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Diff locks (operation)

Tom109

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Part time 4wd is rear wheel drive until you put it in 4wd. So you commute to work M-F in rear wheel drive.

The Gren is All wheel drive all the time. So you commute to work M-F with all wheels driven.

The Gren center diff is open for the same reason to do not lock an axle diff for on-road driving. If you lock the center diff, it is now similar to engaging your part time 4wd.

With part time 4wd disengaged, or rear wheel drive, the vehicle would be stuck if both rear wheels are on rollers.
 

Lollo050968

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I copy and paste it from the German Wikipedia Side, in my opinion a good explanation of above topic.
and the translation by deepl.:

Differenzialgesteuerter (permanenter) Allradantrieb​

Der permanente Allradantrieb mit Zentraldifferenzial
1: Motor
2: Zentraldifferenzial
Bei Fahrzeugen mit permanentem Allradantrieb wird die Motorleistung ständig auf alle vier Räder übertragen und durch die Differenziale wird der vollständige Drehzahlausgleich ohne Leistungsverluste gewährleistet. Um Verspannungen im Antriebsstrang zu vermeiden, wird ein zusätzliches Zentraldifferenzial zwischen Vorder- und Hinterachse eingebaut. Der Nachteil hierbei ist jedoch, dass bei fehlender Bodenhaftung eines einzelnen Rades oder einer Achse das übertragbare Antriebsmoment durch dieses Rad oder diese Achse begrenzt wird, sodass ein Fahrzeug im Extremfall nicht mehr aus eigener Kraft bewegt werden kann. Aus diesem Grund wird bei Fahrzeugen mit permanentem Allradantrieb häufig entweder das Zentraldifferenzial als Sperrdifferenzial ausgeführt (beispielsweise die Torsen-Differenziale in früheren Audi-quattro-Modellen mit längseingebautem Motor), oder es gibt elektronische Traktionshilfen wie Antriebsschlupfregelung (ASR). In Geländewagen wie zum Beispiel dem Land Rover Defender steht dagegen ein manuell sperrbares Mittendifferenzial zur Verfügung, das je nach Untergrundbeschaffenheit vom Fahrer gesperrt werden kann.

Durch die Bauart des Zentraldifferenzials wird bestimmt, ob das Antriebsmoment gleichmäßig (50 : 50) oder ungleichmäßig auf beide Achsen aufgeteilt wird. Die Verteilung ist für das Fahrverhalten maßgebend. Da an Steigungen und beim Beschleunigen durch die dynamische Achslastverschiebung zusätzlich Gewicht auf die Hinterachse verlagert wird, ist es üblich, auch beim Antriebsmoment einen höheren Anteil auf die Hinterachse zu übertragen. Daher werden in vielen Fahrzeugen die Drehmoment-Aufteilungen zwischen Vorderachse und Hinterachse von 45 : 55 (V : H) bis 33 : 67 gewählt. Diese Verteilung ist fest. Lässt sich aber das Zentraldifferenzial sperren, kann die Leistung nach der an den Achsen gegebenen verteilt werden. Auch Zentraldifferenziale mit elektronischer Reiblamellenkupplung lassen Kraftverteilungen bis 100 : 0 und 0 : 100 zu.
Differential-controlled (permanent) all-wheel drive

Permanent all-wheel drive with central differential
1: Engine
2: Central differential
In vehicles with permanent all-wheel drive, the engine power is constantly transmitted to all four wheels and the differentials ensure complete speed compensation without any loss of power. To avoid tension in the drive train, an additional center differential is installed between the front and rear axles. The disadvantage of this, however, is that if a single wheel or axle does not grip the ground, the transmissible drive torque is limited by this wheel or axle, meaning that in extreme cases a vehicle can no longer be moved under its own power. For this reason, in vehicles with permanent all-wheel drive, either the center differential is often designed as a limited slip differential (e.g. the Torsen differentials in earlier Audi quattro models with longitudinally mounted engines), or there are electronic traction aids such as traction control (ASR). In off-road vehicles such as the Land Rover Defender, on the other hand, a manually lockable center differential is available, which can be locked by the driver depending on the surface conditions.

