PTSS, reporting that "The car is operating normally as it was designed to."


REALLY!? My service advisor and service manager were as stunned as I was, since they could clearly see the problem just as I had. But, they told me they could do nothing, since this was ruled as proper operation.



I am honestly floored by three things. #1 that this $50k+ car has an AWD system with a crucial operation flaw. #2 that Mercedes/AMG does not want to acknowledge and fix the problem. #3 that Mercedes/AMG did not test the system in this way.


I have been an embedded software engineer for 20+ years. I mostly worked on data and flight systems for military aircraft. Although I have not worked on automotive stability control systems like this, I have read a great deal about it, and understand how an embedded control system like this works down to the software level.


ABS/Traction Control
The ABS system uses sensors in the wheel to determine the speed of each wheel. If the systems detects the wheel has stopped (locked up), then the system will release the brake pressure to that wheel allowing it to spin. Then reapplying the pressure to slow the wheel again. This happens quickly and is the "pulsing" felt at the brake pedal under these conditions. The traction control system is built on top of the ABS system. The difference is that when is senses a wheel spinning too fast, relative to the other wheel on the same axle, it will apply the brake to that wheel. This redirects the torque to the other wheel on the axle, sending the torque to the wheel that isn't spinning. Stability Control is a system built on top of the Traction Control to prevent a car from sliding, but that does not pertain to this issue.


The GLA has "open" differentials on both axles. Meaning there is no mechanical system that regulates wheel spin. Some vehicles either provide the capability to lock the axle (4WD - LO), while other vehicles may have a "Limited Slip" Differential (LSD) which uses some implementation of wet clutches to mechanically limit how much wheel spin is incurred. Therefore, the GLA must rely on the traction control system to regulate wheel spin. From the tests I've done, it seems clear to me that the traction control system is not functioning properly in this situation. As an embedded programmer, I strongly believe that this is a software(firmware) issue. I think a fix would exist in identifying why the algorithm is not working, and fixing it by updating the software. A software update would also be the easiest for Mercedes to implement. No parts or extensive labor. Just the time for the dealer to program the new software into each vehicle.


After talking to my local service advisor, he said that since this is a new vehicle platform, based on the A45. It is not the same as the legacy 4Matic AWD system. This is a front drive platform that was modified to send power to the rear via a transfer case & rear clutch configuration. He has never seen this issue with the historical 4Matic system.


I found a YouTube video where a test is done on an icy & uneven roadway, with similar results. The rear tire just spins freely. In this case, It seems the momentum of the car & spinning tire brings it forward, but if it had been going slower, I think it would have been stuck as well.
https://www.youtube.com/watch?v=KUVRmOp4IH8



Another off road test:
https://www.youtube.com/watch?v=mOcHiF62SMY



Conclusion
Has anyone else tested their AWD systems and found that it performed similarly? What do you think about AMG's response? I read they would be adding a LSD to this car. That would likely solve the problem. So, in my opinion they looked at this issue and decided it was not worthwhile to fix this since the new model will have a fix already. I think this is a poor decision as a current owner, but so far have no recourse. I just have to live with a vehicle that I bought for its AWD especially in the snow where I drive it. So I am very disappointed and frustrated. I am not sure whether this is a GLA45 or GLA250 issue.
If enough people raise the issue to their dealers, maybe that will compel AMG to address this problem and offer a solution to current GLA owners. What do you all think?
 

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Your description of the system matches what I understand ... the brakes are applied to maintain stability and limit wheel slip.

But I believe there is a minimum speed requirement for this feature to engage.

Off road testing videos I have seen indicate high throttle conditions to get through various courses. Not exactly rock climbing technique!!! Watching these videos left me wondering.

I know LSD is an option on the 2017 45. It is a part of the P71 AMG Dynamic Plus Package offered at $2,800 US. The LSD itself indicates a code 471, but I don't know if it can be ordered separately. I also believe it is front axle only. The 459 Adaptive Suspension in that same package is ala carte.

I skipped this choice (did 459 as a solo), but now you have me thinking. Then again, on a 58 Willys M38A1 I built almost 40 years ago (650HP Sprint car motor with major modifications to everything) I included LSD on the rear and a locker on the front. Both axles would spin one wheel only if I got wheel(s) off of the ground.

I agree with your assessment. It would seem that a software solution would be due. Perhaps it is as easy as lowering the minimum speed requirement. What was the speedometer indicating with the wheels slipping? Did you try a heavy application at the gas pedal (I realize this is counter intuitive to regaining traction, but so is ABS operation). What was AMG's argument that the wheels slipping was OK?
 

