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So done about 50 miles since they changed the tires out, and no error code so far. I'll be confident is solved once I hit 100 miles.
Turned out the tires that were on it were different "models" of the same size tire from Kuhmo, I wish I'd noticed but not something I looked at when I bought the car. (assumed 305/30/20 was on both tires, and they were both kuhmo so both were the same ) Check out the size difference and tread pattern.. it was just over 1/4" taller.. two 305/30/20 but one is just over 1/4" taller
So done about 50 miles since they changed the tires out, and no error code so far. I'll be confident is solved once I hit 100 miles.
Turned out the tires that were on it were different "models" of the same size tire from Kuhmo, I wish I'd noticed but not something I looked at when I bought the car. (assumed 305/30/20 was on both tires, and they were both kuhmo so both were the same ) Check out the size difference and tread pattern.. it was just over 1/4" taller.. two 305/30/20 but one is just over 1/4" taller
Actually provided this isnt inflation differences or something playing with your eye, that's a sizeable difference. The diameter is 1/4" difference which probably translates to about 15% circumference difference and 15% distance travelled variation between each side....there is a calc, but Im too lazy to apply it!
Actually provided this isnt inflation differences or something playing with your eye, that's a sizeable difference. The diameter is 1/4" difference which probably translates to about 15% circumference difference and 15% distance travelled variation between each side....there is a calc, but Im too lazy to apply it!
agreed, wish I'd know months ago before I wasted a bunch of time and money on software updates and anxiety about a potentially bad differential
What was frustrating is the first dealership said it was my nonstandard tires, 305/30/20 vs 295/30/20, they didn't measure them, they just called it not standard because they were wider. it turned out to be either a tire that should have failed quality control or just a variance between years of that tire model. I have Ps4's now
Who’s running kuhmo tires on a GTS? Yeah I am a tire snob, but wtf?
bought it used and planned on replacing them once they were worn out. This car wasn't "taken care of". I had to do rotors, pads, diff and trans fluids when I got it but it was about $15k under market at the time. I'm not a tire snob, but yes, Kuhmo's are not for a GT
Glad to hear thats all that it was. Same size but different tire model, that is something for the WTF book.
Enjoy the hell out of it now.
definitely I'm kicking myself for no seeing the obvious, but I would never have thought someone would put different tires on the rear of a car like this . since two techs didn't notice I guess I shouldn't feel so bad...
definitely I'm kicking myself for no seeing the obvious, but I would never have thought someone would put different tires on the rear of a car like this . since two techs didn't notice I guess I shouldn't feel so bad...
Dont kick yourself, i have bought many cars in my life that were used and never have I had the inclination to go and check the model # of a tire. It was always if the sizes are the same we must be golden.
We need to find the dude that bought and put them on, and kick him and go down the line. LOL
Dont kick yourself, i have bought many cars in my life that were used and never have I had the inclination to go and check the model # of a tire. It was always if the sizes are the same we must be golden.
We need to find the dude that bought and put them on, and kick him and go down the line. LOL
sounds good, a swift kick on the happy sacks will help him rethink his life choices
I'll defend the PO. I don't think someone made a conscious choice to pick two different model tires with the same brand. What happens that is not that uncommon is the owner experiences a blowout on one of two matching tires. The replacement matching tire is not available (or significant delay) and the owner is sold a same brand but similar tire...possible...
On the math, pretty simple - whatever % difference there is on the diameter will be the same % difference on the circumference. In this case, w/o showing the math, just less than 1% difference.
I'll defend the PO. I don't think someone made a conscious choice to pick two different model tires with the same brand. What happens that is not that uncommon is the owner experiences a blowout on one of two matching tires. The replacement matching tire is not available (or significant delay) and the owner is sold a same brand but similar tire...possible...
On the math, pretty simple - whatever % difference there is on the diameter will be the same % difference on the circumference. In this case, w/o showing the math, just less than 1% difference.
sounds legit. definitely less than I thought; I calculate it translating to about 5 extra rotations per KM.
sounds legit. definitely less than I thought; I calculate it translating to about 5 extra rotations per KM.
Some math "fun facts" - a 305/30/20 wheel/tire is close to 25" in diameter (actually 27" but close enough for fun).
There are 100 one quarter inches in 25". If one wheel/tire is 25" and another is 24 3/4" that would be a ratio of 100/99, or 1% difference, but at 27", slightly less than 1%
Multiplying the diameters by Pi (a fixed ratio) = circumferences of larger and smaller tires, but the same slightly less than 1% difference...the magic of Pi...
