open front & rear diffs with computer traction control is not same as locking diffs - Page 2 - Jeep Renegade Forum
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post #11 of 32 (permalink) Old 01-02-2019, 02:01 AM
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You are all forgetting an important consideration. “Available Torque” is determined by the engine and driveline computers. The engine computer is designed to reduce “available” torque in conditions where a wheel is being braked to distribute traction to the remaining wheels. This is done to prevent damage to the drivetrain. The computer can and will reduce available torque to almost zero if, for example, two of the four wheels are being braked and one of the remaining 2 wheels has more traction than the other. If this did not happen and full available torque was applied to one wheel in the drivetrain (braking the other 3) damage will likely be caused. The drivetrain hardware in the Renegade is identical to that in the Range Rover Evoque. It is licensed to Fiat by the same company (GKN Driveline in the UK). However the parts in the Renegade system are much less robust than that in the Evoque.
Secondly braking a wheel does not reduce available torque by 50%. Although it does reduce available torque it s something far less than that. Again it is the engine computer that drops the torque to protect the system.
If you were to disable the engine torque management the system would put all the engine can produce into the driveline. When 4 wheels have traction it will go to all four wheels. If one or more wheels lost traction and were then “braked” the remaining torque would go to the wheel(s) that have traction. The next thing likely to happen is the CV joint on that wheel(s) would be damaged.
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post #12 of 32 (permalink) Old 01-02-2019, 02:54 AM
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I’d also like to add that the system limits torque by reducing the injector output. You might have the accelerator floored but the computer overrides your input to determine how much fuel the engine receives to keep the torque output within its designed limit. The system also considers wheelspeed in the equation. More torque can be applied to a wheel/drivetrain already in motion than one at standstill without damage. So if you are completely stopped the system will reduce initial available torque even more than if the the drivetrain was turning. This is why in a mud situation it’s able to power through if you keep momentum. If you stop then there will be even less power available to get going again.
Keep in mind that the total engine output, particularly the diesel, of this vehicle is more than some V-8 engines during the 70’s and 80’s. It is not a question of how much torque the motor can make, it’s a question of how much torque the computer will allow to the drivetrain.
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post #13 of 32 (permalink) Old 01-02-2019, 03:03 AM
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BTW the new Jeep Grand Cherokee Trackhawk is also using the GKS system under license. With 700 hp and 645 lb/ft of torque you can imagine the system in this vehicle was nbuilt far more robustly than ours.....

http://www.gkndriveline.com/en/newsr...-powerful-suv/
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post #14 of 32 (permalink) Old 01-03-2019, 05:14 AM
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Thanks for your input, really enlightening! I've also experienced the situation you described with the accelerator floored but no rpm response. I guess this torque reduction/cut is really to protect the 4WD system? But then again why bother using the terrain system anyway when in most off-road scenarios you'll often encounter traction loss of at least one wheel , and this is precisely where you need the system to work and get you out...unless I haven't fully understood the protection mechanism. I'm also really puzzled as to why the 4WD system has managed to climb that loose rocky surface the first time clearly indicating that it was well within it's capability limits, but the 2nd time that followed immediately (same day-time and approach used) it failed and power was cut.

Last edited by CyTHdiesel2.0; 01-03-2019 at 05:19 AM.
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post #15 of 32 (permalink) Old 01-03-2019, 08:31 AM
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Agree, and I’d love to have a technical bulletin on specifically how the system works, where does it get its inputs, what are the limitations and triggers on those inputs and what effect the different drive (traction) modes have on the limitations. This way it would be possible to modify the way you take on obstacles to avoid complete loss of available torque.
These systems have been around for a long time in aviation. Here they are commonly referred to as governors. Propeller torque especially is governed in turboprop engines. Over-torque and fail the prop-shaft and you come out of the sky (instead of being stuck on the side of the road). The airplane I currently fly has a maximum torque ability of 1800 ft/lbs but it governs the prop torque to 1012 ft/lbs maximum. It will allow 1350 ft/lbs if the prop is spinning at high rpm and airplane is over 140 KIAS. It also doesn’t give you the full amount on acceleration, allowing only 800 ft/lbs until the props reach full speed. All this is to protect the propeller gearbox and propshafts. Exactly the same as what’s happening in the GKN Renegade drivesystem.
Does the system work? How many reports of broken Renegade CV joints or prop-shaft joints have you heard? I’ve not heard a single one in this vehicle, but if you go to the Jeep CJ or any other older tech 4wd forum you will find countless posts on repairing/upgrading rear ends, transfer cases, CV and universal joints that are grenaded by too much torque. Same thing on hot rod builds, if your going to build the engine, first thing to upgrade is the rear differential.
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post #16 of 32 (permalink) Old 01-03-2019, 08:53 AM
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Originally Posted by Fingers View Post
BTW the new Jeep Grand Cherokee Trackhawk is also using the GKS system under license. With 700 hp and 645 lb/ft of torque you can imagine the system in this vehicle was nbuilt far more robustly than ours.....

http://www.gkndriveline.com/en/newsr...-powerful-suv/
Great article link fingers. I've found short snippets but that had pretty good detail.

