Flat6Estate wrote:
A couple of questions:
Is there a difference in the internal damage problems with a CD that is only bining, compared to one that is binding and cluncking as well?
As the problem mostly occurs when the CD is hot, how did the shop you took it to test this?
Thank you, these are good questions.
First Part.The damage is restricted to the fluid losing its properties and its temperature stability. Once the fluid overheats, the fluid becomes overly tacky, with too high a viscosity.
The viscosity then increases with temperature, hence the binding of the Centre Differential becoming more problematic when the gearbox heats up.
As you note in the second question, "the problem mostly occurs when the CD is hot".
When the Centre Differential is dismantled and cleaned. All the plates are inspected for damage, etc.
While I have seen pictures on the internet of other makes of viscous couplings where the internal plates are bent, I have not seem damaged internal plates on the Subaru Viscous Diffs, albeit that I have only torn down a few.
The Subaru viscous coupling design has an inbuilt flaw (my words) in that the circlip at the end retaining the coupling together will fail by dislodging under pressure before the internal plates become damaged.
At this point, the internals of the gearbox get damaged.
Only "binding" is a failing centre differential that is on its way to becoming worse and becoming "binding and cluncking as well".
Second Part.Even though the "problem" becomes evident on the car when the CD is hot, the base level cold coupling torque already has increased above specification.
It should be noted that the CD unit provided by Brunsy3.0 was measured on my equipment when cold and before disassembly, to establish a base line.
The torque, measured cold, was about double that of the same diff once reassembled, and reassembled back to the same weight as prior to disassembly.
So, the Subaru gearbox specialist that the diff was taken to, were given the diff to test without telling them what it was.
The shop also sell "used differentials" that they have tested. So I trust that they are able to distinguish between a serviceable viscous coupling and a failing/binding differential unit.
Additional Part.The design of the standard 5MT Viscous Coupling Centre Differential and the 6MT unit are essentially the same (one minor difference noted)
The internal components and working parts are the same, no difference, interchangeable.
So one question that I have been considering is "why the apparent difference in Spec-B 6MT Centre Differential failures compared to the 5MT units"?
My only response to date is to consider that the Spec-B cars can produce more power, have better suspension, more grip and can transmit more power through the centre differential on turns, as well as more likely to be driven spiritedly. There is nothing is the construction of the viscous coupling that would lead to differing service life?