LT95 Salisbury Powr-Lok lockable limited-slip center differential - LSD

The legendary first few 1970 Range Rover's with limited slip center differential gearbox

The first Range Rovers built in 1970, both the pre-production with reg numbers YVB1**H, the press 
vehicles NXC2**H and the early vehicles for sale to customers had the legendary Salisbury Powr-Lok lockable limited slip center differential in the LT95 4 speed gearbox.
In addition to the limited slip function in center differential, the center differential could also be locked
via a vacuum actuator knob close to the gear shift lever, exactly as we know about with all later
manual 4 speed Range Rovers during the 1970s.

It is not clear exact how many Range Rover's that had the limited slip differential, but most sources
say it was delivered against the end of 1970. Most probably maximum around 300 vehicles.

The LT95 4 speed gearbox with integrated transfer box for high and low range selection was 
developed to suit two Land Rover projects - the Range Rover and the military Land-Rover 
Forward Control FC101. To achieve best possible strength to withstand the power from the V8
engine the transfer box was designed with permanent four-wheel drive via a third centre differential.
This center differential had both limited slip function with friction discs to secure best possible 
traction during regular driving. In addition the center differential could be locked to secure that both
axles had drive under extreme slippery conditions or if one wheel had no traction at all.

Salisbury Transmissions in UK developed and supplied the center differentials to the new LT95
gearbox. Salisbury had for many years been a trusted supplier of axles and axle differentials to
Land Rover. The Salisbury differentials are easily identified by its red color, either used in the
axles or in the LT95 gearbox.
All Range Rover with Suffix A LT95 gearbox was originally produced with red Salisbury center differentials. Important here is to be aware that the Range Rover #VIN Suffix A serial numbers 
and the LT95 Suffix A serial numbers did not follow the same path and are not compliant to each 
other serial numbers. LT95 Suffix B and later serial number gearboxes had a different type of
center differential.

The red Salisbury center differential that was installed in all Suffix A LT95 gearboxes are identical
either with the Powr-Lok limited slip function of the very early Range Rovers or the later Suffix A            The limited slip friction discs in the Range Rover LT95 gearbox 
LT95 gearboxes without the limited slip function. Only the setup with limited slip friction discs in
limited slip version differ from the later one without the limited slip function. 
The vacuum actuated lockable center differential lock function are identical.

In the 'Range Rover - Technical Details' publication 789 from June 1970 is written;
'Transfer Box: Two-speed reduction on main gearbox output. Front and rear drive permanently
engaged via a lockable limited slip differential.' 

The discontinuation of the limited slip function in the center differential was a last-minute decision 
because already in July 1970 a new 'Range Rover - Technical Details' publication 789A was issued
with this interesting changes:
'Transfer Box: Two-speed reduction on main gearbox output. Front and rear drive permanently
engaged via a lockable third differential.'

In the official workshop manual 'Range Rover - Repair Operational Manual' - Issue 1 from 1970, in
section 37.01 is written:
                                                                                                                                                         Red Salisbury differentials used in all Suffix A LT95 gearboxes
'Gearbox differential lock switch                                                                                                       - visible for the trained eye; a non-Limited Slip version
The Range-Rover has permanent four-wheel drive, achieved by using a limited slip differential
fitted in the transfer gearbox. This allows a high degree of mobility in 'off' road use and also 
prevents transmission 'windup'.
Upon encountering conditions in which the maximum amount of traction is required, the limited
slip differential can be 'locked' by means of the differential lock switch. This ensures that all
road wheels revolve the same speed.                                                                                   

In Issue 2 from October 1970 the 'limited slip differential' was replaced by 'lockable differential'.

