Workshop Tips for SACHS Products
- Installing Clutches
- Changing the release bearing
- Checking DMF
Professional clutch replacement
Replacing the clutch of a manual transmission so expertly that the gears can be perfectly shifted again is mere routine for an automotive technician.
However, as manufacturer analyses of some clutches that are subject to complaint show there are indeed very few exceptions.
In these cases, the damage pattern proves that observing only a few decisive details during component replacement mostly means that a lot of time and high costs can be saved and unhappy customers can be avoided.
Cleaning all relevant parts
Prior to the installation of new clutches, it is for instance required to clean the inside of the bellhousing as well as all components of the clutch actuation system, e.g. the clutch release fork, the guiding sleeve for the clutch release bearing and also the transmission input shaft including the multi-gear spline. This is the only way to identify potential signs of wear on these parts which adversely affect the release behavior of the new clutch. If such signs are identified, the described components must be replaced as well.
Don't forget the axial run-out test
Spare parts from manufacturers of proprietary goods are carefully produced according to state-of-the-art production standards. However, particularly precision components such as clutches may be damaged due to improper handling during transport to the workshop. The packaging condition is a first indication. Therefore, an axial run-out test of the clutch disk should be performed prior to each clutch installation, even if this is not suggested in the service plan. Since clutch actuation is a very precise process, the linings' axial run-out must not be significantly above 0.5 millimeters. Otherwise, shifting is very difficult or entirely impossible.
Precisely centering the clutch disk
If the clutch disk is precisely centered even before it is pressed on by the clutch pressure plate, this means easier installation of the transmission: You have minimized the risk of damaging and/or deforming the hub spline in the process. The tolerance between transmission input shaft and the hub spline is only a few hundredths of a millimeter. Even minimal deformation during insertion of the transmission input shaft reduces the mobility that is required for later declutching.
Keeping clutch linings clean
The clutch function and, therefore, the shifting characteristics are further improved if even specially coated hub splines are coated with the special grease included in the delivery prior to installation – suitable tools would be a brush or toothbrush, for instance. In order to avoid clutch judder, excess lubricant must always be removed and the clutch linings must be kept perfectly clean. For the same reason it is recommended to additionally clean the friction surfaces of the flywheel and the pressure plate.
As for many other areas, the following principle also applies to clutch replacement: Small additional investments at the beginning will ultimately pay off many times over.
Correct assembly is particularly important for the concentric slave cylinder to prevent seal and clutch damage. Furthermore, ZF Services recommends replacing this component every time the clutch is replaced.
Repair not possible
Vehicle clutches are complex systems, which consist of many individual components. These include the concentric slave cylinder (CSC), also known as the central release unit. Like the disks, it is subject to wear. However, when damage occurs, the CSC cannot be repaired – it needs to be completely replaced. CSC replacement is also recommended when replacing the clutch or removing the transmission. Replacing the clutch in isolation may lead to the need for further costly repairs shortly afterwards.
Dirt deposits on the outside of the CSC piston guide can compromise the sealing lip – if the piston returns to its non-extended position, then it passes over the particles. CSC defects are also often the result of assembly errors: The clutch pedal may not be actuated when removing the clutch and transmission with a closed hydraulic circuit – if this does occur, it pushes the CSC piston too far over. This can damage the piston sealing lip, which causes a loss of hydraulic fluid and leads to clutch defects. Also manual compression of the CSC often results in seal damage – likewise with the application of lubricants: These make the seals swell up and cause CSC failures.
Filling and venting
The CSC may only be filled and vented when it is assembled in the installation position: This means, it must be installed in the clutch bell housing on the transmission according to the instructions – at the same time the clutch and transmission should be completely flange-mounted to the engine again beforehand. Hydraulic fluid should only be filled as specified by the manufacturer. Venting is carried out with the equipment on hand. Even under pressure the system may only be implemented when the concentric slave cylinder is in the installation position.
Before checking the DMF...
...you should check whether contaminated ground contacts may be the cause of failure.
Car starting problems are frequently misinterpreted. One of the wrong diagnoses in this context is a defective dual-mass flywheel. Before you remove the DMF, you should therefore exclude other causes in the area of the flywheel. Engine starting problems, flickering headlights or unusual starting noise in the area of the clutch and DMF may also be caused by contaminated ground contacts. The cleaning system for electric contact surfaces from SACHS allows you to easily and efficiently clean all contacts and eliminate many problems.
Checking the DMF – Stress tests on the flywheel
If the fault diagnostics options are not sufficient for the installed DMF, the DMF can also be examined visually, mechanically, or on a test bench, although this latter possibility remains a task for the manufacturer.
Any workshop can check a dismantled dual-mass flywheel with simple tools. It is important to know the right methods.Telltale signs
During a visual inspection, a discolored friction surface on the secondary flywheel (arrow A) indicates an overheated or overloaded DMF. This effect can occur if drivers let the clutch slip for too long. In addition, there is a danger that longer-lasting overheating will harden the special lubricant in the DMF, which frequently ends in component failure. If there are visible fissures in the friction surface (arrow B) , the DMF has to be replaced – otherwise it may crack. Yellowish discolorations in the area below the friction surface (arrow C) are also a reason for replacement, as the DMF axial bearing is then worn. This is audible during vehicle operation. If lubricant with fragments escapes from the inside of the component, a new part also has to be fitted.
Rotate to the maximum extent
For the lever or mobility test, the primary flywheel has to be fixed in a vise – with two bolts through the holes for the crankshaft mounting. A test lever then has to be screwed to the holes for the clutch mounting. If the secondary flywheel is rotated from the loose position to the left and right maximum stop, the angles achieved and the expenditure of force felt in both directions should be identical. In addition, no exceptional movement resistance may occur
If these criteria are not met, this indicates defective or broken components inside the DMF. These may scratch through the housing so that lubricant escapes. If the full rotation angle of around 60 degrees is not reached on each side, there is also a possibility of the lubricant hardening as a result of thermal overloading – this is often visible from the discolorations. In these cases, the DMF has to be replaced.