The design of the center differential determines whether the drive torque is distributed evenly (50:50) or unevenly to both axles. The distribution is decisive for the driving behavior. As additional weight is shifted to the rear axle on inclines and when accelerating due to the dynamic axle load shift, it is common practice to transfer a higher proportion of the drive torque to the rear axle. For this reason, the torque distribution between the front axle and rear axle in many vehicles ranges from 45 : 55 (V : H) to 33 : 67. This distribution is fixed. However, if the center differential can be locked, the power can be distributed according to the power given to the axles. Central differentials with electronic multi-plate clutches also allow power distribution up to 100 : 0 and 0 : 100.

Kupplungsgesteuerter (zuschaltbarer) Allradantrieb​

Der mit Klauenkupplungzuschaltbare Allradantrieb
1: Motor
2: Klauenkupplung
Bei Fahrzeugen mit zuschaltbarem Allradantrieb wird unter normalen Fahrbedingungen nur eine Achse angetrieben. Erst bei glatter Fahrbahn oder auf rutschigem Untergrund wird der Antrieb der zweiten Achse zugeschaltet. Dadurch wird die Traktion des Fahrzeugs erhöht.

Für das Zuschalten der zweiten Antriebsachse gibt es verschiedene Bauarten:

  • Im einfachsten Fall wird eine Klauenkupplung an der Antriebswelle geschlossen.
  • Ein sperrbarer Freilauf an der Radnabe verbindet die Antriebswelle mit dem Rad (bei manuell schaltbaren Naben muss das Fahrzeug verlassen werden).
Durch den fehlenden Drehzahlausgleich der starren Verbindung kann es zu Verspannungen im Antriebsstrang und gelegentlichen Geräuschen beim Einparken kommen. Bei Kurvenfahrt folgen die Vorderräder einem größeren Radius und müssen sich schneller drehen als die Hinterräder, werden aber gezwungen, sich genauso schnell wie die Hinterräder zu drehen. Das führt – je nach Auslegung des Fahrwerks – zu reduziertem Übersteuern oder verstärktem Untersteuern. Dieses kostengünstig zu produzierende Allradantriebssystem wurde häufig in der Kleinwagenklasse verwendet. Als Beispiele hierfür seien der Fiat Panda 4×4 (der Jahre 1983–2003), Subaru Justy und Subaru Vivio sowie der Citroën AX 4×4 genannt.
drive

The all-wheel drive that can be engaged with a dog clutch
1: Engine
2: Claw clutch
In vehicles with selectable all-wheel drive, only one axle is driven under normal driving conditions. The drive of the second axle is only engaged when the road is slippery or the surface is slippery. This increases the traction of the vehicle.

There are various designs for engaging the second drive axle:

In the simplest case, a claw clutch is closed on the drive shaft.
A lockable freewheel on the wheel hub connects the drive shaft to the wheel (in the case of manually shiftable hubs, the vehicle must be left).
The lack of speed compensation of the rigid connection can lead to tension in the drivetrain and occasional noises when parking. When cornering, the front wheels follow a larger radius and have to turn faster than the rear wheels, but are forced to turn just as fast as the rear wheels. Depending on the design of the chassis, this leads to reduced oversteer or increased understeer. This low-cost all-wheel drive system was frequently used in the small car class. Examples include the Fiat Panda 4×4 (1983-2003), Subaru Justy and Subaru Vivio as well as the Citroën AX 4×4.
 

MileHigh

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Rear wheel drive? I think a lot of all wheel drive systems are front wheel biased and the rear is almost a vestigial limb that engages when it has to and allows the vehicle to be called “4WD”…


So the IG has an ESC button that I assume is (electronic stability control) on the over head. And it is marked “Off” so I assume it is on by default and must be turned off to defeat it. So in the german video I assume that they have the ESC off. I also thought when you put it into low it turns off ESC, but that might be my 4Runner.

From what I have seen on videos from vehicles in general is ESC, using the brakes to ‘quiet’ spinning wheels and let wheels with traction transfer power and move the vehicle. That adds heat to the system and you can eventually cause issues.

A lot of vehicles depend on the ESC system for their 4wheel prowess, but it only has limited usability- something that you’d probably never run into in standard driving.

BUT, when you are off road, you can overwork an ESC system. That’s where the IGs system of full time 4WD and lockers doesn’t suffer from that same thing. It will work all day.

At least that is what I see- and I am FAR from a 4WD expert.
 

[ Adam ]

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Honest question: can someone who understands the mechanics of the Grenadier's full-time 4-wheel drive system explain under what conditions it will provide the equivalent of "all-wheel drive", and why it was immobilized by having one tire on a no-traction surface. Thanks.

