"Matthew Maddock" <
[email protected]> wrote in
message news
[email protected]...
>
> Is the disco auto box any good? I'm not bothered from a
> drive-ability point of view, more if it is going to be
> reliable. I don't recall hearing anything specifically
> bad about them.
>
> Cheers,
>
> Matt
Basically, it's a good reliable box. However, it has one main common failing
that is easy to check and easy to prevent, that of premature "A" clutch
wear, and the odd other faults that are also easy to check for.
There are piston-ring type total-seal rings that in theory seal oil passages
within the rotating input shaft and static stator shaft at the front of the
box. As these seals wear, they allow a bleed of oil from the outlet of the
main pump to partially apply the main "A" clutch in the box. This clutch is
permanently applied in all forward gears but released in P, R and N. If it
is applied in P, R or N, the clutch slips, causing wear. This situation can
be caused by excessively (continuously and regularly) revving the engine in
P or N, or sitting the engine at, say, 3000rpm in the case of setting up LPG
on a V8. The "cure" (preventive measure) is to put the transfer box in N and
main box in D, 3, 2 or 1 if you need to sit revving the engine for any
length of time, as this ensures the "A" clutch is applied and not slipping.
The method of checking for the seal wear (carefully), is to drive the
vehicle for at least 20minutes to ensure the oil has thinned out as it
reaches operating temp; then put the box in N, rev to 2400 (ish) rpm and see
if it creeps forward with the brakes off. Please make sure you have LOTS of
clear space in front when you do this, to be safe! No need to hold the revs
up, just accelerate the engine slowly and if you get to 2400ish with no
creeping, back to idle.
Dip the oil, it ought to be clear, translucent red - if it's brown or dirty
this indicates clutch pack wear, walk away!
If the shift from 1 to 2 (D selected) is overly fierce or delayed to quite a
highish rpm, this is an indication of governor sticking issues; quite common
really, a lot of people just don't realise that it isn't meant to be that
fierce a shift!
There is another issue whereby you select D and it doesn't drive, but you
can select 1 and pull away then go manually through the box right up to D
with no problems. This is a sprag (one-way) roller bearing clutch right in
the centre of the box that has "failed". The rollers have "flipped over"
internally and the box needs removing and stripping to fix. This can be
caused by selecting a forward gear at too high an rpm, or over-zealous
acceleration from a standing start with a powerful engine. (won't apply to
TDi's then!!)
Rpm rises with a "squealing" noise before the vehicle drives - normally low
oil level or a very dirty internal filter. Ask yourself why the filter is
dirty though, where is the dirt or debris coming from, the clutch packs
wearing??
Don't let this put you off an autobox though, they are essentially very
reliable bits of kit that will go for easily well over 130,000miles given
the correct routine maintenance. I know of some with nearly 200,000 miles
that have had no issues. The failures quoted here are the typical failure
modes with the box, the overall failure rate is very low when used in the
way the manufacturer designed it. We run a zf box from a range rover classic
in one of our racers, it is abused severely (banged into gear at rpm's well
above idle, held against the brakes with throttle applied etc etc) and it
has survived 3 or 4 seasons of Hillrally and Comp Safari use since fitting,
although the oil is starting to show clutch wear debris now. It was lifted
from a motor with 80-odd thou on the clock, and apart from a mod to the
valve body to prevent the convertor clutch locking in 4th, it was thrown
into the racer with no other work being done to it!
Be warned though, in my opinion a tdi with an autobox is rather lethargic,
especially when towing.
Badger.