Some math "fun facts" - a 305/30/20 wheel/tire is close to 25" in diameter (actually 27" but close enough for fun).
There are 100 one quarter inches in 25". If one wheel/tire is 25" and another is 24 3/4" that would be a ratio of 100/99, or 1% difference, but at 27", slightly less than 1%
Multiplying the diameters by Pi (a fixed ratio) = circumferences of larger and smaller tires, but the same slightly less than 1% difference...the magic of Pi...
You had me right up to using math and fun facts in the same sentence!
It's interesting to me that this would create the error though, given that turning frequency would also create different rates of rotation. When the computers are doing their thing they must take steering input into the calculations.
You had me right up to using math and fun facts in the same sentence!
It's interesting to me that this would create the error though, given that turning frequency would also create different rates of rotation. When the computers are doing their thing they must take steering input into the calculations.
What very odd is the error says electronic differential lock on the dashboard which is a generic error. The actual code stored in the TCU was C072800 which is a speed difference error. but I don't have a reference to that actual code meaning from star/xentry/das
There are two-speed sensors in the transmission that monitor the two clutches, but that would have nothing to do with the rotational speed of the two back wheels. So I really have no idea why the this code was thrown and the tech didn't know either, it was MB who told him to check the tire diameter.
What very odd is the error says electronic differential lock on the dashboard which is a generic error. The actual code stored in the TCU was C072800 which is a speed difference error. but I don't have a reference to that actual code meaning from star/xentry/das
There are two-speed sensors in the transmission that monitor the two clutches, but that would have nothing to do with the rotational speed of the two back wheels. So I really have no idea why the this code was thrown and the tech didn't know either, it was MB who told him to check the tire diameter.
So, a couple of things.
With different rear wheel diameters (albeit only 1/4") this results in a rolling circumference difference of .780". Translated, one rear AXLE half is turning at a slower speed than the other axle half. To toss a little more "silly math" at it - with a .78" difference in rolling circumferences on the rear axles, which physically is absorbed in an open differential (not an issue), every 106 rear wheel rotations one axle has accumulated one more turn than the other axle. One last silly calc - with a difference of .780" in circumferences, there are about 7 more axle rotations per mile, on one half than is on the other half.
Again, an open differential can handle those physical differences, it's the logical side that is struggling. A C07800 is a CHASSIS fault code. C0780 specifically is an ABS fault. Now you have to think about how a modern EMS handles traction control (TC) and Active Handling (AH). In both of these conditions, the ECU and ABS through a body control module control manage skid control (via a number of sensors - wheel speed sensors, yaw rate sensors, TP, etc.). During an "event" the ABS reports different wheel speeds, the ECU can control speed (a number of different ways, the ABS can control different wheel speeds based on the seriousness of the event.
But the question becomes, how is the ABS controller programmed to deal with variances? This is the unknown (for me). usually the way the algorithms are constructed for these controllers there are thresholds established for different events. Another factor is a "memory/time limit" function that works in conjunction with the designed thresholds. So, for example, if the ABS records an "event" where wheel speeds are different (such as turning a corner) but below a threshold, the error is stored, not acted upon and then a timer begins, If the error is not seen again in a prescribed time, the error is deleted.
Now, in the case of a continuous difference in axle speeds (such as in the different wheel circumferences) the ABS continues to record these, but the memory is not allowed to expire. In short, I think this is what was giving the C07800, but I have no idea what the design algorithms are in the MB. When I dealt with tuning American engines, especially when going from NA to an FI engine setup, torque management was an important adjustment to be made; otherwise, the increased torque produced exceeded EMS design. Then TP was always being controlled by the ABS/Body Control/ECU. Until it was fixed, C-codes would pop up.
Here is a pic of one of the torque management pages needing adjustments.
So, here I could eliminate all nanny controls out of limits that utilize the ABS (wheel differences, speeds, etc), by either shutting off enablers or setting extreme high/low limits that physically never could be obtained. I NEVER used a fuel cutoff to control ANY variance while an FI engine is in PE. ETC is the most popular and safest method in responding to variances, but no one likes to be at 67% throttle when they're commanding 100%.
Back to this situation, I think I can kinda understand how the factory was led to different axle speeds in this situation. I'd be very interested to see if the issue is over, longer term.
