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  • Using TC, or rear diff lock, or both together?

    I have been mulling over the question of using TC, or rear diff lock or both together, for some time. As this is a somewhat longwinded discourse, I have hesitated in asking the questions, not wanting to bore many of you. Also, if this is better put in the general areas of the forum rather than the Gen4 specific, please feel free to move.

    As most of you will know, when you activate the rear diff lock, it deactivates the ABS, TC and ASC and I have always wondered why this should be so. The ASC can’t work with the diff lock on, so I had wondered if it being off meant the TC had to be, too, but it is already possible to deactivate ASC and keep TC working. So is the reason because:
    1. it may cause a problem or damage in some circumstances, or
    2. MMC couldn’t be bothered programming it as such, or
    3. TC isn’t effective when the rear diff lock is activated?
    Some time ago I purchased Vladgaun’s traction control activator/deactivator plug-in. As yet I haven’t installed it due to concerns over the first worry and because, to date, the TC has been sufficiently effective that I’ve not needed the diff lock.

    I can envisage that, on sand, activating the diff lock and thereby deactivating TC would be an advantage, whether in 4HLc or 4LLc. However, I assume usually the rear diff lock is activated when in 4LLc and needing to go uphill. So, what are the circumstances that mean, were it available, TC for the front wheels might be required or desirable with the rear diff lock activated? If both front wheels are not, or either one of the rear wheels is not, slipping (that is, the wheels are rotating faster than the corresponding ground speed) then TC wouldn’t come on because, as the centre diff is locked, if either pair is not slipping the other pair won’t be either (even if one of the front wheels is in the air). So, the only circumstances when the TC for the front wheels might activate is if the rear wheels are slipping and one or more of the front wheels is slipping, at a different rate of slippage (or in the air). Any forward motion is generated by the slipping torque from the rear wheels plus the transferred torque on the other front wheel.

    If one is cross axled and rear diff lock is on, then only one rear wheel will be providing forward push. If, as before, there is some slippage in the rear wheel in ground contact, having TC and rear diff activated might be an advantage but is it better than just TC alone, which would still give you 2 wheels pushing?

    Are there other circumstances where TC and rear diff lock operation would both be required or desirable? Does this give rise to potential damage or problems?

    Surely it isn’t that MMC are just slack. Other car makers do the same (deactivating TC with rear diff lock).

    Is any of the above just bovine excrement? Please feel free to argue or provide insight.


    Admin: moved to "Where the rubber hits the road", as the forum nominated for discussion of locking diffs - and this discussion is relevant to more than just Gen 4 Pajero.

  • #2
    I don't know about insight, but I'll add my 2c.

    I bought my NT already fitted with Air Lockers front & rear, and the factory ECU doesn't know they exist. My lockers are always used with the TC enabled.

    I suspect the traction control looks at unexpected acceleration, polling each wheel sequentially, rather than simply speed differences - with all diffs locked on a gnarly climb, my traction control system will randomly flash one or more wheels. So this is one potential reason for Mitsubishi to disable it - if drivers notice only one rear wheel light flashing with the diff locked they would be entitled to believe something has failed. Turn off the TC and the issue can't occur.

    But, personally, I believe your option 2 is most likely - not so much that MMC couldn't be bothered, but couldn't afford it. With a limited R&D budget, I suspect that refining the TC system with rear diff locked was too far down the list of priorities.

    Is both at once required?

    You are correct that, with one front wheel lifted and no wheels turning, the TC on the front won't make a difference either way - only the rear axle would be driving. With open rear diff, if one rear wheel starts to spin, the rear wheel with traction will receive limited torque (limited to whatever torque is required to make the other wheel spin). Braking of the two spinning wheels (one front & one rear) will send more torque to the two wheels with traction, which may or may not be enough for the vehicle to drive through. How much braking force will the TC system apply? Will the brakes apply enough braking torque to load the engine to maximum output?

    Once the rear diff is locked, the rear axle can receive all the torque coming through the drivetrain - regardless of what the front axle is doing (yes, the centre diff must be locked too). As long as one rear wheel has grip the rear axle can drive with all that torque - so locking the rear diff in this scenario can be better than relying on traction control. But with a lifted front wheel, the front axle isn't driving, and until the rear wheels begin to turn, the TC won't do anything - the vehicle is, to all intents and purposes, a rear wheel drive.