Open differentials (without traction control) send power to the axle (or drive shaft) with the least amount of grip or resistance. Whether that be three open differentials (like the Grenadier or Defender) or a 2wd Jeep with only the rear differential powered. Only by locking ANY differential (front, center or rear) do you split the delivery of the power across the differential. By locking the center differential you split torque 50/50 between the front and rear, and by locking the rear you will split torque between the rear left and right. The same applies for the front.

This is why, based purely on mechanical, open differentials the Grenadier was immobilized by lifting a single wheel.

The Grenadier does employ "brake traction control" or ETC, as illustrated in this video:
View: https://youtu.be/lLFzv7FJy3g?si=oDZ94pqqzP1PudvU&t=332


My assumption is that in the previously referenced video, they have turned ETC off (if this is possible) for purposes of illustration. Brake traction control, coupled with a locked or unlocked center differential can be quite capable. - as illustrated here:
View: https://youtu.be/LUdQ4WgRbgE?si=T9sxYdoBiFYn82q-&t=60


Regarding "all-wheel drive", it really depends on what system you are using. Modern "all-wheel drive" systems simulate locking differentials by monitoring the wheel speed sensors and applying the brakes and/or by using limited slip and or electronic locking differentials in the front, center and rear.
 
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Krabby

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What Setup did RRCs and D Is and IIs use? The same as the Gren type?
 

[ Adam ]

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What Setup did RRCs and D Is and IIs use? The same as the Gren type?
I'm not sure when the Range Rover got traction control, but for Discovery it was the II (~1999) and for the Defender it was the Td5 (~2006) era trucks.
 

Tom109

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What Setup did RRCs and D Is and IIs use? The same as the Gren type?
D1's had locking center diff and open axles.

RRC had a Borg Warner viscous coupling in the T-case, and open diffs. Later RRC's added rear axle traction control.
 

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Do we have an answer why front and rear diff locks only work in low? Can someone please explain me the logic behind it? I am sure Ineos has a very good reason but I don't get it.
 
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Part time 4wd is rear wheel drive until you put it in 4wd. So you commute to work M-F in rear wheel drive.

The Gren is All wheel drive all the time. So you commute to work M-F with all wheels driven.

The Gren center diff is open for the same reason to do not lock an axle diff for on-road driving. If you lock the center diff, it is now similar to engaging your part time 4wd.

With part time 4wd disengaged, or rear wheel drive, the vehicle would be stuck if both rear wheels are on rollers.
Yes, all this I understand. You are describing 4-wheel drive at its most basic level, and that is not what I am asking about. Four wheel drive vehicles are not new to me. I learned to drive in a 1966 International Scout, driving in 4-wheel drive on the beach when I was 12. I've been driving 4x4s since 1983 (1975 FJ-40, 1978 CJ-7, 5.9 Cummins, 3 Toyota pickup trucks, 2008 Jeep JKU); every one of these was manual transmission, manual transfer case, and half of them had manual locking front hubs. I've also owned a couple of Audi all-wheel drive sedans (2008 RS4, 2012 S4). While I've never owned a full-time four wheel drive 4x4, I understand the concept. What I don't understand is how a full-time 4x4 can be stopped dead in its tracks with one tire on a roller. I appreciate your reply, but it hasn't helped answer that question.

Based on the section of the video that starts at 3:30, in which the Grenadier was unable to make forward progress while in "street mode" (full-time 4-wheel drive, center diff open), with one tire on a roller, we can imagine four scenarios in which one tire loses all traction. In all four scenarios, lets compare the "full-time 4-wheel drive" Grenadier to a "part-time 4-wheel drive" Jeep Wrangler. In all four scenarios, the Grenadier is in full-time 4-wheel drive, with the center diff open, and the Jeep is in rear-wheel drive:

1: front left tire with no traction
Grenadier is stuck
Jeep Wrangler drives away in rear wheel drive

2: front right tire with no traction
Grenadier is stuck
Jeep Wrangler drives away in rear wheel drive

3: rear left tire with no traction
Grenadier is stuck
Jeep Wrangler is stuck

4: rear right tire with no traction
Grenadier is stuck
Jeep Wrangler is stuck

The Grenadier is stuck in all four scenarios - just like it was at 3:30 in the video. One would then need to lock the center diff, thereby sending power 50/50 to both the front and rear axles.

In two of the four scenarios the rear-wheel drive Jeep is able to drive away - no need to shift into 4-wheel drive.

