After a bit of searching around, here (pasted images below) is the definitive explanation of how the 4x4 Panda works, direct from the Fiat eLEARN documentation. The system is indeed hydraulically operated and electronically controlled, with hydraulic pressure generated by a pump within the unit at the rear end of the propshaft. In this explanation it refers to the electronic control unit being under the driver's seat. In a RHD car this will be under the passenger seat.
The reference to two differentials: the first, the front one, is there on all front wheel drive cars and the second is at the back beyond the end of the propsahft. Crucially there is not a third, centre diff (as is found on Land Rovers), so there is a risk of transmission wind-up if 4x4 is forced to operate when not needed (such as on tarmac). As an aside, most modern 4x4x pickups (eg Ford Ranger/VW Amarok, Navara and D-max) also do not have a centre diff, and so also run as 2WD most of the time -- usually to the rear -- and (unlike teh Panda) cannot automatically engage 4x4. They also have a clear instruction to not turn on 4x4 on tarmac)
The TwinAir 4x4s have the 90 degree gears to drive to the propshaft (the idler gears) internally in the gearbox/front diff, and so share the same oil. Any damage to this drive requires the whole gearbox to be removed. On the diesel 4x4s, there is a separate power transfer unit (PTU) which has its own separate oil fill and drain and which can be removed independently from the gearbox/front diff.
Where it says here that upto 100% torque can be transmitted to the rear, it surely means that the maximum is a 50/50 split between front and rear?
The reference to two differentials: the first, the front one, is there on all front wheel drive cars and the second is at the back beyond the end of the propsahft. Crucially there is not a third, centre diff (as is found on Land Rovers), so there is a risk of transmission wind-up if 4x4 is forced to operate when not needed (such as on tarmac). As an aside, most modern 4x4x pickups (eg Ford Ranger/VW Amarok, Navara and D-max) also do not have a centre diff, and so also run as 2WD most of the time -- usually to the rear -- and (unlike teh Panda) cannot automatically engage 4x4. They also have a clear instruction to not turn on 4x4 on tarmac)
The TwinAir 4x4s have the 90 degree gears to drive to the propshaft (the idler gears) internally in the gearbox/front diff, and so share the same oil. Any damage to this drive requires the whole gearbox to be removed. On the diesel 4x4s, there is a separate power transfer unit (PTU) which has its own separate oil fill and drain and which can be removed independently from the gearbox/front diff.
Where it says here that upto 100% torque can be transmitted to the rear, it surely means that the maximum is a 50/50 split between front and rear?
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