First, a bit of background .....
Pierre Lepelletier is one of my heroes. He was born in 1928 and spent the whole of his career working for Ferodo (now Valeo) in France. At the age of 59 he took early retirement and then, two years later, filed a patent application for a new geartrain arrangement for automatic transmissions that could produce six well-spaced ratios using only 5 clutches, a Ravigneaux epicyclic and a simple epicyclic.
This was in December 1989. A few years later he had signed licenses with Aisin-AW (the world’s largest automatic transmission manufacturer), ZF, Ford, Jatco & GM allowing them all to manufacture transmissions using his geartrain system.
Why was his invention so important? Well, in ZF’s case - compared with the 5-speed 5HP24 unit that it replaced - the 6HP26 had 29% fewer parts (down from 666 to 470), was 44mm shorter (reduced from 697mm to 653mm) and weighed 12% less, despite its higher torque capacity and increased ratio spread. Most of the world’s OEMs switched to 6-speed transmissions as a result.
A colleague of mine was lucky enough to meet and have a chat with Monsieur Lepelletier at a conference a few years ago and told me that he’s a perfect gentleman. Now in his mid-eighties, I hope he enjoys a long and healthy retirement. His patent will have expired 21 years after its priority date (i.e. in December 2010) but hopefully he’s made a few quid from the 20 million or so 6-speed transmissions that have so far been produced using his system!
The ZF 6HP26 transmission first appeared in 2001 in the 7 Series BMW, the year before the L322 Range Rover was launched, but since Ford had taken Land Rover off BMW’s hands in 2000, the L322 driveline development was frozen resulting in the Range Rover being launched with the already out-of-date 5-speed ZF unit in 2002. It wasn’t until 2006MY, when the Jaguar petrol engines were introduced, that the Range Rover finally caught up and the 6-speed unit became available. The diesel version had to wait yet another year for the TDV8 to be introduced before the infamous GM 5L40-E was finally abandoned in favour of the 6HP26.
Strictly, the Range Rover is fitted with the ZF 6HP26X transmission (the ‘X’ stands for external 4WD i.e. fitted with a separate transfer box) with part numbers starting 1068 020 XXX, the rear-wheel drive variant being the ZF 6HP26 (part numbers starting 1068 010 XXX) and the ‘internal 4WD’ version for Audi & Bentley being designated the ZF 6HP26A61 with part numbers starting 1068 030 XXX.
Anyway, on with the teardown ….
I saw this particular transmission on eBay, actually from a supercharged 4.2l V8 Range Rover Sport, with a starting price of £50 – and I turned out to be the only bidder.
After sliding the torque converter off its splines, and placing it safely to one side, I drained out the transmission fluid. The drain plug is at the back of the plastic sump pan.
Incidentally, the filler/level plug is on the right side of the casing, in a location similar to the 5-speed units
Sump pan removal next – there are 21 screws in all and it’s good to see that the head size has been increased to Torx T40, from T27 used on the 5HP24, to make the heads less likely to strip
These screws are tightened to 8Nm. ZF warn against removing the sump pan at fluid temperatures above 40 deg. C due to the risk of the plastic distorting at higher temperatures!
The sump pan then just pulls off
The filter is an integral part of the sump pan and the photo shows where the oil enters the filter at the very bottom of the sump and also the suction tube which engages in a hole in the oil pump housing
The electrical connector sleeve is removed next, by first pulling out its retaining clip
and the connector sleeve will then come out of the casing
It’s easy to see, from the casing corrosion which has coated the two O-ring seals, why this is a very common source of oil leaks in the 6HP26 transmission.
The electrical pins on the Mechatronic module can be seen through the hole in the casing where the sleeve was extracted
There are ten screws in all (Torx T40) which bolt the Mechatronic module to the casing - seven long and three short - and these are easy to distinguish from the screws which hold the two valve block halves together which are only T27 and have a smaller head size
The rooster comb pin engages in a slot in the manual valve which also drives the slider on the (brown) potentiometer so that the controller knows which gear position has been selected by the driver
The Mechatronic module can then be lifted out
The Mechatronic module is a combined hydraulic valve block and transmission ECU. It also houses both the turbine and output speed sensors and also the fluid temperature sensor. There’s a risk that electro-static discharge could damage the ECU so ZF recommend that a ground strap is worn when handling it
This is how the underside of the casing looks with the Mechatronic module removed
Next, the bridge seal to the pump housing just lifts off
and then each of the four rubber jump tubes has to be extracted
In terms of length, there are two small, one medium and one large tube
The park lock engagement mechanism can be seen in the photo above and this is removed next, together with the selector shaft
So that’s it for the underside, next our attention turns to the front of the transmission.