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I do agree that software updates are probably required but i think the AWD on the 45 is more oriented towards performance and traction on paved roads not to climb icy hills. Im sure the STI/EVO crowd would be disappointed but this platform was probably a front wheel drivetrain first and "AWD" as an afterthought. Kinda like the AWD in honda CRVs (youtube have plenty vids) Yea kinda disappointing.

I have no excuse for the AWD 250 however
 

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I do agree that software updates are probably required but i think the AWD on the 45 is more oriented towards performance and traction on paved roads not to climb icy hills. Im sure the STI/EVO crowd would be disappointed but this platform was probably a front wheel drivetrain first and "AWD" as an afterthought. Kinda like the AWD in honda CRVs (youtube have plenty vids) Yea kinda disappointing.

I have no excuse for the AWD 250 however[/Q

I know this thread discusses an issue with the AMG 45 4Matic, but I am going to mention a situation that m-type alludes to above. I have a 2015 GLA250 with FWD. When I start off, if I give it some gas in S, the ESP system keeps switching power from one side to the other, making handling interesting; this also happens a little when accelerating through a turn. If I shut off the ESP the car is far easier to handle from a start. Unfortunately, you have to defeat the ESP every time you start the car, which means going through the steering column switch, and when ESP is defeated, cruise control is non-functional.
 

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Conclusion
Has anyone else tested their AWD systems and found that it performed similarly? What do you think about AMG's response? I read they would be adding a LSD to this car. That would likely solve the problem. So, in my opinion they looked at this issue and decided it was not worthwhile to fix this since the new model will have a fix already. I think this is a poor decision as a current owner, but so far have no recourse. I just have to live with a vehicle that I bought for its AWD especially in the snow where I drive it. So I am very disappointed and frustrated. I am not sure whether this is a GLA45 or GLA250 issue.
If enough people raise the issue to their dealers, maybe that will compel AMG to address this problem and offer a solution to current GLA owners. What do you all think?
Did you do your testing with the original summer high-performance tires on the car?
If so, I'm not surprised at all by your results.
 

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Discussion Starter · #6 ·
Your description of the system matches what I understand ... the brakes are applied to maintain stability and limit wheel slip.

But I believe there is a minimum speed requirement for this feature to engage.

Off road testing videos I have seen indicate high throttle conditions to get through various courses. Not exactly rock climbing technique!!! Watching these videos left me wondering.

I know LSD is an option on the 2017 45. It is a part of the P71 AMG Dynamic Plus Package offered at $2,800 US. The LSD itself indicates a code 471, but I don't know if it can be ordered separately. I also believe it is front axle only. The 459 Adaptive Suspension in that same package is ala carte.

I skipped this choice (did 459 as a solo), but now you have me thinking. Then again, on a 58 Willys M38A1 I built almost 40 years ago (650HP Sprint car motor with major modifications to everything) I included LSD on the rear and a locker on the front. Both axles would spin one wheel only if I got wheel(s) off of the ground.

I agree with your assessment. It would seem that a software solution would be due. Perhaps it is as easy as lowering the minimum speed requirement. What was the speedometer indicating with the wheels slipping? Did you try a heavy application at the gas pedal (I realize this is counter intuitive to regaining traction, but so is ABS operation). What was AMG's argument that the wheels slipping was OK?
I think I was in the 10-20 mph speed range. This should be far above the threshold speed. Yes, I know that ABS has a minimum threshold speed as well, under which it will disengage the ABS system. A good point to bring up.
My vehicle has none of the off-road options installed, but I maintain the system should be working independent of that. The LSD could be a pricey option, and would certainly help, but shouldn't be required in a situation like this.
 

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Discussion Starter · #7 ·
I do agree that software updates are probably required but i think the AWD on the 45 is more oriented towards performance and traction on paved roads not to climb icy hills. Im sure the STI/EVO crowd would be disappointed but this platform was probably a front wheel drivetrain first and "AWD" as an afterthought. Kinda like the AWD in honda CRVs (youtube have plenty vids) Yea kinda disappointing.

I have no excuse for the AWD 250 however
My test was not on an icy road, that was a test I found on YouTube.

Did you see the video I posted which was done on gravel?
https://drive.google.com/file/d/0B8URuP4vPk8kbjFOaVpNcXVHRlk/view?usp=sharing

The other test was done on a rural paved road & snow covered soft shoulder.

Even if geared for performance on paved roads, the wheels should not slip as seen in my video, IMO.
 

