Marius, unfortunately I know very little about it. Mitsubishi was one of the first manufacturers that fitted an ABS system to its 4x4 vehicles that functioned when 4HLc or 4LLc (equivalent to part time in 4wd mode) was selected.4ePikanini wrote:Can you (or anyone else) expand on this? I have never heard of this before. I'd like to know more about it. Gen3/Gen4?Gerrit Loubser wrote:....... Also, the ABS brakes are never de-activated, but the Pajero has a so-called Multimode ABS with different callibration settings for 2H and 4H than for 4HLc and 4LLc.
I will try to illustrate why this was significant:
One ABS control strategy would be to estimate the true vehicle speed in some way and then to compare this value with the wheel speeds in order to decide how to modulate the brake pressure on individual wheel brakes. It is possible to use the wheel speed sensors themselves to estimate the true vehicle speed, especially as the ABS system knows what the brake pressure is at all wheels and can determine which wheel is most likely to represent the vehicle speed most accurately... that is, if the wheels are each free to accelerate and decelerate at their own rates.
It is then also relatively easy to determine whether wheel deceleration is the result of the whole vehicle decelerating without wheel slip or whether it is the result of impending wheel lock-up and slip; in the latter case the rate of deceleration is much greater, because essentially it is the inertia of only the wheel and sideshaft that is resisting deceleration, whereas it is the inertia of the whole vehicle in the former case.
In a 2 wheel drive vehicle (or a full time 4wd with all open differentials), all of the wheels are relatively free to decelerate at their own rate under the action of the brakes, but once a part time four wheel drive drivetrain (or Super Select drivetrain in 4HLc or 4LLc) is thrown into the mix, the wheels at the front and those at the rear are connected together and this means that it becomes more difficult to estimate the true vehicle speed using wheel speed sensor data. Applying brake torque to a wheel at one axle also results in brake torque being transferred to the other axle via the drivetrain and this might cause slip at a completely different wheel. This makes it difficult to determine how to modulate the brake pressure to rectify the situation. In such a scenario the brakes are also called upon to decelerate a much more substantial inertia than only a wheel and sideshaft; now propshafts, differentials, transfer box gears and more wheels with sideshafts suddenly enter the equation, which means that the deceleration rate might be much lower (much more like the rate achieved when the vehicle comes to a panic stop without slip on a high grip surface, for example) and impending slip much more difficult to identify as a result.
In the earlier days of ABS, many manufacturers simply decided that they would simply turn the ABS off when 4 wheel drive was selected and that would be that. Later ABS systems were developed that incorporated accelerometers to determine the vehicle's rate of deceleration. This value could then be compared with individual wheel rates of deceleration and greatly assisted in the determination of the brake pressure modulation strategy to follow in a particular situation, even if the vehicle speed could not be determined accurately. The Mitsubishi Multimode ABS system is such a system incorporating an accelerometer rendering it operable in any of the Super Select modes (hence the name). The system was launched when the Gen 2 Pajero was introduced in 1991 and has featured in both the Gen 3 and Gen 4 as well.
The transfer case switches that indicate whether four wheel drive is selected and whether the center diff is locked and also the switch in the rear diff that indicates whether the rear diff is locked all feed their information into the ABS ECU to enable it to act appropriately. I don't know the details of the Multimode ABS control strategies, but clearly it would be possible for the ABS ECU to react differently when in 2H than when in 4HLc or when the rear diff is also locked. It would also be possible to alter the control strategy to allow more wheel slip when in in 4LLc (say) than in the other modes (something that is done in some modern ABS systems), but I don't know if this is the case with the Mitsu system.
There has been a lot of debate of the years over whether ABS is actually a help or a hindrance in off-road situations. I have, however, never found the ABS in my Gen 3 to let me down in the rough. I have definitely felt the ABS working in the sand (say), but have never felt that it caused a run away situation, something that the Colt guys has mentioned several times (As far as I know, the Colt's system is very similar to the Pajero's).
