diff locks explained

[stoop44rio]

hi, i'm going to look at a 90q next week and want some info on the diff locks please- how they work,how to check they're working,possible problems to look out for,etc. thanks for replies - its a type 85/c reg if its makes a differance.

[Booty]

The diff locks are pneumatic or air controlled if you like. They are fed by a small diameter pipe from the engine bay, around the side of the battery box ( yours will be in the engine bay on the passenger side ) through the bulkhead and into the back of the switches on the centre console. You have two locking positions.

1, Position 1 - first click - This locks the centre diff on the back of the gearbox, you should get a little green LED light up. This setting is used for driving in slippy conditions like snow, icy roads or wet roads etc. You can drive perfectly safe on tarmac with it in position one.

2, Position 2 - second click - This 'totally' locks the system all together, centre and rear diffs together. So in other words, as you go round a reasonably tight corner, all four driven wheels are locked together and will not 'yield'. This setting is for when you are up to your neck in mud, sand e.t.c. It will dig it's way under ground on this setting!

The centre console control distributes two further air pipes to the centre and rear diffs. These end up attached to plastic actuators that have an arm attached which dissappear inside the diffs. These can seize and the acctuators can brake and seize too. To test they all work you need to sit very quietly, and I'm not joking here, with the engine running and turn the rotary knob on the dash to position one. You should hear a faint hissing sound as the air runs and operates the centre lock. The green LED should light up too. Turn it back to the off position and the noise should be heard again, and the light should go off. Try this again for the rear diff. Now, sometimes they wont come off if they are a bit 'sticky' and you may need to drive a few yards to unlock them but you'll get the idea wether they work or not from the above. To test that they do work put the car onto some gravel and lock them all totally, then try and turn the car 'semi - sharply', the car will just dig and the inside wheels will try and 'bounce' as they are fully locked.
A few people do say ' well why do you need diff locks, I,ve never used them' ' who cares if they work or not'?! Well to me they are a fundamental part of the quattro system and should work, if you drive on snow the difference that having the centre lock in is amazing, real rally driving stuff!
Don't let it put you off though if they are not working as they can be fixed / un-freed.

Hope this helps.

Rich.

[stoop44rio]

cheers booty,i don't think i could've got a better explaination! thanks!!

[msh]

Originally Posted by Booty

2, Position 2 - second click - This 'totally' locks the system all together, centre and rear diffs together. So in other words, as you go round a reasonably tight corner, all four driven wheels are locked together and will not 'yield'. This setting is for when you are up to your neck in mud, sand e.t.c. It will dig it's way under ground on this setting!

How front wheels can be locked together if front differential isn't lockable?

[Booty]

Originally Posted by msh

How front wheels can be locked together if front differential isn't lockable?

Buy a quattro and you'll find out.

[MikeNZ]

Another explanation can be found here:

http://www.kvquattro.com/techdocs/diffLocks.html

[20vcqdriver]

Occasionally depending on the positions of the locking rings in the gearbox and diff, selecting the diff lock will not immediately bring on the green lamp, however moving forward or back slightly on lock should engage them and bring on the lamp.

[Martc]

If you have engaged the diff locks and are moving along and then disengage them then the light may not go out straight away and if you are travelling too fast not at all. There is a maximum speed limit for disengaging, I think its 15 mph.... Perhaps someone can confirm that

Also if the car has ABS, when you turn on the diff locks the ABS is turned off, so you should expect to see the ABS light, unless the ABS is broken or it does not have ABS in the first place...

and some useful links to buying a type 85

http://www.kvquattro.com/buyersguide.html

[MikeNZ]

there is no stated speed for engaging/disengaging the locks.

The 15mph thing comes from the later Type 89 cars with Torsen centre diff. On these cars, the rear diff alone remained driver-operated, but it automatically unlocked when 15mph was exceeded (as it has no function at higher speeds and can produce some 'interesting' effects on handling if used inappropriately). The Type 85 diff locks do not have this feature and will only lock/unlock when selected by the driver. Needless to say, if you are using diff locks it means you are driving on a slippery surface so you should not be travelling too fast!

As stated in http://www.kvquattro.com/techdocs/diffLocks.html, if you drive with them locked on dry tarmac you can experience difficulty unlocking them due to transmission wind-up, as the wheels cannot rotate at different speeds when going around a corner for example, so a considerable amount of stress can be generated. The tyres will be scrubbing against the road surface too. To avoid this, always lock/unlock them on a surface with slip, such as mud, grass or gravel - they fulfil no purpose on dry tarmac anyway. While the drivetrain is very tough, too much wind-up could cause something to break.

If the light will not go out, make sure you are on some grass or gravel, and move the car backwards and forwards. If it will still not go out, let the clutch out suddenly to encourage one wheel to break traction to release the wind-up. You will feel one wheel spin slightly and the light will go out.

[Isimmo]

Reversing has always cleared any experience of sticky diffs for me...

I like manual diff-locks a lot. In the early 80's, the Germans (German Press) recommended running ur quattros on wet autobahns with the centre diff locked to encourage directional stability. This, ironically, I can completely understand, as a by-product of 'quattro' is the masking of aquaplaning. By locking the centre diff, you negate the power being moved between axles at high speed and de-stabilising the chassis. And believe me, there have been plenty of ur-quattros that have been written off or bent/spun on wet motorways/dual Carriageways etc. I know of at least 5.