I suspect there was a wiring connection issue to the electro pneumatic valve.
There have been reports of problems, I think water gets into the connector.
Some have been fine after a clean and a fiddle, others have needed replacement as they failed again soon after.
I wrote this a while ago and it hopefully explains the differences between the old, viscous system and the newer, electro pneumatic valve system.
Both systems front to rear torque systems perform similarly, though one is mechanical and the other electrically controlled.
The old viscous coupling on the 169 "Climbing" model is basically two shafts joined together by plates in a tube filled with thick fluid.
Every second plate is locked into one shaft, the others in between are locked into the other shaft. (very similar to a motorbikes wet clutch).
With no pressure on the plates, they slip past one another and no torque transmits from one shaft to the other.
When it all starts slipping the thick fluid heats and expands to drag on the plates, thus sending torque through it.
The more slip, the more drag on the plates, the more torque.
It's totally automatic and self regulating.
The later models with the electric system uses a power transfer unit on the back of the gearbox, similar to the viscous system but with an electro pneumatic actuator on the rear diff instead of a viscous coupling to engage and transmit power to the rear axle.
This valve uses small electric currents to vary the valves position thus controlling variable power to the rear axle as needed.
The valve is not just a on or off switch, but will vary depending on the differing levels of torque between the prop shaft and the diff.
It will transmit up to 50% of torque to the rear when needed.
Now where the two systems differ is with this electro pneumatic valve system, it's easier to over ride and pre lock the torque 50/50 than it is to pre lock a viscous coupling, it's just a matter of signalling this valve.
To pre lock a viscous coupling, there needs to be some way of altering the state of the fluid in the system (with a pump) or some other way of locking the two shafts together.
I believe due to weight/cost/reliability Fiat did not include a front to back lock on the old viscous system.
This pre locking is part, but not all of what the ELD system
So both can alter power between front and rear axles, though only the later electronic system can pre lock the front to back transfer of torque.
Due to the other systems fitted to the later models, like the stability control (ESC) and ABS, they can also "lock" the diffs side to side, though it's not a true lock, just the effects are similar.
Under normal conditions, if one wheel on a powered axle spins due to lack of grip, the open diff will transmit all the power on that axle through the path of least resistance, ie to the spinning wheel. The other wheel on that axle, even through it might have traction, doesn't receive any power and sits there idle.
By using the ESC it can detect a slipping wheel and using the ESC & ABS systems it can grab the brake for that wheel, thus sending the torque to the other wheel which otherwise would be left unpowered, so now this other wheel has a chance at gripping and moving you.
The ELD is speed limited, as what works for low traction situations doesn't for higher speed cornering.
So by pressing the ELD button you pre lock the power to the back axle with 50% of power and alter the ESC system, already to grab the brakes on slipping wheels making it more more capable, grip wise, than it otherwise would be.
The ELD a bit of a step up from the viscous coupling, as unlike the pre lock, electro pneumatic valve system, the viscous's front to back torque is a reaction to low grip, so you are already loosing it before it actually operates fully.
Though the side to side "lock" using the brakes is a reaction system rather than a pre locking system, the old system had nothing similar.