    But if the rear wheels (both of them) start spinning, the lifted front wheel must also spin, and traction control would then have an opportunity to intervene, and transfer some drive torque to the grounded front wheel - back to 3wd. Whether or not it would transfer enough drive torque to make a difference is another issue, and would most likely depend on the specific circumstances.

    So, in my opinion, there are circumstances where rear diff lock & front traction control will get you through, where either system on its own wouldn't. How often a Pajero encounters those circumstances is a different discussion.

    NT Platinum. DiD Auto with 265/70R17 ST Maxx, Lift, Lockers, Lockup Mate, Low range reduction, LRA Aux tank, bull bar, winch, lots of touring stuff. Flappy paddles. MMCS is gone!

    Project: NJ SWB. 285/75R16 ST Maxx, 2" OME suspension, 2" body lift, ARB 110, 120l tank, bullbar, scratches, no major dents. Fully engineered in SA. NW DiD & auto in place - a long way to go....

    Scorpro Explorer Box

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    • #3
      Thanks for your thoughts, NJ. I think we are on the same page, mostly.

      However, your paragraph below:

      Originally posted by nj swb View Post
      .....

      Once the rear diff is locked, the rear axle can receive all the torque coming through the drivetrain - regardless of what the front axle is doing (yes, the centre diff must be locked too). As long as one rear wheel has grip the rear axle can drive with all that torque - so locking the rear diff in this scenario can be better than relying on traction control. But with a lifted front wheel, the front axle isn't driving, and until the rear wheels begin to turn, the TC won't do anything - the vehicle is, to all intents and purposes, a rear wheel drive.
      ......
      has made me wonder whether the rear drive axle can accommodate all engine power to one rear wheel and that, therefore TC is better than rear diff lock. That is, if cross axled, rear diff locked and with no rear wheel slippage on the grounded wheel, all engine power is going to that one rear wheel. However, if no rear diff lock and TC on, the engine torque will be shared by two grounded wheels. Torque delivered will be that which the braking of the two spinning wheels achieves. So, less total torque and less torque on each axle?

      Is the rear axle strong enough to take all available torque on one wheel? If not, operationally, depending on the hill ground conditions, is it best to start out without using the rear locker, but if one fails to proceed, engage the rear locker and potentially the TC as well?

      Comment


      • #4
        Originally posted by Mundy55 View Post
        ... made me wonder whether the rear drive axle can accommodate all engine power to one rear wheel and that, therefore TC is better than rear diff lock. That is, if cross axled, rear diff locked and with no rear wheel slippage on the grounded wheel, all engine power is going to that one rear wheel. However, if no rear diff lock and TC on, the engine torque will be shared by two grounded wheels. Torque delivered will be that which the braking of the two spinning wheels achieves. So, less total torque and less torque on each axle?

        Is the rear axle strong enough to take all available torque on one wheel? If not, operationally, depending on the hill ground conditions, is it best to start out without using the rear locker, but if one fails to proceed, engage the rear locker and potentially the TC as well?
        Good question.

        No, the rear axle probably isn't strong enough to take the entire engine output through one wheel. An open diff, even with TC, will never send more than half the available engine output to one wheel. If one wheel has good traction, and the other has none (the scenario under which a locked diff will send all output to one wheel) the wheel with traction will only receive as much torque as the TC can apply braking. So TC will never be as hard on the axles as the locked diff, in a worst-case scenario. But how often will that worst-case scenario eventuate, where the full engine output goes to one wheel, and that wheel doesn't turn?

        TC is reactive - a wheel must spin before it can do anything. With a locked rear diff, the vehicle has a better chance of driving through without spinning a wheel.

        I like my TC, I think it works well, but I've never had to choose between TC and locked diff. I often use my rear locker on steep & loose climbs so that my TC doesn't need to work so hard, but in those circumstances, I have both.

        We need input from somebody with a factory locker and the Traction Control mod.
        NT Platinum. DiD Auto with 265/70R17 ST Maxx, Lift, Lockers, Lockup Mate, Low range reduction, LRA Aux tank, bull bar, winch, lots of touring stuff. Flappy paddles. MMCS is gone!

