Or: why you want a 3rd diff, and not a coupling.

"Ami in Euro-trim", final test report on the Jeep Grand Cherokee, 4.7L.
Translated from the German OffRoad magazine, issue 11/98.

[jumping the first sections of the article, quite positive, no snide 
remarks other than the note that US industry had to improve the road-going 
qualities of their 4x4's due the general increase of US speed limits]

....[end of positive remark #27]....

....But the future Jeep-leader dissapoints because of a serious deficiency:
 
It is not a true full-time vehicle, not a permanent on all 4 wheels driven 
fourwheeler, like Mercedes ML, Mitsubishi Pajero, Toyota Land Cruiser and 
others, which are significantly lower priced as well. The Jeep has to live 
with a cheap compromise, in which the front axle is engaged automatically, 
continuous variable.

On-road, be it driving through corners or straight ahead, only the rear 
axle is driven, for the full 100%. When the rear wheels spin, the front 
will be engaged when a difference of more than 20rpm occurs.

The new system with the title Gerotor-pump-coupling in the T-case luckily 
comes without electronics. The pump-rotor is connected to the rear axle, 
the pump-housing is connected to the front drive shaft.

When both axles rotate with the same speed, there won't be a pressure 
difference in the system, and the front wheels are only driven by the road. 
Slip on the rear axle and rpm difference in the Gerotor pump causes a 
pressure difference, which engages a clutch plate coupling, to drive the 
front axle.

The responsible drive-train developer Harry T. Page quotes a response time 
of 0.5s before maximum pressure is generated. Chrysler has named this 
drive-split system Quadra-Trac II.

The same Gerotor-pump-couplings can also be integrated in the axle diff's, 
called Vari-Lok.
Quadra-Trac II in the T-case plus 2x Vari-Lok in the axles is then called 
Quadra-Drive by the marketing specialists. You don't need to remember this 
math, but it explains what marketing departments do all day.
(note WJ: I just love that author....:-))

On the next gravel dirt road the Quadra-Drive will be able to prove what 
the theory promises. 
Simply going fast through the corner, and on the sharpest point in that 
corner, also flooring the pedal: 
The rear of the rig breaks loose, the heart stands still, the vehicle at 90 
degrees, and with only futile centimeters away from the ditch, the front 
axle engages and pulls us around the corner in a drift. Stopping....sitting 
down....10 minutes deep breathing...and trying again. But the end result is 
always the same.

Whether the Gerotor-pump-coupling really only needs 0.5s to tranfser the 
2x4 Grand Cherokee in a real 4x4, can neither be proven nor be falsified.

But the problem is not this 0.5s, the problem is that single second of 
fear, in which one does not know how the Grand will finally react. At first 
one drives a VW bug, a rear-skidder, and the next moment a true 4x4 
vehicle. Which of the two must one be prepared for?

Offroad Quadra-Drive operates several classes better than the electronic 
systems in Mercedes ML-class and the new Land Rover Discovery, which in 
fact are only by-products of ABS, or better: a few extra software rules in 
the ABS-programming code. Offroad the Grand Cherokee pilot drives in low 
gear, with a forced lenght-wise locked drive train. And there it doesn't 
matter whether the Gerotor-couplings in front and rear axle require 0.5s or 
1s to lock up completely. One can be sure of 4 driving wheels. Passing 
ditches diagonally, where only two wheels have contact with the ground, has 
suddenly lost its fear. Even the theoretical situation of 3 wheels on ice, 
and one front wheel on grippy surface, can be mastered with the Grand.

But even more important, on loose gravel or sand, where the 4-ETS in the 
Mercedes ML strangles the engine with smoking brake pads, the Grand keeps 
pushing over all 4 wheels and gets through.

Is there a solution to this dilemma?

In the USA the 4.0L is supplied with the real Quadra-Trac [note WJ: error 
author, should be Select-Trac], which has been offered since mid-96. This 
Quadra-Trac T-case (NV242) consists of a real planet-gear differential, 
with a front/rear split of 50/50, exactly what a noble (expensive) offroad 
vehicle is entitled to. Chrysler Germany has a different meaning, and won't 
import this vehicle.

[addendum from the Dutch 4x4 mag 'Captain', issue 985/October, in an 
article about a Jeep winter demo in the French alps, organized by the Dutch 
distributor]

....we will only learn later on that there are good reasons to exchange the 
drive train system of the Grand Cherokee with a faster acting 
mechanisme....

....the second fluid coupling in the drive train, unique for the Grand, 
appears to be a handicap sometimes. The treacly visco drive locks up slower 
than the front wheels dig into the snow. When the front wheels are engaged 
through the visco coupling, there has already a snow ramp been build where 
the Grand subsequently gets stuck in. We know now by own experience why 
Jeep has chosen for a new technology to enage the front wheels 
automatically. From model year 1999 there will be new hydraulic couplings 
who can engage faster than the classic visco coupling.
(how little do these poor guys know....;-((


[final note WJ: it doesn't happen every day that people on Usenet see a 
homepage chapter being born under their very eyes. But this is one such 
day....;-))....]

If you have any question, remark, comment, want to share some philosophy or just want to express your opinion about these pages, feel free to send email to: w.j.markerink @ a1.nl