Rebuilding of ZF 6HP converter explained

ZF6HPxx series torque converters – features and problems of rebuilding.

KINERGO article for Transmission Digest magazine
Source: Transmission Digest 

Modern car market requires manufacturers to design powerful vehicles, which should be driven with comfort. Usage of modern high-performance engines gave engineers the task of developing a fundamentally new approach to the transfer of torque from the engine to the wheels. Therefore, some time ago the concept of transferring torque to vehicle wheels had to be revised. Especially revision of the link, which acts as an intermediary between the engine and the wheels was needed.  This is why a hydrodynamic torque converter for decades of usage in automatic transmissions has undergone such a number of changes in construction.

Revolutionary construction of torque converters appeared together with a design of partial lockup mechanism or “controlled slip mode” of friction plate, which led to the management of the force by pressing and/or locking the clutch plate or clutch plate pack. Depending on the driving mode and other conditions, solenoid can modulate more or less pressure on the clutch plate giving driver more control and power of the car. To ensure such proper operation of modern torque converter new requirements to the material of lockup plates were developed and physical and mechanical characteristics of the transmission fluid were changed.

Software that controls the process of such plate locking allows selecting the conditions of transmission work. For instance, in some cases during acceleration, the friction plate can provide up to 80% of all produced torque from the engine, and only 20 % will be transfered by the turbine wheel of the converter. This gives dynamics of acceleration in combination with the comfort of movement ( the gear shift is almost insensible). Most common representative of transmission with such technology is the six-speed ZF.

The Six-speed ZF transmissions began their “invasion” to the world market since 2000. They were  appreciated by car manufacturers (they are light and environment friendly) and by the consumers (they are economic and have fast gear shift).

However, high performance of these transmissions has a downside. Early involvement of the torque converter lockup, which ensures both fast gear shift and high energy efficiency, cause overheating and thus more rapid wear of this unit. Elements that suffer from increased load are freewheel and the locking unit. The premature failure of these elements is also provoked by severe usage environment if one drives without changing ATF and has an aggressive driving style. Consequently, snatching and vibration during even movement begin to appear. Sometimes the tachometer needle on the dashboard starts jumping within + – 200 rpm – this indicates the moment of the torque converter lockup failure and the need for repair.

Let us look inside some of the typical representatives of ZF 6HPxx family.

There are several types of torque converters made by subsidiary Sachs and those of LUK.


Design solution to make locking unit with preloaded clutch greatly complicated the possibility to remanufacture this item to the factory specifications. It should be noted that the pressing force in the same modification of the torque converter might vary depending on engine capacity and model of a car. It is not possible to determine the identity of the torque converters by external examination – it is necessary to compare part numbers.

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Consequence of excessive loads and “controlled slippage” is overheated surfaces of the friction material, which loses its properties. Engineers made the possibility of automatic pressure adjustby ECU, but its possibilities are not limitless. There comes a time when pressure on the clutch disk is not enough to keep the torque and there is a failure, then the clutch again locks and then it breaks again – it happens over and over. Friction provokes overheating, which starts to break down the binder of the material itself. As a result of snowballing process of lockup destruction, not only a torque converter usually suffers, but also parts of automatic transmission. Burnish on the surface of the friction disk also indicates improper operation. Such disk also must be replaced.

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Another weak link of lockup unit are leaf springs for preloading the clutch disk. Hard temperature regime affects even elasticity of the metal. 90% of torque converters that come to our workshop for repair of this unit do not provide clamping force required for proper operation of the lockup.

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Problems with freewheel of ZF6HPxx do not occur as often as problems with lockup, but this unit also fails. Externally you may not see problems if the destruction has just begun, but if you open the cover of the stator, the changes are immediately apparent. Even after the revision of the separator of the freewheel during repair, there is no guarantee that it will not break down after 1,000 km. Therefore, we recommend to change this unit.

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Seal system of blocking unit in this series is well-chosen and durable, keeping all its properties. However, every repairer knows that these parts must be replaced. Ingress of moisture into ATF, wear or destruction particles of transmission elements causes wear and deterioration of the sealing elements. The replacement cost of these items is not so high as to sacrifice the quality of repair and, when properly performed, new seals and o-rings will work longer and better than the old ones.