The oil pump and stator support assembly is retained by 13 Torx T27 screws
and each of the (identical) screws is fitted with a washer with a rubber insert to prevent leaks
The assembly can then be drawn out from the casing
Next out is the A & E clutch assembly, which includes the front epicyclic
Followed by the B clutch assembly
To remove the static C clutch (& D clutch) housing, the large snap ring has to be extracted first
and it can then be drawn out
The two rectangular blocks in slots on the outside of the drum prevent the assembly rotating in the casing
Now the Ravigneaux epicyclic, together with the D clutch pack, follow
Which just leaves the output shaft
The intricate basket weaving around the annulus gear is actually the target wheel for the output speed sensor
Turning to the back of the transmission, the rear output seal is removed
to reveal a snap ring that retains the insert which locates the rear axial bearing
A couple of threaded holes allow extraction bolts to be screwed in to assist with its removal
So that’s just about it. The casing is now empty
A very simple process requiring no special tools.
Next step is to disassemble each of the subassemblies in turn. I’ll follow up with reports on progress with those.
Okay, so making a start with pulling apart the various sub-assemblies.....
First the A & E clutch assembly, which includes the front simple epicyclic
This assembly comes apart by simply releasing the snap ring which holds the B-clutch hub to the A-clutch drum
The various components which make up the assembly can be seen below. On the right is the E-clutch hub which drives the carrier of the rear Ravigneaux epicyclic – ZF call this the intermediate shaft. The drum with the holes in it is the A-clutch hub which, through the hollow shaft, drives the rear sun gear of the Ravigneaux unit.
So here is the A-clutch assembly with the front simple epicyclic attached
The clutch pack slides out after removing the snap ring and this plastic component is an oil catcher that channels fluid down the centre of the planet pins to lubricate the planet gear bearings
It took me a little while to fathom out how to detach the epicyclic carrier and sun gear from the A-clutch. Although you probably can’t make it out in the photo there’s a clip which holds the carrier in place and it is removed by levering it out of its groove using a small screwdriver through the gaps down the side of two of the planet gears
The clip can be seen more clearly below which shows the epicyclic once removed
and this is how the A-clutch assembly now looks
The clutch piston return spring then has to be compressed in order to remove the retaining circlip and I was pleasantly surprised to find that one of the tools I’d had made up for stripping down the 5HP24clutches fitted this one fine
So the oil dam plate can be removed together with the circlip
revealing the clutch piston return spring
To remove the piston it’s easiest to place the A-clutch drum back on to the oil pump assembly and then apply air pressure into the feed hole
The piston then pops out and these are all the components that make up the A-clutch and front epicyclic assembly
Moving now onto the E-clutch assembly. The E-clutch assembly is part of the ‘input shaft’ of the transmission which is driven directly by the torque converter turbine
The photo below shows that the annulus gear of the front simple epicyclic is EB welded to the E-clutch drum and so is permanently driven by the transmission input shaft
Once the clutch pack had been lifted out of the drum, I was again pleasantly surprised to find that one of my 5HP24 clutch spring compression tools fitted the E-clutch spring retainer perfectly, allowing me to remove the circlip without difficulty
and then the spring and piston could be removed
Like yourself, I haven’t seen any particular mechanical/hydraulic issues with the 6HP26 being raised on any of the forums so it would appear to be an extremely durable transmission. It would be easy to say “still too early to tell” but the fact that the transmission has been around since 2001 means that there must be plenty of high mileage ones around and I think we’d know by now if there were any major issues. The only recurring niggles seem to be the 2-1 rollout shift clunks (which can usually be sorted by changing the fluid and reflashing the ECU software) and the ‘dropping into neutral when accelerating or cornering hard’ problem due to the cable clamp bolt corroding at the transmission selector shaft lever.
Let’s put it this way. If anyone was thinking of setting up in the ZF 6HP26 repair trade – I think business might be a bit quiet!
The seller of this particular unit, which I bought off eBay, stated that the transmission was ‘working fine when removed’ (though why anyone would wake up in the morning thinking “Hmm, my transmission’s working well at the moment – I think I’ll remove it and sell it on eBay for £50” I’m not sure) so I’m not necessarily going to find anything wrong with it as I work towards completing the teardown.