full-time all wheel drive with viscous coupling locking planetary gear center differential. 37/63 front/rear ratio.
The constant drive was realized as follows. Behind the common 5-speed gearbox, the ZF transfer gearbox with interaxle differential was mounted. Planetary gear set of the interaxle differential distributed the torque in 37/63-front/rear ratio. The differential included viscous coupling with lock-function. To the rear, the transfer gearbox extended in two-section cardan shaft propelling the rear axle with bevel gear differential and axle shafts with CV joints. The rear axle differential included viscous coupling with lock-function for improved traction. Front axle propulsion comes out of the transfer gearbox aside the engine axe. There is a 1:1 chain transmission.
The transfer gearbox is connected to front bevel gear differential using short cardan shaft. The front differential is mounted on crankcase, therefore the propulsion of front left wheel must have been constructed using a shaft leading through crank case. The shaft was split into two parts to ensure smooth and reliable operation. The parts used in this configuration are rather special and therefore expensive.
On all 4WD cars, the most powerful motors were mounted only, at first 2.8 (till 1987, very rare car), then 2.9litr (engine equipped with catalys. has torque - 229 Nm, than 222 Nm, at standart engine). Also You can understand that you have business with 4x4 before 1990, thanks to black windows ornament. Also 4x4 version was produced as GL and as Ghia set. After 1990 only in GL version (not in England). All 4x4 cars was equipped with 5-speed manual gearbox. Main bodyes was: hatch and saloon, but there was also Tournier version, but it's a very rare car, I have met only 1 in my life.
Oh, yes, 4x4 technic was designed by Fergusson, it's very famous firm, and it's helped to Ford with Sierra 4x4 also.
Figure: ford sierra 4x4 (38KB)
A - rear axle
B - epicycle drive
C - viscomate
D - drive chain in the distributive drive
E - gearbox's band wheel shaft
F - driving shaft to the front axle's distributive drive
G - front axle's distributive drive
H - front axle