Starter

A starter brings the diesel engine up to sufficient speed during starting to initiate compression, fuel injection and self-ignition. In MAN and Mercedes trucks, buses and vans, this electric motor must handle high peak currents, often at low ambient temperatures, with a heavy flywheel and high compression pressures. Starter motors differ in nominal voltage (12 V or 24 V), power consumption, number of teeth and module of the drive pinion, direction of rotation, mounting flange, bolt pattern and main power cable connections. Therefore, always identify a replacement starter motor using the chassis number; visual comparison or selection based solely on the model name is never sufficient.

Starters can vary considerably depending on the vehicle series, year of manufacture, emission class, gearbox and engine family. For example, a starter motor for a MAN D08 or D20 engine is not automatically interchangeable with a version for a D26, D38 or older D28. Mercedes applications also have significant differences between engines including the OM 904, OM 906, OM 457, OM 460, OM 470, OM 471 and OM 473. Even with similar engine blocks, the ring gear diameter, tooth pitch, installation depth of the drive pinion and required starting power may differ.

The Bendix, also referred to as the drive mechanism or overrunning clutch, temporarily engages the small starter pinion with the ring gear on the flywheel. When the ignition key or start button is operated, the electromagnetic starter solenoid first moves the Bendix forward. Only when the pinion engages sufficiently with the ring gear do the heavy main power contacts close and the starter motor armature rotate. The integrated overrunning clutch prevents the already running combustion engine from overspeeding the starter motor after starting. Wear to the splines, overrunning clutch rollers, spring or tooth flanks causes free-spinning, rattling, slipping or a starter motor that turns without rotating the crankshaft.

A single distinct click without the starter motor turning often indicates insufficient battery voltage, contact resistance in the earth or positive cables, worn solenoid contacts or sticking carbon brushes. Repeated rapid clicking generally indicates a voltage drop under load. Therefore, first measure the resting voltage of both 12 V batteries and then the voltage during starting. In a 24 V system, the voltage must remain sufficiently stable under load; also measure directly at the starter motor between B+ and the starter housing. A significant difference from the measured battery voltage indicates resistance in the cables, battery terminals, earth straps or main switches.

If the starter motor turns slowly, check the mechanical load as well as the electrical supply. Measure the current draw using a DC current clamp and compare it with the vehicle specifications. High current draw at low speed may indicate a short circuit in the armature windings, worn bronze plain bearings, a rubbing rotor or an engine that is mechanically difficult to turn. Normal current draw without crankshaft rotation requires inspection of the Bendix and ring gear. Through the starter motor opening, inspect the tooth flanks of the flywheel ring gear for broken, rounded or locally worn teeth.

Always disconnect the batteries before dismantling and secure the vehicle against unexpected starting. After removal, check the cable terminals for corrosion, heat discoloration and loose crimp connections; a new starter motor will not solve a poor main power connection. Clean the mounting surfaces, as the attachment to the flywheel housing often also forms an essential earth path. Use the specified bolt grade and correct tightening torque to prevent misalignment in relation to the ring gear.

Replace starter motors preventively when the Bendix repeatedly slips, the solenoid contacts are burnt or the carbon brushes reach their wear limit. At Braem, starter motors are available with technical support for correct selection based on chassis number and version.