In the other two scenarios, the Jeep is stuck - just like the Grenadier - and you would need to shift into 4-wheel drive, thereby sending power 50/50 to both the front and rear axles.

I am really surprised by the poor performance of the Grenadier on a single roller - with the center diff open. Of course, Audi Quattro is a road-biased all-wheel drive system, designed for different use than the Gren or any other off-road 4x4. But if one was to compare apples to oranges, the Audi can drive away with three tires on rollers (i.e. only one wheel getting power).

So what's my point? Its not to bash on the Gren - so no need for anyone to feel defensive. On the contrary, full-time 4-wheel drive was one of the features of the Gren that - in my mind - put it above part-time 4x4s like the Wrangler. But the demonstration in this video suggests to me that - when driving on roads that alternate between "decent traction" and "poor traction" (common in the winter around here) - one will need to lock the center differential in the same way that one shifts into 4-wheel drive in a Wrangler, Tacoma, or any other part-time 4-wheel drive. To me, that is both surprising and disappointing. It had been my assumption that the Grenadier's "full-time four wheel drive" system was going to be better than this test makes it seem, and that it would be a significant step up from a part-time four wheel drive vehicle that needs to be shifted in and out of 4-wheel drive on wintry roads.
 
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[ Adam ]

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I am really surprised by the poor performance of the Grenadier on a single roller - with the center diff open. Of course, Audi Quattro is a road-biased all-wheel drive system, designed for different use than the Gren or any other off-road 4x4. But if one was to compare apples to oranges, the Audi can drive away with three tires on rollers (i.e. only one wheel getting power).

I would advocate for engaging the center diff lock as soon as you go off a paved surface. As you've illustrated, an open center differential is a detriment off road.
 
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Open differentials (without traction control) send power to the axle (or drive shaft) with the least amount of grip or resistance. Whether that be three open differentials (like the Grenadier or Defender) or a 2wd Jeep with only the rear differential powered. Only by locking ANY differential (front, center or rear) do you split the delivery of the power across the differential. By locking the center differential you split torque 50/50 between the front and rear, and by locking the rear you will split torque between the rear left and right. The same applies for the front.

This is why, based purely on mechanical, open differentials the Grenadier was immobilized by lifting a single wheel.

The Grenadier does employ "brake traction control" or ETC, as illustrated in this video:
View: https://youtu.be/lLFzv7FJy3g?si=oDZ94pqqzP1PudvU&t=332


My assumption is that in the previously referenced video, they have turned ETC off (if this is possible) for purposes of illustration. Brake traction control, coupled with a locked or unlocked center differential can be quite capable. - as illustrated here:
View: https://youtu.be/LUdQ4WgRbgE?si=T9sxYdoBiFYn82q-&t=60


Regarding "all-wheel drive", it really depends on what system you are using. Modern "all-wheel drive" systems simulate locking differentials by monitoring the wheel speed sensors and applying the brakes and/or by using limited slip and or electronic locking differentials in the front, center and rear.
Thanks for the reply! Unfortunately, I understand all this. I just assumed that the Gren would have some sort of brake traction control when in "full-time four wheel drive" or "all-wheel drive" or whatever we are calling its "street mode" (center diff open). I am sure that the Gren has some sort of traction control when in street mode, but it doesn't seem capable of coping with a situation in which one tire has lost all traction. So that still leaves me wondering under what conditions is it actually going to be useful, or will one have to lock the center diff whenever road conditions are low-traction. NOTE: I am just talking about on-road driving, and specifically, when driving in winter conditions.
 

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1: front left tire with no traction
Grenadier is stuck
Jeep Wrangler drives away in rear wheel drive

2: front right tire with no traction
Grenadier is stuck
Jeep Wrangler drives away in rear wheel drive

3: rear left tire with no traction
Grenadier is stuck
Jeep Wrangler is stuck

4: rear right tire with no traction
Grenadier is stuck
Jeep Wrangler is stuck
1 & 2 above, I would say the Gren center diff should be locked to equate to the jeep system.

Observation - If you were going to be on-wheeled in the jeep, you shift to 4wd. If you’re going to be one-wheeled in the Gren, you just lock the center diff (which is simpler operation and possible at speed).

All I can tell you is in 27 years of driving them, I have never been one-wheel-stranded in a D1 or RRC. Yes, fancy ETC systems are like behind the scenes magic, but do have their limitations. While the Gren system requires driver intervention (throw a lever, push a button) it is mechanical and will work as long as you have fuel.
 
