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-   -   Driving all four wheels: how is it done? (https://www.team-bhp.com/forum/4x4-technical/90454-driving-all-four-wheels-how-done-28.html)

I think the original question was towing with all wheels on the road, AWD powered off.

Regards
Sutripta

You could start off in 3rd gear in 4L.. Not a problem in most situations

I think you should consider 4L just as reduced gearing. So, I would say, you can drive in which ever gear you want. Lower the gear, lesser the speed and more wheel control.

Thanks for replies :)

Why would one want to drive in 4L in 3rd gear? What speed would be in 3rd gear at 4L? No issues with drive-train? What advantage would it have over 4H in 1st or 2nd gear?

TIA

Consider an obstacle, where you need to gain adequate momentum, but then towards the apex, need more climbing torque. You can start of in 3rd low, and shift to 2nd low, all the while keeping the vehicle moving. 1st high is similar to 3rd low (gear ratio) , but then you need to stop to change from high to low and then to 2nd gear. After which the vehicle might not clear the apex.

Same case with 3rd low then 4th low and then back to 2nd.

During a few OTRs that I went to, while climbing a hill from stand-still, the advice from UB Sir (of Kolkata offroaders) was to engage 3rd in 4L, rev up the engine, then release clutch to get the momentum enough to climb to the top. That almost always worked. Did that many times in my stock Scorpio 4wd. I guess engaging 3rd directly gives the momentum (having to change from 1st to 2nd to 3rd in the middle of the climb invariably makes you lose the momentum and you can't make it), and 4L gives much more torque than 4H, and the combination of above gets you through. While climbing up the Sandakphu trail, most of the times I was on 2nd in 4L (except the hairpins), and even some parts of the last 3 kms. Till now, no damage to drive-train detected.

This thread has been a revelation. Thanks to all who wrote and explained, that after 23 years of driving, I finally understand (well, kind of) what that football-like thingy found on the underside of vehicles actually does.

However, funny thing is, there is a down-side for me now after reading this thread. You see, before I read up all the details here about open and limited diffs, and Sushmita Sen's brother, and scenarios of one wheel hanging in the air and two others slipping, and the torque-genie ignoring the 4th wheel and blah, blah, blah, I never thought twice about taking my Scorpio through different kinds of roads and terrain, including loose sands of Puri and Baruva beaches, or top of the dunes at Hunder, or some really nasty patches of deep mud in Medinipur, or a snow-bound Kunzum pass. Maybe I'm lucky that I didn't get stuck even once (touch-wood) or maybe I knew my limits, or maybe even a remote possibility that I am a damn good driver :D. However, going forward now, I will probably be much more cautious while confronting such terrain, as Shamindra-da's opening question "driving all four wheels: how is it done?" and all 28 pages of this thread will run through my mind first. Maybe for the better. :)

I have a question about the Storme shift-on-fly part time 4x4 system.

Per my understanding, 4H light on center console glows ONLY after
1. transfer case has connected the front output shaft to rear output shaft
AND
2. the front axle disjoint pieces are connected.

A move from 2H to 4H , would need front disconnected shaft's splines to be aligned, before connecting sleeve can move over the splines and join the two parts.
But how do splines align if the car is stationary? And how does the 4H light come up indicating that the second step is complete?

I have tried moving my car from 2H to 4H when stationary, and 4H light glows solid every time. How?
Is there a gap in my understanding? Something that I am missing?

The 4H light comes on as soon as the TC engages the front drive shaft. When the front drive shaft is disengaged, the light goes off. That is all.

I have an XUV500 AWD and this is what the user manual says:

AWD vehicles have to be towed only with the rear wheels lifted off the ground
or on all four wheels. If vehicle needs to be towed by lifting the front wheels,
then the propeller shaft needs to be disconnected in order to protect the
coupling.

All this is fine if the vehicle is towed in front of us. But what if the vehicle is towed away by the cops? How does one ensure that they follow the correct procedure?

I have driven only simple 4WD vehicles, where the differentials can be locked or unlocked, or have LSD. The newer models with terrain modes sounds neat, but I don't understand what they really do.

For example, let's consider the Jeep Compass 4WD system, which has auto/sand/snow/mud modes. I have done extensive driving in snow, sand and mud, and know how to change my driving style to suit the terrain. And it is a combination of how I accelerate, brake, steer and choice of gear ratio.

