WORKSHOP TIP FROM ZF AFTERMARKET
Conduct a professional DMF Stress Test before time-consuming component removal
In spite of ever greater horsepower and higher torques, today’s vehicles actually run very smoothly. The dual-mass flywheel (DMF) plays a crucial role in this by absorbing and almost completely eliminating torsional vibrations with its integrated spring and damping system.
If workshops hear unusual noises from a vehicle, the DMF is usually the first port of call, even though in some cases it may not be the source of the problem.
However, ZF recommends workshops carry out a test drive. If unusual noises are heard when the engine is started, it may indicate a defective DMF, but it could also be due to the engine cranking speed being too low.
Other causes could be poor starter performance, low battery voltage due to oxidation, or corrosion of the electrical contact surfaces. To rule this out, ZF Aftermarket recommends a professional clean is undertaken.
Another possible cause of unusual noises could be defective bearings in the belt tensioner freewheel.
It is reasonable to suspect a faulty DMF if the noises occur during normal acceleration and gear changes. To determine this, workshops should accelerate the vehicle in a high gear above idle speed from approximately 1200 rpm, with constant full throttle. If there are no banging noises, unusual vibrations or vehicle bucking while doing this, the DMF can be largely ruled out as a source of error.
Engine diagnostics, such as the measurement of engine starting speed and the injection quantity regulation can provide additional information to workshops.
After the test drive and engine diagnosis, ZF Aftermarket recommends that workshops perform a visual and mechanical inspection of the actual DMF. A discoloured friction surface on the secondary flywheel is an indication of a heavily overheated or overloaded DMF, which can occur when the clutch is allowed to slip for too long.
Such overheating over an extended period of time can cause the special grease in the DMF to harden. If there are visible cracks in the friction surface, the DMF must be replaced as it may crack at certain speeds and in the worst case scenario, total failure can occur.
ZF Aftermarket offers its workshop partners a high-quality bespoke tool for quick and professional inspection of Sachs dual-mass flywheels in passenger cars (PC) and light commercial vehicles (LCV). The tool allows workshops to check the free travel of the torsional damper, the regularity of the torsional damper’s spring force, the axial bearing condition and the displacement travel of the radial bearing position.
An additional tip from ZF experts: A ‘smacking’ sound during the mobility test is a positive sign – indicating the right consistency of the special DMF grease.
For more from ZF, visit www.zf.com/au