Problems with pump hubs in these torque converters do not occur as often compared to a 5-speed gearbox, but still they exist.

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Pay attention to the version with the extended plug. When disconnecting engine and transmission carelessly torque converter plug can bend due to the large cantilever load. As a result, the main parameter is violated – the axis of rotation of the whole unit moves, which necessarily leads to problems both in the TC and the transmission. Be sure to check the condition of the mounting and plug in such TC models.

Lockup unit rebuilding

This unit is made indecomposable by the factory. To get to the lockup clutch and O-ring, it is necessary to cut the mounting area of the piston to the housing. It’s important to do it very carefully to keep the contact surfaces on the housing and the pressure plate. A lathe should be used for it.

Once opened, we can inspect internal parts of the unit itself and the clutch disk.

Even if the clutch disk has no visible damage, do not leave it as it is – replace it with a new part. Used disk will no longer provide the necessary grip to the housing.

If there is a complete destruction of the friction material, clutch plates begin to slip metal to metal. Surfaces wear also. To return the necessary flatness and grain piston and torque converter housing are lathed.

After the restoration of surfaces, replacement of the seals and the clutch disk – it’s necessary to assemblethe locking unit.

Here lies the main problem of the rebuilding of this TC type. Given that after finishing geometrical dimensions of the body and piston have changed, different friction plate is used and we have shrunken springs, we cannot assume with reasonable certainty what will be the preloading force. To indirectly measure this force without welding locking unit, you can use a special stand.


This stand uses method of indirect measurement of contact force in relation to the force of friction. This stand allows you to measure, define and control down force without the use of welding. It shows the strength of resistance to uniformly rotating friction disk, sandwiched with a certain force between two stationary surfaces.

After Installing on the stand the housing of the TC, it’s necessary to press manually the plate with leaf springs and pressure plate to the locking ring using a hydraulic press, at the same time, using a torque wrench to control the moment of lockup failure, causing the failure at the level of 5-15 Nm depending on the clamping force parameter for a particular type of TC.

By varying the amount of clearance between the piston and the housing, achieve the desired value and fix the plate with three welding points.

Check the readings on the stand once again and compare with the values ​​of a new torque converter. Take values ​​from the manual to the stand. We get regularly updates of the values on new models by email. Now you can continue to assemble the TC by sending it to the welding machine.

Some models of ZF6HPхх use lockup with two clutch disks.

In these models you should pay attention to the plate with external teeth connected to the turbine. The fact is that if the lockup unit has been damaged, the teeth of the plate can be deformed. Damaged edge of a tooth can cause slipping of friction disk and thus lead to breakage of this unit again.

Typical errors in the rebuilding of torque converters of ZF six-speed automatic transmissions

We will not focus on standard errors that may happen during rebuilding of «classical» torque converters. The main problem remains the locking unit.

In torque converters with one clutch after lathe turning there were occasions when excessive removal of stock from the piston provokes a snug fit to it of the clutch disk and the point of metal-metal contact appears.

A little more about it. The edge of the piston has a bevel, which can be cut off during lathe turning, as a result the outer diameter of the friction surface of the piston increases. In turn, the diameter, at which the bend of clutch tooth starts, remains unchanged. The result is that we get a source of noise, metal shavings and lop-sided pressure.

The clutch disk itself and attempts to bring it back to life by grinding it with sandpaper or re-bonding friction linings can be an additional problem.

Typically, these manipulations result in breakage after 200-1000 km of run. Car starts to shake again, lockup breaks and tachometer «bounces» even more than before the repair.

Attempts to restore the down force at the lockup unit by artificial deformation of leaf springs (stretching of springs) lead to the same sad result. After a while they regain their former geometry (as before the repair, but not as a new part).

There are other mistakes in the rebuilding: the piston or thrust plate with leaf springs pressed on the bias, preliminary measurement of the pressing force on dry friction plate, overheating of the housing, forget to put o-ring before final assembly and welding…

Sometimes there are bad parts. For example, the clutch disk, where the contact surfaces of friction elements have failures and excessive projections, or the o-ring with a smaller diameter.