Repair and Installation of the Mercedes OM906 Engine in the Demag AC 40 City Crane – Comprehensive Service by WIBAKO

Introduction – Demag AC 40 City mobile crane and Mercedes OM906 engine

Mobile cranes, such as Demag AC 40 City, are designed to work in tight spaces, near buildings, where maneuverability, precision, and reliability matter. The heart of the presented machine is the Mercedes OM906 engine, a popular diesel unit valued in off-highway applications for its durability, flexibility, and work culture. We undertook a comprehensive repair, testing on the dynamometer, and reinstallation of the engine in the machine. The order was executed by the WIBAKO service, and we carried out the fieldwork for the client in the Łódź Voivodeship, in the vicinity of Łódź, with the transport of the engine to our headquarters in Kojszówka.

The aim of the actions was to restore full functionality of the powertrain and ensure service predictability for another long period of operation. In the article, we describe the entire process: from diagnostics in the machine, through disassembly, verification, and repair, to tests on the engine dynamometer, installation in the crane, and maintenance recommendations. Additionally, we indicate in which other machines the OM906 engine is found and what benefits our working standard brings to the user.

Preliminary diagnostics in the machine – cause of the report and confirmation of faults

The machine came to us with symptoms of low oil pressure and occasional, increasing knocking under load. We performed a basic review of operating parameters and a preliminary listening test. The preliminary analysis indicated possible worn crankshaft and connecting rod bearings. To confirm, we measured the oil pressure on a warmed-up engine, checked the condition of the oil filter, and conducted a quick endoscopic screening through service openings. Metal particles were detected in the filter, and the pressure values were below the norm for the given operating temperature, confirming the direction of further actions.

We planned the in-depth diagnostic stage on the workbench – not all components can be assessed in the vehicle. After consulting with the client, we proceeded to disassemble the powertrain, adhering to safety principles and preparing the machine for service downtime.

Engine disassembly from the crane – WIBAKO mobile service and logistics of the work

The disassembly was carried out on-site, near the client's base. According to our standard, the engine disassembly is performed by a crane provided by the client. Our mobile service arrived on-site with a full set of tools and workshop equipment. We started by disconnecting the batteries, securing the electrical system, and draining the operating fluids. We disassembled the intake and exhaust piping, fuel and cooling lines, as well as accessory components that could interfere during the removal of the powertrain.

We placed a strong emphasis on protecting sensitive components – we capped the intake and cooling system ports, marked the harnesses and wires to shorten the later assembly time. After unscrewing the mounts and checking the suspension points, the engine was safely removed and prepared for transport to our headquarters. We documented the entire process photographically to ensure that the reconfiguration in the machine proceeded flawlessly.

Verification in the workshop – confirmation of bearing wear and assessment of other components

On the workbench, we conducted a full disassembly of the unit. After removing the oil pan, we found clear signs of scoring on the main and connecting rod bearings and metal particles in the oil sump. Verification of the crankshaft included visual inspection, runout measurement, journal dimensions, and checking oil passages. We noticed signs of wear qualifying the crankshaft for grinding. Verification of the connecting rods showed no cracks, but the need to check geometry and weight – we restored the components to the selection class.

The cylinder head underwent a leak test in a water bath with pressurized air. We found no cracks, but standard machining, replacement of guides and valves was required. Verification of the turbocharger showed exceeded lateral clearances – we recommended regeneration. The injection system was tested on the test bench – atomization and correction doses deviated from nominal values, justifying comprehensive regeneration at a specialized partner facility.

Repair process – crankshaft-piston system, cylinder head, injection, accessories

Crankshaft-piston system

We performed grinding of the crankshaft to the first oversize and cleaning of the oil passages. We used new main and connecting rod bearings in the appropriate assembly class. The piston-ring-bushing sets were selected according to specifications and nominal working clearances, ensuring proper sealing of the combustion chamber and stable oil pressure. We also checked the axial clearance of the crankshaft and the parameters of the oil pump.

Cylinder head

The cylinder head underwent machining, replacement of guides, valves, and seals. We set the valve clearance according to the service card. The cooling and oil passages were flushed and blown out. We verified the flatness of the mating surfaces and the tightness of the seats after machining.

Fuel system

The injectors underwent full regeneration on the test bench, restoring atomization parameters, doses, and return tightness. The injection pump was regenerated at an external, certified facility and delivered with a settings protocol. We checked the high-pressure lines for micro-cracks and deformations – questionable components were replaced.

Accessories and professional assembly

The turbocharger underwent regeneration combined with core balancing. We tested the starter and alternator for current draw and charging efficiency. Belts, rollers, tensioners, and auxiliary pumps (coolant, power steering – if present in this configuration) received new consumables. We assembled the engine according to the procedure – with tightening controlled by torque and angle, using appropriate assembly materials and maintaining assembly cleanliness. All seals and gaskets were new, compliant with quality references.

