Zf 6hp Converter Rebuild

8/17/2019 Zf 6hp Converter Rebuild

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ZF6HPxx series torque converters - features and problems of rebui lding.

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

Revolutionary construction of torque converters appeared together with a design of partial lockupmechanism or "controlled slip mode" of friction plate, which led to the management of the force bypressing and/or locking the clutch plate or clutch plate pack. Depending on the driving mode andother conditions, solenoid can modulate more or less pressure on the clutch plate giving driver morecontrol and power of the car. To ensure such proper operation of modern torque converter newrequirements to the material of lockup plates were developed and physical and mechanicalcharacteristics of the transmission fluid were changed.

Software that controls the process of such plate locking allows selecting the conditions oftransmission work. For instance, in some cases during acceleration, the friction plate can provide upto 80% of all produced torque from the engine, and only 20 % will be transfered by the turbine wheelof 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 suchtechnology is the six-speed ZF.

The Six-speed ZF transmissions began their "invasion" to the world market since 2000. They wereappreciated 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 overheatingand thus more rapid wear of this unit. Elements that suffer from increased load are freewheel andthe locking unit. The premature failure of these elements is also provoked by severe usageenvironment 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 needleon the dashboard starts jumping within + - 200 rpm - this indicates the moment of the torqueconverter 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 toremanufacture this item to the factory specifications. It should be noted that the pressing force in thesame 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 isnecessary to compare part numbers.

Consequence of excessive loads and "controlled slippage" is overheated surfaces of the frictionmaterial, which loses its properties. Engineers made the possibility of automatic pressure adjust byECU, but its possibilities are not limitless. There comes a time when pressure on the clutch disc isnot enough to keep the torque and there is a failure, then the clutch again locks and then it breaksagain - 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 torqueconverter usually suffers, but also parts of automatic transmission. Burnish on the surface of thefriction disc also indicates improper operation. Such disc also must be replaced.

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





After the restoration of surfaces, replacement of the seals and the clutch disc - it’s necessary toassemble the locking unit.

Here lies the main problem of the rebuilding of this TC type. Given that after finishing geometricaldimensions of the body and piston have changed, different friction plate is used and we haveshrunken springs, we cannot assume with reasonable certainty what will be the preloading force. Toindirectly 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 showsthe strength of resistance to uniformly rotating friction disc, sandwiched with a certain force betweentwo stationary surfaces.



After Installing on the stand the housing of the TC, it’s necessary to press manually the plate withleaf springs and pressure plate to the locking ring using a hydraulic press, at the same time, using atorque wrench to control the moment of lockup failure, causing the failure at the level of 5-15 Nmdepending 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 valueand 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 modelsby 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 discs.

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 disc and thus lead to breakage of this unitagain.



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

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 removalof stock from the piston provokes a snug fit to it of the clutch disc and the point of metal-metal contactappears.

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, atwhich 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 disc itself and attempts to bring it back to life by grinding it with sandpaper or re-bondingfriction 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 thebias, 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 disc, where the contact surfaces of frictionelements have failures and excessive projections, or the o-ring with a smaller diameter.

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Zf 6hp Converter Rebuild