Visco diff problem caused by big rear tyres on RRC 3.9

[RR_Cas]

I own a Range Rover Classic 3.9 automatic '92 with Borg Warner centre diff. Last week I replaced the rear wheels by bigger wheels. The difference in diameter between front and rear wheels was about 2 inches .
A hugh mistake :doh:.

After driving for about 1 mile the car slowed down and came to a full stop while the steering wheel forced to the right. I waited a few seconds, slowly accelerated again ander after 500 meters the same occurred. Very, very scary. I turned the car to go back home, and after about 500 meters there was a snapping sound (a loud and short tick) coming from the back of the car. Immediately the problem seemed gone: no more slowing down, no more stopping, no more pulling to the right. Everything seemed normal again. Carefully I speeded up: at speed around 50 mph I hear soft ratling sounds coming from the back, while the car is lightly vibrating. I refitted the original tyres, the sounds and vibrations at 50 mph are still there.
Checking the internet, I now realize I haven't been particularly smart. Trying to locate the problem, I first checked the visco-coupling: I jacked up the left front wheel, with force I can turn it round, 6, 7 times and more, it does NOT lock up. I read a lot about locked-up visco's, but mine isn't locked. So temporarily removing the front prop shaft (to force 4WD into 2WD) is of no use.
Who has a good idea of what's damaged in my car?

 

[fanatic]

I'm guessing you have broken something in the rear axle , the viscous locking up is like driving with diff lock on and would cause transmission wind up. (classics crab badly on full lock if the viscous s shagged from experience)

My mate took a crown wheel tooth of a classic rear diff, it had an intermittent whine and vibration too

 

[RR_Cas]

Hi Fanatic,
So you think the visco did actually lock (although it doesn't appear to be locking anymore) and that because of that the rear diff is faulty. So I should replace the Visco and check the diff?

 

[fanatic]

Hi Fanatic,
So you think the visco did actually lock (although it doesn't appear to be locking anymore) and that because of that the rear diff is faulty. So I should replace the Visco and check the diff?

Your asking for a remote diagnosis, my advice is you obviously caused the viscous to lock up by your selection of wheels.
HowStuffWorks "Viscous Coupling".

Now before throwing money at it, I'd recommend inspecting half shafts and rear diff as you say the noise came from the rear see what you find and go from there

 

[jamesmartin]

viscous do eventually sieze but yours sounds okay ,they must be run with 4 equal tyres ,viscous will turn to allow some differential but will stiffen with torque applied through it the more force and the faster to try to turn it the stiffer it gets ,jacking one wheel up in turn will show if half shaft or diff bust ,rear output shaft in transfer box has splines that strip easy to check just undo speedo housing behind hand brake assembly and pull out 2nd set of splines from inside end go easy to replace shaft approx £70