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post #17 of 32 (permalink) Old 01-03-2019, 09:04 AM
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Originally Posted by CyTHdiesel2.0 View Post
Thanks for your input, really enlightening! I've also experienced the situation you described with the accelerator floored but no rpm response. I guess this torque reduction/cut is really to protect the 4WD system? But then again why bother using the terrain system anyway when in most off-road scenarios you'll often encounter traction loss of at least one wheel , and this is precisely where you need the system to work and get you out...unless I haven't fully understood the protection mechanism. I'm also really puzzled as to why the 4WD system has managed to climb that loose rocky surface the first time clearly indicating that it was well within it's capability limits, but the 2nd time that followed immediately (same day-time and approach used) it failed and power was cut.
Well I am no 4-wheeler, but I have had a similar experience in my old Grand Cherokee SRT. I can say that the torque management is there to protect the 4wd, and also the brakes if equipped with BLSD (brake limited slip differential). Playing around learning its limits and repeated spirited starts around corners was intoxicating. Doing it one time too many would result in a 1-wheel peel with seemingly no movement. Like the awd/blsd just shuts down until things cool off.

There is a look-up table somewhere in the pcm that determines the heat load on the brakes based on how much and how long the BLSD is in use was the consensus... But I wouldn't think slow rock crawling with a wheel in the air would not generate enough heat on the BLSD to effect your 2nd climb.
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post #18 of 32 (permalink) Old 01-03-2019, 09:12 AM
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Originally Posted by CyTHdiesel2.0 View Post
Thanks for your input, really enlightening! I've also experienced the situation you described with the accelerator floored but no rpm response. I guess this torque reduction/cut is really to protect the 4WD system? But then again why bother using the terrain system anyway when in most off-road scenarios you'll often encounter traction loss of at least one wheel , and this is precisely where you need the system to work and get you out...unless I haven't fully understood the protection mechanism. I'm also really puzzled as to why the 4WD system has managed to climb that loose rocky surface the first time clearly indicating that it was well within it's capability limits, but the 2nd time that followed immediately (same day-time and approach used) it failed and power was cut.

You have the diesel, so you have wayyy more torque but I recently experienced how protective the torque reduction is (which is good in this case) when playing around with 9th gear on the highway. I was worried if it lug the engine if speed dropped and sure enough it only allowed enough throttle to keep the engine happy regardless of throttle input.

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post #19 of 32 (permalink) Old 01-03-2019, 01:12 PM
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Originally Posted by 18Rene View Post
Well I am no 4-wheeler, but I have had a similar experience in my old Grand Cherokee SRT. I can say that the torque management is there to protect the 4wd, and also the brakes if equipped with BLSD (brake limited slip differential). Playing around learning its limits and repeated spirited starts around corners was intoxicating. Doing it one time too many would result in a 1-wheel peel with seemingly no movement. Like the awd/blsd just shuts down until things cool off.

There is a look-up table somewhere in the pcm that determines the heat load on the brakes based on how much and how long the BLSD is in use was the consensus... But I wouldn't think slow rock crawling with a wheel in the air would not generate enough heat on the BLSD to effect your 2nd climb.
Got it! I wish there was some kind οf thermometer indicator provided for the 4wd system as well, similar to the coolant, transmission, oil etc. that you can check on driver screen. At least you'll know in advance and decide, or avoid getting stuck due to unexpected 4WD failure.
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post #20 of 32 (permalink) Old 01-03-2019, 01:13 PM
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Originally Posted by Fingers View Post
This is done to prevent damage to the drivetrain. The computer can and will reduce available torque to almost zero if, for example, two of the four wheels are being braked and one of the remaining 2 wheels has more traction than the other. If this did not happen and full available torque was applied to one wheel in the drivetrain (braking the other 3) damage will likely be caused.
That isn't actually correct.
Just considering one axle:
When both wheels grip the diff applies equal torque to both half-shafts - half the propshaft torque to each.
f there is a locking diff and one wheel has no traction the full torque from the propshaft will be applied to the gripping wheel's half-shaft.
With a braked system with open diff the two half-shafts always get half the torque - the slipping wheel is prevented from spinning by the brake, which from the drive line's point of view is the same as being on the ground.
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