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Discussion Starter · #8 ·
Did you do your testing with the original summer high-performance tires on the car?
If so, I'm not surprised at all by your results.
Yes, the OEM summer tires. I think you should be surprised. It's true that the summer tires will not grip the dirt/sand/gravel surfaces as well as paved surfaces per their intended design.
But, more importantly, its the fact that the car can sense a single tire spinning, since it has lost traction,
and has not sent any torque to the opposite wheel that is stationary and may have the available traction.
If both wheels on an axle were spinning, then yes, I would agree with you that the tires at that point would be the issue in not moving the car forward. In this situation, its the fact that the system is not trying to send power to the other wheel that may have that traction. (what if the not spinning wheel was on pavement? The car would not have moved in that case either)

My other test was on snow with Dunlop Winter Sport 3D's. The 2 left side tires were on pavement, and the 2 right side tires were on a snow covered soft shoulder. Same results. Only the tires in the snow spun, while the tires on the pavement did not move.

See the pics here:
https://drive.google.com/open?id=0B8URuP4vPk8ka1dUci0zY05mc1E
 

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If there's no LSD, either mechanical or electronic, then I don't know how the car would shift torque to the wheel with traction. All the electronic traction control does is apply brake pressure to the slipping wheel. That would do no good in these situations. Disappointing the car didn't do better on the gravel, but probably not surprising. I think what you've highlight is that the AWD system has fairly limited capabilities.

The Forester XT the GLA replaced for us had a viscous LSD so it's disappointing Mercedes couldn't put one at least in the front. We have the 250 not the AMG, but if Subaru can put one in their cars you'd think Merc could do the same.

David
 

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Discussion Starter · #10 ·
If there's no LSD, either mechanical or electronic, then I don't know how the car would shift torque to the wheel with traction. All the electronic traction control does is apply brake pressure to the slipping wheel. That would do no good in these situations. Disappointing the car didn't do better on the gravel, but probably not surprising. I think what you've highlight is that the AWD system has fairly limited capabilities.

The Forester XT the GLA replaced for us had a viscous LSD so it's disappointing Mercedes couldn't put one at least in the front. We have the 250 not the AMG, but if Subaru can put one in their cars you'd think Merc could do the same.

David
David,

My explanation of how a traction control system works, may have been insufficient.
It's true that that the traction control system applies the brake to the slipping wheel. The action of that is what forces the torque to the opposite wheel. The torque coming from the driveshaft must apply equal torque to a single wheel or both wheels. If one tire is spinning, while the other tire is stopped, then 100% of the torque is driving the spinning wheel. Normally driving down the road, with both tires rotating in a straight line, 50% of the torque is applied to each tire. So, as you slow down a spinning wheel from 100% to say 70%, then the other wheel will get the other 30% of the torque coming from the driveshaft. The torque at both wheels must add up to 100%.

This is the basis of how traction control systems work. Its true that the system will slow down the wheel that's spinning, in an effort to regain the traction that it has lost on that wheel. Importantly though, slowing down the spinning wheel will send more of the force coming from the driveshaft to the opposite wheel and provide the traction that wheel can provide.

Although an LSD would work better, this method should work here as well, it just seems to not have been implemented correctly.

Here are some links to "How Traction Control Systems Work" to further explain this, if I was still unclear.

From Wiki.. "Braking action on slipping wheel(s) will cause power transfer to wheel axle(s) with traction due to the mechanical action within the differential."

https://en.wikipedia.org/wiki/Traction_control_system

Traction Control Explained | HowStuffWorks
 

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Thanks for the correction. I was thinking an open diff would not transfer torque to the other wheel, but I was incorrect. An open diff applies equal torque to both wheels. By applying the brake on the spinning wheel the same amount of torque should get applied to the non-spinning wheel. So you would think in the situation where one wheel is on snow/ice and the other is on the road that the car should move by putting some torque to the wheels with grip. I would agree something is not working right.

David
 

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Discussion Starter · #16 ·
does all the 45s are same like that? If yes, I should reconsider to buy this car.
I think this is a design flaw that would affect all 45s and it seems the 250s as well.

I would like to hear from other 45 owners. Has anyone does tests like I have? Would you be willing to, then post the results?

Turning off the traction control will yield the same results. The car will not control wheel spin.
 

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Discussion Starter · #17 ·
There's an interesting video of an AWD test conducted on a GLA250. Is this not the expected behavior of our 4MATIC system ?

https://m.youtube.com/watch?v=KUVRmOp4IH8
Yes, I posted that same link in my original post. No it is not the expected behavior of the 4Matic system, if you read what I wrote about how the traction control is supposed to work.
 

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I used my gla45 last winter extensively with no issues. It stopped and accelerated on ice with no problem. In that respect it was similar to my old a3 quattro. You could provoke a little wheelspin if clumsy but it's worlds better than other fwd cars I've driven in snow .This awd system is similar to , and actually might be haldex which has been extensively tested and developed. As far as getting stuck i don't know because it never happened to me.
 

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Discussion Starter · #20 ·
thanks Wayne, and no it is not.
But I was told by AMG in Affalterbach that "the car is operating normally as it was designed to."
How can that be when it directly contradicts the release you posted?
 
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