The first Range Rover's with limited slip gearbox had different Recommended Lubricants than the 
later vehicles.
In the 'Range Rover - Repair Operational Manual' - Issue 1 1970 the following oils where required:
Engine:BP Super Visco-Static 10W/40Castrol GTXDuckhams Q20/50 Motor Oil
Main gearbox:BP Gear oil SAE 80EPCastrol Hypoy-LightDuckhams Hypoid 80
Transfer gearbox:BP Limslip Gear Oil 90/1Castrol Hypoy LS Limited Slip Duckhams ?

Already in the Issue 2 from October 1970 the 'Range Rover - Repair Operational Manual' changed
the Recommended Lubricants and Fluids away from light EP80 hypoid gear oil in the main gearbox 
and EP90 hypoid gear oil special version for limited slip differential in the transfer gearbox - to 
recommend regular SAE20 motor oil in the engine, main gearbox and transfer gearbox with regular
center differential:                                                                                                                               Technical; the friction discs structure - from RR Workshop book
Engine:BP Super Visco-Static 10W/40Castrol GTXDuckhams Q20/50 Motor Oil
Main gearbox:BP Super Visco-Static 10W/40Castrol GTXDuckhams Q20/50 Motor Oil
Transfer gearbox:BP Super Visco-static 10W/40Castrol GTXDuckhams Q20/50 Motor Oil

Why was the limited slip function discontinued ?
The idea with the limited slip function in the center differential was to achieve best possible traction
with a permanent 4x4 drive system. On the market in mid-60s there were not many modern leisure
four-wheel drive vehicles, called SUV today. Most common were the US Ford Bronco, Chevrolet
Blazer, Jeep Wagoneer and Toyota Land Cruiser, in addition to the Land Rover Series II. All of those
rather rugged driveline system compared to the refined and sophisticated driveline for the projected
Range Rover. 
Only really sophisticated four-wheel drive system on the market was the Ferguson Formula - FF -
which was low scale production, more seen on Formula 1-testing and Jensen FF in the 60s.
The Ferguson Formula system is explained in detail here.
Since there was not many comparable permanent four-wheel-drive system on the market with center 
differential, the development engineers at Land-Rover worked with the permanent 4x4 driveline to
both have the limited slip function as well as the lockable function to totally lock the center differential, the latter to have direct drive front and rear like the Land-Rover system and the US systems.

Back in the later part of the 60s the new Range Rover was seen as a more luxury vehicle for the 
farmer, engineering companies etc for with need for off-road driving or driving on rough roads.
What went out to be instantly after the launch of Range Rover in 1970 was that the new vehicle
was used more than 99% on paved roads, often by people the never before had drove a 4x4.                   Later in 1970 the friction discs was replaced by distance discs.
Maybe the need for better wheel traction was only needed on a slippery car-park at a sport event             No longer with the limited slip function, reason described to left.
or gymkhana.
In the time of launch, just before and right after, at lot of the automotive magazines made test drives
of the new Range Rover. The reaction to the new Range Rover was very positive, but the gearbox 
sometimes got comments about gear whine noise and rather rugged agricultural feeling.
Another comment was problems often found around the steering. In 1970 Range Rover had only a 
manual steering box no servo-assisted steering, and the Range Rover felt often problematic to hold
a straight steering at speed. 
When driving with full steering to left or right around corners or obstacles, the vehicle starts with 
uncomfortable jerking in the driveline system. 
Also when driving at speed on slippery ground the car likely tended to get fish-tail feeling.

The limited slip in the center differential made the driveline system too 'stiff', so when the Range
Rover were driven on delicate surfaces or in special conditions like described here the vehicle 
became too uncomfortable.
The engineers at Rover found out that the limited slip in the center differential made problems. The 
recipe to eliminate these unwanted driveline behaviors was to withdraw the friction discs and only
have the space rings instead. Then the jerkiness and fish-tail feeling disappeared. 

Normally the very slippery conditions, like a muddy park yard or icy road, the length and speed 
to get back to normal conditions were so short and solved by the lockable differential function.
So it was decided that limited slip in the center differential gave more discomfort than extra benefit.