A properly reconditioned Viscous Coupling.
The output graph produced by the computer associated with the test rig, shows the viscous coupling response to increasing turning resistance (and temperature), over its working range. This is the same as the front wheels turning faster or slower than the rear wheels. The coupling's oil is thickened by the process, which also produces heat. The left scale is measured in Newton Metres and the bottom scale is the temperature. The coupling, which is the most important part of the transfer box must perform within the two green lines.