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Used Porsche and Prestige Cars For Sale | Harbour Cars

Diff/4wd unit servicing

D

david_a

Silverstone
Joined
20 Jun 2012
Messages
108
With 996s being at least 10 years old and some on good mileage (75knin my case )

Was wondering what the consensus or reccomendation on servicing/fluid change in these units was ?
 
Do it on every car I get now cos you don't know what's in it when you buy it and gearbox/diff repairs can be quite pricey.

Everytime I've dropped gearbox oil it's been like plss.
 
Although if you mean the VC, you can't do anything with it.
 
Did the gearbox and front diff oil on my 4S after 50k miles. It can only be a good thing. The manual said 96k, but for a few quid, I feel it is worth it. Preventative maintenance and all that. :D
 
Did the tip box last year which felt worth it.

VC - is that the 4wd unit? Is it sealed then? Curious what is reccomended - I'm a big fan of keeping things tip top and preventative maintenance.
 
david_a said:
VC - is that the 4wd unit? Is it sealed then? Curious what is reccomended - I'm a big fan of keeping things tip top and preventative maintenance.
Click to expand...

The AWD system (as it isn't strictly 4WD) works by an in-built diff inside the gearbox housing for the rear wheel drive and then the prop shaft is connected to the opposite end of the shaft that drives the rear diff.
This prop shaft travels under the length of the car to the VC (Viscous Coupling) that acts as the clutch/drive for the front wheels. The opposite end of the VC connects to the front diff. unit (which can be oil changed just like your gearbox/rear diff.) which then drives the front wheels.

The Viscous Coupling is filled with a special fluid known as a dilatant and is a sealed unit so not serviceable. The fluid is silicone based and can best be described as porridge or something similar to silicone window seal. The input and output shaft are allowed to turn slowly independently of each other without creating a full locked AWD system, but as soon as one shaft trys to spin at a faster speed than the other (eg. rear wheel spinning when setting off), then due to the properties of the fluid, it prevents this from happening and creates the fluid to 'lock', thus transferring torque from the prop shaft to the front diff.

Here's a snap of a cross-sectioned VC and front diff unit

882048d1414637881-headlights-and-awd-porsche-911-turbo-2007-front-differential.jpg


and here's my favourite video displaying the properties of a dilatant fluid so you can imagine how this fluid works inside your VC to create a lock in the coupling when one end spins greater than the other




All this is the theory of how it works, in reality, whether it does or not is open to discussion, especially on older VC units (with old fluid in).
 
david_a said:
Did the tip box last year which felt worth it.

VC - is that the 4wd unit? Is it sealed then? Curious what is reccomended - I'm a big fan of keeping things tip top and preventative maintenance.
Click to expand...

As per Porsche, the viscous coupling is a life of vehicle, unserviceable part; anecdotally they have typical useful service life of less than 100k miles.
 
mcbit said:
david_a said:
Did the tip box last year which felt worth it.

VC - is that the 4wd unit? Is it sealed then? Curious what is reccomended - I'm a big fan of keeping things tip top and preventative maintenance.
Click to expand...

As per Porsche, the viscous coupling is a life of vehicle, unserviceable part; anecdotally they have typical useful service life of less than 100k miles.
Click to expand...

So what happens then ? Stop working or lock on awd all the time?
 
david_a said:
mcbit said:
david_a said:
Did the tip box last year which felt worth it.

VC - is that the 4wd unit? Is it sealed then? Curious what is reccomended - I'm a big fan of keeping things tip top and preventative maintenance.
Click to expand...

As per Porsche, the viscous coupling is a life of vehicle, unserviceable part; anecdotally they have typical useful service life of less than 100k miles.
Click to expand...

So what happens then ? Stop working or lock on awd all the time?
Click to expand...

Stop working, well not actually STOP working, but not delivering the amount of torque transfer that it should.

If it did lock, your tyres would rapidly morph into the Mitchell Brothers.
 
david_a said:
mcbit said:
david_a said:
Did the tip box last year which felt worth it.

VC - is that the 4wd unit? Is it sealed then? Curious what is reccomended - I'm a big fan of keeping things tip top and preventative maintenance.
Click to expand...

As per Porsche, the viscous coupling is a life of vehicle, unserviceable part; anecdotally they have typical useful service life of less than 100k miles.
Click to expand...

So what happens then ? Stop working or lock on awd all the time?
Click to expand...

With these, they apparently jus turn into a 2wd. :dont know:
 
I imagine that there will be more road feel if the coupling stops working and becomes permanent 2wd?
 
chriscoates81 said:
I imagine that there will be more road feel if the coupling stops working and becomes permanent 2wd?
Click to expand...

How do you mean?
 
alex yates said:
chriscoates81 said:
I imagine that there will be more road feel if the coupling stops working and becomes permanent 2wd?
Click to expand...

How do you mean?
Click to expand...

What i mean is that when i upgraded from a 996 C2 to a turbo i noticed there was a bit less road feel i.e. the RWD gave a bit of a purer experience. Its a bit hard to describe but im hopeful somebody else can explain better than i can.
 
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