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I would advocate for engaging the center diff lock as soon as you go off a paved surface. As you've illustrated, an open center differential is a detriment off road.
Yes, of course. Driving off-road with the center diff open is not good for the drive train.

I should have made it clear that I am talking about driving on-road, and specifically in winter conditions. I had hoped that the Grenadier's "street mode" (full-time 4-wheel drive with center diff open) would provide good traction on snowy and icy roads, and that it would not be necessary to shift the transfer case (locking and unlocking the center diff as conditions vary). I have spent the last 40 years of my life shifting in and out of 4-wheel drive in winter conditions (i.e. variable traction). That's just what you do in a part-time 4-wheel drive (Jeep Wrangler, etc.). I was really looking forward to owning a 4x4 with a full-time four wheel drive system, and it had been my assumption that the Grenadier's full-time 4-wheel drive system (with the center diff open), would provide good traction for on-road driving in the winter.
 

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The answer
Yes, all this I understand. You are describing 4-wheel drive at its most basic level, and that is not what I am asking about. Four wheel drive vehicles are not new to me. I learned to drive in a 1966 International Scout, driving in 4-wheel drive on the beach when I was 12. I've been driving 4x4s since 1983 (1975 FJ-40, 1978 CJ-7, 5.9 Cummins, 3 Toyota pickup trucks, 2008 Jeep JKU); every one of these was manual transmission, manual transfer case, and half of them had manual locking front hubs. I've also owned a couple of Audi all-wheel drive sedans (2008 RS4, 2012 S4). While I've never owned a full-time four wheel drive 4x4, I understand the concept. What I don't understand is how a full-time 4x4 can be stopped dead in its tracks with one tire on a roller. I appreciate your reply, but it hasn't helped answer that question.

Based on the section of the video that starts at 3:30, in which the Grenadier was unable to make forward progress while in "street mode" (full-time 4-wheel drive, center diff open), with one tire on a roller, we can imagine four scenarios in which one tire loses all traction. In all four scenarios, lets compare the "full-time 4-wheel drive" Grenadier to a "part-time 4wheel drive" Jeep Wrangler. In all four scenarios, the Grenadier is in full-time 4-wheel drive, with the center diff open, and the Jeep is in rear-wheel drive:

1: front left tire with no traction
Grenadier is stuck
Jeep Wrangler drives away in rear wheel drive

2: front right tire with no traction
Grenadier is stuck
Jeep Wrangler drives away in rear wheel drive

3: rear left tire with no traction
Grenadier is stuck
Jeep Wrangler is stuck

4: rear right tire with no traction
Grenadier is stuck
Jeep Wrangler is stuck

The Grenadier is stuck in all four scenarios - just like it was at 3:30 in the video. One would then need to lock the center diff, thereby sending power 50/50 to both the front and rear axles.

In two of the four scenarios the rear-wheel drive Jeep is able to drive away - no need to shift into 4-wheel drive.

In the other two scenarios, the Jeep is stuck - just like the Grenadier - and you would need to shift into 4-wheel drive, thereby sending power 50/50 to both the front and rear axles.

I am really surprised by the poor performance of the Grenadier on a single roller - with the center diff open. Of course, Audi Quattro is a road-biased all-wheel drive system, designed for different use than the Gren or any other off-road 4x4. But if one was to compare apples to oranges, the Audi can drive away with three tires on rollers (i.e. only one wheel getting power).

So what's my point? Its not to bash on the Gren - so no need for anyone to feel defensive. On the contrary, full-time 4-wheel drive was one of the features of the Gren that - in my mind - put it above part-time 4x4s like the Wrangler. But the demonstration in this video suggests to me that - when driving on roads that alternate between "decent traction" and "poor traction" (common in the winter around here) - one will need to lock the center differential in the same way that one shifts into 4-wheel drive in a Wrangler, Tacoma, or any other part-time 4-wheel drive. To me, that is both surprising and disappointing. It had been my assumption that the Grenadier's "full-time four wheel drive" system was going to be better than this test makes it seem, and that it would be a significant step up from a part-time four wheel drive vehicle that needs to be shifted in and out of 4-wheel drive on wintry roads.
You are correct... and the vehicles reacted mechanically exactly as would be expected in the absence of any other interventions. I don't speak German so I don't know if there was anything said which indicated what they'd done in terms of disabling the traction control on the Grenadier. Under normal circumstances the Grenadier's traction control system should enable it also to drive off three rollers when only one wheel has traction. (The sophistication of the traction control is most evident in its "downhill assist" mode.) See also the video posted by @junx on the comparison done by Robert Pepper where the Grenadier's traction control system was on display)
Perhaps one of our German-speaking forum friends can enlighten us as to whether the exact conditions of the 4 x roller test were explained.
 