However, I am unable project this into modes like sand/snow/mud. What does the 4WD system do when each of these modes are selected? And what does it do in auto?

Both my SUVs are AWD with a viscous coupling and no low range option, however, I have rented Wranglers & Grand Cherokees a number of times to drive in treacherous conditions that included a 120 kmph blast on the Coquihalla highway in -20C with snow on the ground. Even if the vehicle had one of these systems (Grand Cherokee has a "Selec-Terrain" system) I have never felt the need to use one and have relied on my personal driving style and experience that was acquired over the years (limited to snow/ice, very rough surface, no rock crawling, very little sand or mud). Having said that I am not implying that these systems do not deliver what they promise; I have just not used one. Yes, I do have a Gypsy back home that we all know does not have any of these fancy gizmos other than a low range transfer case.

A few years ago while working on the B2C side when I was dealing with FCA, I asked a similar question to one of their technical guys and here’s the gist of what I understood/remember:

Snow/Gravel: On any hard but low traction surface such as gravel or compact snow the system changes the throttle mapping to provide a gradual response that will help prevent the wheel spin. The transmission kicks into 2nd gear at take-off that again helps reduce the wheel spin. The electronic centre/rear differential switch to higher preload setting and will come into action sooner. The traction control also switches to steeper response curve. Hill Descent Control that uses the ABS to maintain a slow speed on steep declines is automatically engaged in low range if the vehicle is equipped with one.

Mud: In this setting also the throttle mapping is altered to reduce wheel spinning however the transmission takes off in 1st gear and will stay in 1st under light throttle. The transmission will shift into a higher gear when the throttle is increased. Traction control curve is less aggressive to allow slightly more tire slippage to help generate traction.

Sand: Though in the initial stages one requires gentle throttle, however, once you have gained momentum you would want to maintain enough speed to avoid getting bogged down. In this mode, the system kicks off the throttle gradually but ramps up the response with increased velocity. In the sand mode, the transmission both upshifts & downshifts quickly to keep the engine within its power band. Traction control curve is linear.

Rock: Throttle mapping is very gradual and progressive in this mode as we all know precise throttle application is critical along with brake and steering input. Transmission kicks off in 1st gear, stays there longer and downshifts much quicker to maintain slow speeds. Both HDC & TC have a very aggressive curve to reduce speed quickly on steep declines.

As for the auto mode, I guess the system algorithm takes inputs from engine speed, throttle position, steering movement and the vehicle velocity in addition to other sensors to provide an optimum setting for various drive train systems that would best serve the terrain in question.

I must also mention that this description pertains to a generic "Terrain Response" systems and not specific to Jeep Compass/other FCA vehicles. Such systems on vehicles equipped with locking differentials, LSDs, low range option would function similarly, however, will have pronounced effect on account of these additional 4WD oriented enhancements.

Hope this helps somewhat.

Cross posting from this thread:

What a treasure trove of knowledge this thread is clap: A BIG thanks to the thread starter as well as all those who contributed to it! Read through all of the 28 pages in one go!! Can't believe I missed it all this while.

I have driven part time 4WD vehicles most of my driving life - Classic, Safari 4x4, and Gypsy - and have understood the underlying mechanics quite well. In my humble opinion, these are the simplest of all 4WD systems and by the virtue of it, probably the most reliable and easy/cheap to fix.

My latest acquisition has been the Pajero SFX, which comes with the Super Select 4x4 (SS4-II, as per the user manual) system, bringing along with it a center differential which enables the SUV to be driven in 4WD mode on tarmac as well.

Now for the questions :D

• Is being able to drive on tarmac in 4WD mode the only advantage of having a center differential? If so, what do I gain by this? Better traction on, say, wet highways? Anything else?
• Given that the center differential in a Pajero is LSD, does it make the vehicle any more "off-road capable" compared to the 2H mode?
• What is the torque split across front and rear driveshafts in 4H (open center differential) mode?
• Is the center differential in SS-II system VCU based or planetary gear based ?
• Does the Pajero SFX have a rear LSD as well?

Calling upon Pajero gurus to provide their inputs...

Thanks. Please advise how can we get a Manual Locking rear differential installed from Mahindra in 1. 4/4 scorpio, amd in one 4/2 scorpio. What's the part number. Service center says they can't do it.