6HP series automatic transmission design

Automatic transmissions of the 6HP series are a joint development of ZF and BMW engineers. Many rear-wheel drive models of the concerns BMW, Jaguar, Ford, Audi and Range Rover were equipped with transmissions of this series. The design of this automatic transmission allows it to “digest” a fairly high torque – up to 750 Nm, which made it an excellent choice for most premium models with powerful power units up to 6 liters and any type of drive.

The main design feature of this transmission is the presence of pre-tightened one or more clutches in the torque converter lockup unit (TC). This solution allows blocking the torque converter in the earliest gears, synchronizing its rotation with the engine crankshaft speed, which provides outstanding dynamic performance of the car and reduced fuel consumption. It also makes gear shifting almost imperceptible.

1. Impeller hub 2. Rolling bearings 3. Sprag 4. Friction clutch 5. Rivets 6. Petal spring 7. Piston 8. Centering shank 9. Oil seal 10. O-ring

Obviously, such a solution significantly increased the load on the lock-up mechanism, which, in turn, inevitably led to an increase in the number of its breakdowns and cases of incorrect operation.

The second negative factor is the setting of the electronic control unit: the designers abandoned the principles of the fool-proof philosophy and allowed the automatic transmission to be operated in modes close to the limit. As a result, the combination of a sporty driving style, city traffic jams and high summer temperatures led to the fact that the transmissions of the first years of production very often failed.

The most common problem with the ZF 6HP is a decrease in the amount of torque transmitted by the tightened clutch of the lockup unit. This problem in most cases appears due to two main reasons:

1. Loss of friction material properties (so-called surface glazing or scorching) caused by severe operating conditions in partial blocking mode, often without ATF replacement.

2. Reducing the stiffness of the petal springs that pre-press the piston.

In the first and second cases, the restoration of the torque converter is required, which can cause difficulties for many services specializing in the repair of automatic transmissions. However, with the availability of special equipment, it will not be a problem to carry out a high-quality repair.

Below is the process of repairing a 6HP series torque converter using Kinergo equipment.

Stage one: cutting open the torque converter

The torque converter is mounted on a lathe using special accessories. Next, you should mat the neck of the torque converter with sandpaper to reveal deep scratches, cavities or dents.

After that, you can proceed to the cutting procedure: using the through cutter, it is necessary to separate the cover and the body. It is important to remember that the body must be cut strictly along the equator, avoiding excessive metal removal. The correct type of cutter or carbide inserts for this type of work, as well as cutting conditions, are selected according to special reference books.

For the final separation of the cover and body, you can resort to light tapping with a hammer or a punch from a soft material in the seam area. After depressurization of the housing, it is necessary to drain the remaining transmission fluid. Now you can start disassembling and troubleshooting the torque converter.

Stage two: washing and troubleshooting

The body and internal parts of the torque converter are thoroughly washed in a special chamber. Inspection of thrust bearings, blades of wheels of torque converters is carried out, the operation of the sprag of the reactor wheel is checked.

Further, it is necessary to clean the joints of the body from burrs. Upon completion of stripping, you need to check the fit of the halves of the body – they should be freely combined, without skewing and friction.

Stage three: removing the lock-up unit

To do this, the cover is mounted on a lathe, where the operator cuts off the weld between the cover and the platter. The cutter feed point is shown in the diagram below.

Stage four: replacement of components

After the lockup unit is removed, you can proceed to replace the worn parts. The most common failure is the friction plates of the lockup mechanism. Before installing new friction plates, they must be placed in a container with transmission fluid for at least 2 hours so that the material is well saturated. Pre-impregnation of the friction lining is necessary for further testing of the lockup mechanism, since the dry material differs in frictional properties, which will give an error when tested on the bench.

We replace oil seals and rubber seals with new ones, if necessary, we change the thrust bearings. We install a previously prepared friction lining and install the locking mechanism in its original place.

Step five: measuring the preload value

After assembling the torque converter, it is necessary to check the preload value on the bench. The pneumatic cylinder, through a special adapter, presses the spring support plate to the pump casing. Using a special adapter, we turn the friction plates. It is necessary to rotate the discs smoothly, without jerking. The value of the transmitted torque by the pressed friction discs is measured with a dynamometer. For different models of torque converters, the nominal torque value may differ.

If the indicators are within the factory tolerances, it is necessary to weld the pressure plate to the torque converter cover.

Welding must be carried out directly on the measuring stand, without removing the clamp from the supporting disk. The stand has an adapter for supplying compressed air to the cavity of the torque converter cover. This will not allow the rubber products in the torque converter cover to overheat due to more intensive cooling. Also, in order to avoid overheating, welding must be done gradually, in small segments.

After welding, the resulting frictional torque (preload) must be re-checked to ensure that it is within the normal range.

Stage six: body welding

The assembled torque converter is installed on a welding machine, where its body will be welded. Before welding, it is necessary to set the value of the thermal gap between the two halves of the torque converter body within 0.5-0.6 mm. If the air temperature in the room is not stable at different times of the year, then in the cold season it is recommended to increase the thermal gap by 0.2-0.3 mm.

The welding process takes place in a fully automatic mode. At the initial stage, the machine symmetrically fixes the parts to be welded by installing 8 welded points, which prevents the occurrence of thermal deformations of parts during continuous welding of the torque converter body.

At the end of welding, the axial runout tolerances must be checked. The operation to check the axial runout is carried out on the welding stand. The allowed value is up to 0.2 mm.

Stage seven: checking the thermal gap

The next stage of the repair is to check the resulting thermal gap. During the operation of the torque converter, an inevitable thermal expansion of the body and its parts occurs. The stand allows you to determine the size of the thermal gap in torque converters of various designs, both before and after repair, with an accuracy of 0.01 mm.

Stage Eight: Leak Test

The final stage of the torque converter repair is a leak test. The assembled torque converter is placed on the Air test stand, which allows you to quickly identify places of possible leaks along welded joints or in other parts of the body.

For testing, air is injected into the body of the torque converter under pressure up to 8 Bar, and then the torque converter is immersed in water. The presence of bubbles will indicate the location of possible leaks. It is advisable to hold the tested torque converter at the stand for some time. possible violations of the integrity of the case may not appear immediately. The stand does not require an electric power supply, all mechanisms are driven by pneumatics powered by an air compressor.

After final testing, the repaired torque converter is ready for installation.