Originally Posted by 33Permanent4
How does the p4 system really work? I'd just had mine some days before the engine went to hell.
When i started the car the lamp "4x4" lights. Should this light when the rwd starts?
Is there a extra clucth inside the gbox which regulates the rwd?
No, no clutch.
At the back of the gearbox is an electromagnetic coupling, that's the bit you've got the broken wire on. This turns constantly and is connected to the front prop, which is the connected through a viscous coupling to the rear prop, which is connected through a weight (to absord vibration) to the rear diff.
The electromagnetic coupling only couples when there is power to it, when there is no power there is no coupling and the car is 2wd only. Under heavy braking the coupling will disconnect to avoid excessive strain on the 4wd system, and if it detects a fault in the sensors it will also disconnect to prevent any damage happening. When the coupling is disconnected due to a fault the 4x4 indicator lights up on tachometer.
So, when you turn the ignition on the 4wd ecu checks the system for faults, while this happens the 4x4 indicator lights up to show it is working. If the system finds no faults it connects the electromagnetic coupling and the 4x4 light goes off, if there is a fault the light stays on and the coupling isn't coupled.
When running in 4wd mode the default power split is 95/5 front/rear. When the front wheels start to slip and spin quicker than the rear wheels power is taken away from the fronts and directed to the rears, not sure what the maximum split to the rear can be but i seem to remember reading 40/60 front/rear somewhere. (Anyone know?).
The change in power is controlled by the viscous coupling which basically consists of 2 sets of plates separated by oil, as the set of plates connected to the front prop, and hence the front wheels, starts to rotate quicker than the plates connected to the rear prop/rear wheels the oil thickens and drags the rear plates round with them, thus transferring more power to the rear wheels.