Cable bundle

Wiring harnesses in the engine electrical system connect the engine control unit to sensors, actuators and power supply points. In MAN and Mercedes trucks, buses and vans, they handle signals for crankshaft and camshaft sensors, coolant temperature sensors, oil pressure sensors, rail pressure sensors, injectors, glow systems, EGR valves, turbo boost controllers and engine wiring pass-throughs, among other things. A break, increased contact resistance or damaged insulation can therefore cause faults that at first glance appear to be due to a defective sensor, injector or control unit. Correct identification based on the chassis number is always essential; visual comparison or selection based solely on the model name is never sufficient.

Engine wiring harnesses differ significantly depending on the vehicle series, emission standard, engine variant, injection system and the location of control units. MAN D08, D20, D26 and D38 engines, for example, have different injector connectors, branches and lengths than older D28 configurations. For Mercedes, wiring harnesses vary considerably between engines including the OM 904, OM 906, OM 457, OM 460, OM 471 and OM 473. Vehicles with common-rail injection, EGR control, SCR aftertreatment or additional PTO equipment also often require specific loom configurations.

A wiring harness consists of wires with different conductor cross-sections, usually copper conductors with heat- and oil-resistant insulation, shielding for sensitive sensor signals, and connector housings with seals. Power supply wires for injectors or heating elements require a larger cross-section than signal wires for, for example, a pressure sensor. Twisted and shielded pairs limit electromagnetic interference in CAN communication and pulse signals from crankshaft and camshaft sensors. When replacing, pay attention to connector coding, number of pins, wire cross-section, retaining clips, grommets and the exact routing in relation to exhaust components, the cylinder head and engine mounts.

The most common damage is caused by heat ageing, vibrations, chafing against engine mounts or line brackets, oil exposure, water ingress and rodent damage. Insulation can harden or wires can break internally, especially near the cylinder head, injectors, EGR cooler, turbocharger and connector pass-throughs. Typical fault symptoms include difficult starting, uneven idling, cylinder misfires, reduced engine power, limp mode, intermittent CAN faults and fault codes for open circuits, short circuits to ground or short circuits to supply voltage.

First read out all fault codes and freeze-frame data using a suitable diagnostic tester. Then visually inspect the wiring harness along its entire length for melted insulation, damaged corrugated conduit, loose clips, oil in connectors and corroded connector pins. Next, use a multimeter to measure the continuity and contact resistance of every suspect wire, preferably between the sensor or actuator connector and the relevant ECU connector. A continuity test alone is insufficient: also carry out a load test with a test lamp or controlled current draw, because a partially broken copper wire may still show correct resistance when unloaded. Also check for short circuits to ground and supply voltage.

For intermittent faults, a wiggle test is valuable: carefully move the wiring harness while monitoring live data, fault status and supply voltage. Compare sensor values with reference values and check the integrity of the shielding on shielded circuits. Use an oscilloscope on CAN wiring to assess the signal waveform, reflections and voltage level; with the system de-energised, a resistance measurement between CAN-H and CAN-L can also reveal incorrect termination.

Replace a wiring harness when multiple wires are affected, the insulation has become brittle, connectors are thermally damaged or previous repairs do not provide sufficient strain relief and sealing. Individual repairs are only justified when the damage is local and can be repaired using wires of identical cross-section, correct crimp connections, adhesive-lined heat-shrink tubing and restored shielding. Avoid soldered connections on heavily vibrating engine components: they can become rigid and break again next to the connection.

Install the new loom according to the original routing, with sufficient slack for engine movement and clearance from hot exhaust components. Replace damaged retaining clips and seals, and check all fault codes, live data and actuator functions after installation. Braem can assist with chassis-number-based parts identification and technical selection, helping to minimise downtime caused by an incorrect wiring harness.