        Project: NJ SWB. 285/75R16 ST Maxx, 2" OME suspension, 2" body lift, ARB 110, 120l tank, bullbar, scratches, no major dents. Fully engineered in SA. NW DiD & auto in place - a long way to go....

        Scorpro Explorer Box

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        • #5
          Gee I was hoping of just using D and low4, after that if it got to hard use the locker. The rest is irrelevant.

          A bit off track but I watched a 4x4 vid on youtube the other day. For better to worse it was from those lads who also produce a magazine. They were at some pretty amazing tracks, ones like how the hell does a car go up that? Ruts 700mm + deep, holes everywhere, tree roots, 1m plus embankments, off camber, gradients that they had to slide down as they could not walk, that type of stuff.

          They had the usual 6' lift on 35's locked and bombed 4.2 patrols to the same 4" lifted 80LC, all the day down to an Isuzu twin cab on 31's. I would expect the standard 35 to 50mm lift. For what its worth that Isuzu did 75% of what these other highly modified cars did. The winch did the other 25%. No snatching as it was too steep. But it showed line choice can be worth more than many $1000's of modifications. The Isuzu has a rear locker and what ever comes standard. Of course the driver was happy to commit and accept what ever damage was inflicted. To the drivers credit this appeared to be nothing of any significance.

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          • #6
            Originally posted by jaffles View Post
            But it showed line choice can be worth more than many $1000's of modifications.
            Correct. But if you don't know how your 4wd puts its power to the ground, how will you pick the best line?

            NT Platinum. DiD Auto with 265/70R17 ST Maxx, Lift, Lockers, Lockup Mate, Low range reduction, LRA Aux tank, bull bar, winch, lots of touring stuff. Flappy paddles. MMCS is gone!

            Project: NJ SWB. 285/75R16 ST Maxx, 2" OME suspension, 2" body lift, ARB 110, 120l tank, bullbar, scratches, no major dents. Fully engineered in SA. NW DiD & auto in place - a long way to go....

            Scorpro Explorer Box

            Comment


            • #7
              Originally posted by nj swb View Post

              Correct. But if you don't know how your 4wd puts its power to the ground, how will you pick the best line?
              Yes I suppose your right, and yet to find that out in the Pajero. I have not had traction aid before.

              My old car articulated with low gearing, had a live axel open front and a locked bum, and it went everywhere I need to even up some stuff like in the vid lol. Amazing what a rear locker will push through. Must admit a well articulating car with a locker or lockers is a bit of a point and shoot type of deal. Not much though on line required.

              Comment


              • #8
                I'm a little confused here. NJ: are talking about traction control operating on the axle that is locked?

                I have nothing I can add to this conversation, but followingalong with interest

                Comment


                • #9
                  Originally posted by disco stu View Post
                  I'm a little confused here. NJ: are talking about traction control operating on the axle that is locked?

                  I have nothing I can add to this conversation, but followingalong with interest
                  Yes. Those with aftermarket lockers or the traction control mod can use lockers with traction control still active.

                  With all three diffs locked, on rough & loose climbs, my traction control flashes one or more lights randomly. It's hard to tell if it makes any difference to anything.

                  Without diffs locked, on steep & loose climbs, my traction control intervenes a lot, and will flat line easily. If I notice it flashing a lot I'll typically engage the rear locker - after this, the lights flash a lot less, but they do still flash.

                  I can't turn my TC off, so I don't know how much difference the rear locker alone will make.
                  NT Platinum. DiD Auto with 265/70R17 ST Maxx, Lift, Lockers, Lockup Mate, Low range reduction, LRA Aux tank, bull bar, winch, lots of touring stuff. Flappy paddles. MMCS is gone!

                  Project: NJ SWB. 285/75R16 ST Maxx, 2" OME suspension, 2" body lift, ARB 110, 120l tank, bullbar, scratches, no major dents. Fully engineered in SA. NW DiD & auto in place - a long way to go....

                  Scorpro Explorer Box

                  Comment


                  • #10
                    I thought a locked diff was directly opposed to how traction control works. My understanding was that traction control works on single spinner diffs by applying brakes on the slipping wheel to send the power to the other wheel that obviously has traction, exploiting the operation of the diff that normally sends all power to the slipping wheel.

