Flywheel

The flywheel connects the engine crankshaft to the clutch and ensures stable transmission of engine torque to the gearbox. For this purpose, the flywheel is mounted to the crankshaft using flywheel bolts. In MAN and Mercedes trucks, buses and vans, this solid component also dampens torsional vibrations from the crankshaft and supports a smooth idle speed. Depending on the engine, gearbox and torque concept, these may be rigid flywheels, stepped versions for specific clutch diameters or dual-mass flywheels with an integrated spring-damper system. The flywheel is fitted with a starter ring gear on which the starter motor acts to start the engine. The correct flywheel is always determined using the vehicle chassis number.

Flywheels differ in, among other things, outside diameter, mass, crankshaft flange, centring, bolt pattern, starter ring gear and the position of reference marks. For engine families such as MAN D08, D20, D26 and D38 or Mercedes OM 904, OM 906, OM 457, OM 471 and OM 473, these parameters are linked to the exact engine type, emission level, clutch type and transmission version.

In more modern engines, EDC, or Electronic Diesel Control, plays an important role. The EDC engine control unit determines injection timing, injected fuel quantity and idle control based, among other things, on the crankshaft speed signal. This signal is generally provided by an inductive or Hall-effect crankshaft sensor that detects the teeth or reference holes of the starter ring gear. A different number of teeth, damaged trigger points, an incorrect mounting position or insufficient sensor air gap can therefore lead to crankshaft synchronisation fault codes, starting problems, limp mode, irregular idling or an incorrect tachometer display. When replacing, always check that the starter ring gear, signal pattern and any EDC reference marking exactly match the original version.

Common fault symptoms include rattling noises at idle, vibrations when pulling away, knocking during load changes, clutch slipping, a burning smell or starting problems caused by a damaged starter ring gear. In dual-mass flywheels, worn arc springs, internal plain bearings and grease loss cause excessive rotational play or tilting play between the primary and secondary masses. In rigid versions, heat spots, blue discolouration, hairline cracks and grooves in the friction surface are usually caused by an overheated clutch disc or a clutch pressure plate that does not release sufficiently.

First read out EDC and transmission fault codes and check live data for crankshaft and camshaft synchronisation during starting. Then measure the battery voltage and voltage drop across the earth and starter cables, so that a faulty starter motor is not incorrectly diagnosed as a defective starter ring gear. After removal, check the friction surface for distortion using a straightedge and feeler gauge, inspect the starter ring gear for broken or rounded teeth and check the crankshaft flange for damaged locating surfaces. For a dual-mass flywheel, measure radial play, axial movement and rotational angle according to the vehicle-specific tolerances.

Do not replace only the flywheel when the cause lies within the clutch system. Always check the clutch disc, clutch pressure plate, release bearing, clutch release fork, crankshaft oil seal and pilot bearing. As flywheel bolts are stretch bolts that can only be used once, they must be replaced with new ones every time the flywheel is refitted. Use the correct tightening torque and angle of rotation. A leaking crankshaft oil seal can contaminate the friction material with engine oil, while a defective hydraulic release bearing causes overheating and uneven loading. Clean the crankshaft contact surface carefully. Avoid grease on the friction surface and, after installation, check the operation of the starter motor, crankshaft sensor and EDC synchronisation. Braem can identify parts by chassis number and provides support with the technical assessment of new, used or reconditioned flywheels.