are you talking about the old county 4x4.

New Transit All Wheel Drive premieres at 2006 Hanover Commercial Vehicle Show

Under normal driving conditions, the Ford Transit AWD only uses rear wheel drive. However, should road surfaces become slippery with rain, snow, ice or split friction, a loss of traction might occur and the rear wheels are likely to slip. Ford's mechanical torque sensor will automatically engage the two clutches inside the front transmission, which at the same time limits the rear axle's overspeed and transfers torque towards both front wheels for additional traction. At the same time the de-stabilising effect of spinning rear wheels is being re-balanced from oversteering to a neutral driving behaviour by the physical torque flow to the front wheels.
A core element of the system is a hydraulically controlled torque sensor inside the front axle transmission. By sensing speed difference between front and rear axles, the integrated oil pump will continuously lock the multi plate clutch to one of the front wheels and gently convert torque to the front wheel.
The system is completely mechanical and free of wear – no service is required during its lifetime, apart from oil level control inside the clutch housings. Another core element of the system is an Eaton-type oil pump inside each of the clutch housings. It uses a small volume of oil to engage the clutches whenever required (transmission oil type 75W-90 BO).
As long as the front wheels rotate on dry and solid ground at the same speed as the rear wheel drive axle, both clutch pumps remain static and the entire hydraulic system is without function – the clutch locking system remains without pressure and both clutches are open. In case of minor speed differences typical for cornering situations, the pumps begin to rotate gently and pump a small amount of oil through the valves of the clutch system, without closing them for straight torque transmission.
As soon as the rear wheel drive shows significant slip, both clutches inside the system engage as the inner rotor and outer rotor begin to rotate at different speeds. At an appropriate speed difference the two hydraulic valve systems close the clutch system to each front wheel, delivering enough torque to either wheel to stabilize the drive situation and reduce overspeed effects at the rear axle.
Once all wheel speeds are adjusted to a close to identical level, the system disengages automatically by releasing oil pressure from the clutch system. In order to adjust the clutch system to different oil temperatures – which may vary from -40 °C to around 100 °C – a viscosity compensation valve is embedded into the system.