                    Is my understanding correct there?

                    If you have a locked diff then it will send power to both wheels regardless, and the brakes on one wheel would just effect both wheels on that axle. Wouldn't it? That's the part that I'm confused about, but I don't have a lot of knowledge of traction control etc

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                    • #11
                      You're only thinking about the rear axle. With that locked, the front axle is still open. Traction control can still make a difference there.
                      NT Platinum. DiD Auto with 265/70R17 ST Maxx, Lift, Lockers, Lockup Mate, Low range reduction, LRA Aux tank, bull bar, winch, lots of touring stuff. Flappy paddles. MMCS is gone!

                      Project: NJ SWB. 285/75R16 ST Maxx, 2" OME suspension, 2" body lift, ARB 110, 120l tank, bullbar, scratches, no major dents. Fully engineered in SA. NW DiD & auto in place - a long way to go....

                      Scorpro Explorer Box

                      Comment


                      • #12
                        Originally posted by disco stu View Post
                        I thought a locked diff was directly opposed to how traction control works. My understanding was that traction control works on single spinner diffs by applying brakes on the slipping wheel to send the power to the other wheel that obviously has traction, exploiting the operation of the diff that normally sends all power to the slipping wheel.

                        Is my understanding correct there?

                        If you have a locked diff then it will send power to both wheels regardless, and the brakes on one wheel would just effect both wheels on that axle. Wouldn't it? That's the part that I'm confused about, but I don't have a lot of knowledge of traction control etc
                        If you have the rear diff lock engaged and you get to the point where you lose traction on a rear wheel, all the power will go to the other rear wheel. My reading of this is that the Traction Control will not see either rear wheels as locking up. Now, if you lose traction on a front wheel and have the read diff lock engaged, then the traction control will sense this and lock up the spinning wheel, so at least you should have 3 wheel drive. The only time I can foresee problems is if both front wheels are slipping, but if the centre diff is locked at this stage, the front wheels should try to spin at the same speed as the rears. Is a puzzlement...

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                        • #13
                          Yes, cheers. That was why I asked about TC on the locked axle

                          Can you turn it off for one particular axle?

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                          • #14
                            Originally posted by erad View Post
                            The only time I can foresee problems is if both front wheels are slipping, but if the centre diff is locked at this stage, the front wheels should try to spin at the same speed as the rears.
                            Sort of...

                            Front & rear drive shafts are locked in the transfer case, so they will turn at the same speed. Diff output shafts will average the drive shaft speed (divided by the diff ratio).

                            Both front wheels will only spin at the same speed if they both have the same amount of traction. If traction varies between wheels, the wheel with less traction will be easier to spin, reducing the amount of torque it receives from the open diff centre. The wheel with more traction will lose torque also, but because it has more traction, it won't spin at the same speed as the wheel that is spinning with less traction. This should induce a difference in speed - the one with less traction spinning faster, as the one with more traction slows down (due to loss of torque).

                            While both front wheels are spinning at the same speed, theoretically the traction control won't see any need to intervene. It's only once one wheel begins to spin faster than another that TC intervenes - and because the output must average the input speed (divided by the diff ratio), if one wheel begins to spin faster, the other must spin slower, amplifying the speed difference for the TC to detect.

                            But, as I have posted above, with all my diffs locked, my TC sometimes imagines it sees something worthy of its intervention.
                            NT Platinum. DiD Auto with 265/70R17 ST Maxx, Lift, Lockers, Lockup Mate, Low range reduction, LRA Aux tank, bull bar, winch, lots of touring stuff. Flappy paddles. MMCS is gone!

                            Project: NJ SWB. 285/75R16 ST Maxx, 2" OME suspension, 2" body lift, ARB 110, 120l tank, bullbar, scratches, no major dents. Fully engineered in SA. NW DiD & auto in place - a long way to go....

                            Scorpro Explorer Box

                            Comment


                            • #15
                              Originally posted by nj swb View Post

                              SNIP

                              But, as I have posted above, with all my diffs locked, my TC sometimes imagines it sees something worthy of its intervention.
                              Do you think traction control could be that sensitive it may be detecting some type of wear? ​

                              On the other hand there is a lot of gearing to run through, and should not be so tight that some movement is not present.

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