Test on the engine dynamometer – 16 hours of running-in and full loads under control

As one of the few companies in the industry servicing construction machines, we have an engine dynamometer that allows us to conduct load tests outside the machine. This significantly reduces the risk of surprises after installation and shortens the downtime of the crane. The procedure includes two phases: running-in and full loads.

The first phase is a minimum of 16 hours of operation with constant monitoring of basic parameters. We maintain a load of about 20–30% of nominal power, monitoring oil pressure, temperature, smoke, fuel corrections, idle stability, and response to added load. During this time, the initial seating of the rings in the bushings occurs, stabilization of clearances, and confirmation of system tightness. We record the temperature profiles of the coolant, oil, and intake air, as well as observe the operation of the turbocharger.

After successfully completing the running-in, we proceed to the loading stages. We apply multiple cycles from partial to full load, simulating various operating scenarios. We check maximum boost pressure values, fuel doses, behavior under continuous load, and repeatability of performance. We analyze any vibrations and unusual sounds. Confirmation of compliance of parameters with reference values is recorded in the dynamometer protocol. This way, the client receives an engine that has undergone objective verification on the stand, and our assembly teams are assured that the unit is ready for installation in the Demag AC 40 City.

An additional advantage of the dynamometer is the ability to quickly detect even minor irregularities in the accessories – such as small leaks in the fuel circuits under load, which could go unnoticed at idle. In practice, this reduces the number of corrections after installation and shortens the startup of the machine in the field.

Engine installation in the crane – precision, piping, and tests with the operator

The installation is a stage where we pay special attention to the compliance of the configuration with the disassembly and to assembly hygiene. Together with the client, we ensured access to an auxiliary crane for introducing the unit into the compartment. After positioning the engine on the mounts and preliminary tightening of the fasteners, we proceeded to connect all circuits: cooling, oil, fuel, intake, and exhaust. Air piping was thoroughly cleaned and checked for micro-cracks and the condition of clamps – leaks in the intake threaten false air and power loss. We replaced questionable sections, used new gaskets and clamps.

We bled the fuel system, ensuring the cleanliness of the ports and lines. In the cooling system, we installed new clamps and seals, and filled the coolant according to the manufacturer's specifications. We assembled the exhaust system considering thermal compensation. We checked electrical connections, grounds, sensors, and control elements. All critical bolts were tightened to catalog torque. After filling with fresh oil and filters, we proceeded to pre-start tests: cranking the starter without fuel to preliminarily supply the oil system and verify tightness.

The first start-up took place in the presence of the operator and the user representative. We monitored operating parameters when cold and after reaching working temperatures. After a short test drive with load functions of the crane, we reviewed the connections for any leaks and looseness. If necessary, we made minor adjustments to idle settings and throttle response to ensure smooth engine operation throughout the range. At this stage, ergonomics of operation is also important – together with the operator, we checked the machine's response to precise movements to confirm that the delivery of torque and characteristics meet the expectations of urban work.

The final step was a functional test of the entire powertrain with a check for tightness of all connections. After positive verification, we prepared the assembly protocol and provided the user with the as-built documentation along with operational recommendations.

Engine maintenance after repair – operational and service recommendations

All engines after repair at WIBAKO are covered by warranty and full service support. We recommend the first filter change and connection check after 50 hours of operation. This inspection is carried out by our mobile service at the client's location – it is not only a filter change but also an assessment of the overall condition of the accessories, checking tightness, and operating parameters. We suggest subsequent inspections every 250 hours, but no less than every 3 months. Regular checks help maintain stable oil pressure, proper temperatures, and work culture, minimizing the risk of unplanned downtimes.

We also train the operators in daily checks: observing sounds, smoke, fluid levels, and any leaks. If necessary, we provide quick access to spare parts, regenerated components, and diagnostic support remotely or on-site.

Information about machines that use the Mercedes OM906 engine

The Mercedes OM906 engine is widely used in specialized and off-highway applications. In practical service, we have encountered it in self-propelled cranes, municipal machines, some loaders, and OEM equipment carriers. This means good availability of parts and proven repair procedures, which shortens downtime and facilitates service planning. As a result, users of models such as Demag AC 40 City can count on stable service support and predictable maintenance costs.

Summary – benefits for the user of Demag AC 40 City

The comprehensive repair and tests on the dynamometer allowed us to restore full functionality of the Mercedes OM906 engine. Key benefits for the user include: stable oil pressure and reduced noise after replacing bearings and grinding the crankshaft; improved performance due to restored compression and injection parameters; reduced risk of failure after turbocharger regeneration and verification of accessories; shorter startup time after installation due to precise documentation and markings; and assurance resulting from the dynamometer protocol. WIBAKO guarantees not only the execution of the repair but also full after-sales care – including an inspection after 50 hours and regular periodic checks.