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1 & 2 above, I would say the Gren center diff should be locked to equate to the jeep system.

Observation - If you were going to be on-wheeled in the jeep, you shift to 4wd. If you’re going to be one-wheeled in the Gren, you just lock the center diff (which is simpler operation and possible at speed).

All I can tell you is in 27 years of driving them, I have never been one-wheel-stranded in a D1 or RRC. Yes, fancy ETC systems are like behind the scenes magic, but do have their limitations. While the Gren system requires driver intervention (throw a lever, push a button) it is mechanical and will work as long as you have fuel.
I remember driving over the Middlebury gap in an snow storm in my fwd volvo V70. the Traction cotrol tried to work but the engine was not powerful enough to both use the brakes to slow the spinning wheel and still lift the Volvo up the steep slope. That is the edge case for brake powered traction control.
 

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To answer why we need open center diffs, it's because when driving around all 4 wheels might be traveling at different speeds. You can't lock a diff when driving on roads for the reason. It might damage the drivetrain or it might make you spin out of control.
 

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Yes, of course. Driving off-road with the center diff open is not good for the drive train.

I should have made it clear that I am talking about driving on-road, and specifically in winter conditions. I had hoped that the Grenadier's "street mode" (full-time 4-wheel drive with center diff open) would provide good traction on snowy and icy roads, and that it would not be necessary to shift the transfer case (locking and unlocking the center diff as conditions vary). I have spent the last 40 years of my life shifting in and out of 4-wheel drive in winter conditions (i.e. variable traction). That's just what you do in a part-time 4-wheel drive (Jeep Wrangler, etc.). I was really looking forward to owning a 4x4 with a full-time four wheel drive system, and it had been my assumption that the Grenadier's full-time 4-wheel drive system (with the center diff open), would provide good traction for on-road driving in the winter.
Hello @stickshifter
I believe you are confusing drive and traction. If one wheel looses traction, it doesn't mean that the other three have lost traction, they just loose drive, as all the drive is going to the wheel that has lost all traction. One of the benefits of a constant four wheel drive set up like the Grenadier is that the torque from the engine is being spread across all four wheels, which means that there is less torque per wheel than a two wheel drive, which means less chance of spinning.

As an example, I had a Range Rover break it's rear differential on a trip. We removed the rear axles and prop shaft so that it was only front wheel drive. For the drive out, I left the centre diff unlocked in my Land Rover, so I would be able to see where he might have trouble. My Land Rover climbed all the greasy hills on the road out without spinning a wheel, but I needed to tow the Range Rover up two of the hills, as in two wheel drive, he could not climb them.

As others have stated, the centre diff should be locked when used on any low traction surface as they are only designed to handle the small differences in axle speeds when cornering on high traction surfaces. As long as you do not have wheel spin, you should be able to lock and unlock the centre diff at any speed without lifting off the accelerator.

Regards
Craig Murray
 

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I remember driving over the Middlebury gap in an snow storm in my fwd volvo V70. the Traction cotrol tried to work but the engine was not powerful enough to both use the brakes to slow the spinning wheel and still lift the Volvo up the steep slope. That is the edge case for brake powered traction control.
Middlebury gap brings back painful memories. Every year, I would do Killington stage race and stage 3 had us go over Middlebury gap.
 

MileHigh

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The answer

You are correct... and the vehicles reacted mechanically exactly as would be expected in the absence of any other interventions. I don't speak German so I don't know if there was anything said which indicated what they'd done in terms of disabling the traction control on the Grenadier. Under normal circumstances the Grenadier's traction control system should enable it also to drive off three rollers when only one wheel has traction. (The sophistication of the traction control is most evident in its "downhill assist" mode.) See also the video posted by @junx on the comparison done by Robert Pepper where the Grenadier's traction control system was on display)
Perhaps one of our German-speaking forum friends can enlighten us as to whether the exact conditions of the 4 x roller test were explained.

Your comment about the hill descent control implies that there is an electronic aid working. that teh Hill descent button is right next to the ESC button makes me think that they are related.
 
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