With different rear wheel diameters (albeit only 1/4") this results in a rolling circumference difference of .780". Translated, one rear AXLE half is turning at a slower speed than the other axle half. To toss a little more "silly math" at it - with a .78" difference in rolling circumferences on the rear axles, which physically is absorbed in an open differential (not an issue), every 106 rear wheel rotations one axle has accumulated one more turn than the other axle. One last silly calc - with a difference of .780" in circumferences, there are about 7 more axle rotations per mile, on one half than is on the other half.
Again, an open differential can handle those physical differences, it's the logical side that is struggling. A C07800 is a CHASSIS fault code. C0780 specifically is an ABS fault. Now you have to think about how a modern EMS handles traction control (TC) and Active Handling (AH). In both of these conditions, the ECU and ABS through a body control module control manage skid control (via a number of sensors - wheel speed sensors, yaw rate sensors, TP, etc.). During an "event" the ABS reports different wheel speeds, the ECU can control speed (a number of different ways, the ABS can control different wheel speeds based on the seriousness of the event.
But the question becomes, how is the ABS controller programmed to deal with variances? This is the unknown (for me). usually the way the algorithms are constructed for these controllers there are thresholds established for different events. Another factor is a "memory/time limit" function that works in conjunction with the designed thresholds. So, for example, if the ABS records an "event" where wheel speeds are different (such as turning a corner) but below a threshold, the error is stored, not acted upon and then a timer begins, If the error is not seen again in a prescribed time, the error is deleted.
Now, in the case of a continuous difference in axle speeds (such as in the different wheel circumferences) the ABS continues to record these, but the memory is not allowed to expire. In short, I think this is what was giving the C07800, but I have no idea what the design algorithms are in the MB. When I dealt with tuning American engines, especially when going from NA to an FI engine setup, torque management was an important adjustment to be made; otherwise, the increased torque produced exceeded EMS design. Then TP was always being controlled by the ABS/Body Control/ECU. Until it was fixed, C-codes would pop up.
Here is a pic of one of the torque management pages needing adjustments.
So, here I could eliminate all nanny controls out of limits that utilize the ABS (wheel differences, speeds, etc), by either shutting off enablers or setting extreme high/low limits that physically never could be obtained. I NEVER used a fuel cutoff to control ANY variance while an FI engine is in PE. ETC is the most popular and safest method in responding to variances, but no one likes to be at 67% throttle when they're commanding 100%.
Back to this situation, I think I can kinda understand how the factory was led to different axle speeds in this situation. I'd be very interested to see if the issue is over, longer term.
Thanks for the theory / hypothesis, makes sense to me in terms of your explanation, Historically when the code was cleared it came back within 50 miles or so, I've done 100 and no error so far. I got a new scanner that's supposed to be able to read the TCU so I'm going to see if it sees anything.
Just an update, have done about 200 miles since the codes were cleared and the tires changed and the error/code has not come back so I'd consider it fixed.
Glad they got it sorted out for you! The guys @ RMB in ATL seem to be a very good dealership as far as communicating with customers. My E550 I purchased used from another used European dealer in ATL was delivered and serviced there from new. After getting it home I detailed it and while the front seats were out I found a gold earing under the drivers side track. I looked at all the service records that came with the car and the previous owner had torn off the owner info on all the invoices. I called RMB and told the service manager what I had found and asked if they could contact the previous owner. About 2 hours later I recieved a phone call from a SA named Biruh. I told him what I had found and he said he knew the previous owner well and gave me RMB's FedEx account # to mail the earing. About 10 days later I recieved a FedEx box from RMB, it contained a nice Thank You card from the previous owner......2 brand new keys she had @ home and a MB baseball hat!
Glad they got it sorted out for you! The guys @ RMB in ATL seem to be a very good dealership as far as communicating with customers. My E550 I purchased used from another used European dealer in ATL was delivered and serviced there from new. After getting it home I detailed it and while the front seats were out I found a gold earing under the drivers side track. I looked at all the service records that came with the car and the previous owner had torn off the owner info on all the invoices. I called RMB and told the service manager what I had found and asked if they could contact the previous owner. About 2 hours later I recieved a phone call from a SA named Biruh. I told him what I had found and he said he knew the previous owner well and gave me RMB's FedEx account # to mail the earing. About 10 days later I recieved a FedEx box from RMB, it contained a nice Thank You card from the previous owner......2 brand new keys she had @ home and a MB baseball hat!
02-08-2021, 10:06 AM
) Check out the size difference and tread pattern.. it was just over 1/4" taller..
