Engine sensors

Engine sensors provide the engine management control unit with the measured values required for precise fuel injection, turbo boost pressure control, exhaust gas aftertreatment and thermal protection. On MAN D08, D20, D26 and D38 engines and Mercedes OM 904, OM 906, OM 457, OM 471 and OM 473 engines, sensors differ not only by engine family, but also according to emission standard, software version, wiring harness and fitted injection or aftertreatment systems. Therefore, always identify each sensor using the vehicle chassis number. Visual comparison, connector colour or selection by model name alone is never sufficient.

A modern diesel engine uses several inductive, Hall-effect, piezoresistive and NTC sensors. The engine control unit continuously compares these signals with calculated reference values. An abnormal signal may limit power, block regeneration, activate a limp-home strategy or, in severe cases, make starting impossible.

• Crankshaft position sensor: records crankshaft speed and the exact crankshaft position. This signal determines the injection timing and is essential for starting and synchronisation.
• Camshaft position sensor: identifies the cylinder phase and supports the correct control of the injectors. A fault often causes extended cranking time, irregular running or synchronisation faults.
• Boost pressure sensor: measures the absolute pressure in the intake manifold or charge air pipe. The control unit uses this value for turbo boost pressure control and dosing of injected diesel fuel.
• Intake air temperature sensor: corrects the calculated air mass according to the temperature and density of the intake or cooled charge air.
• Coolant temperature sensor: monitors engine temperature and controls, among other things, cold-start enrichment, fan activation and overheating protection.
• Engine oil pressure sensor: checks engine oil pressure in the lubrication circuit. A plausibility fault must be distinguished from genuinely low pressure caused by a worn oil pump, leakage or bearing clearance.
• Fuel pressure sensor: measures rail pressure in common-rail systems. An abnormal value can result in poor starting, power loss, pressure build-up fault codes or irregular combustion.
• NOx, exhaust gas temperature and differential pressure sensors: monitor nitrogen oxides, thermal load and the pressure differential across the particulate filter or catalytic converter respectively. They are crucial for EGR, SCR and DPF control.

Common complaints include an illuminated engine warning light on the dashboard, limited torque, increased fuel consumption, difficult starting, black smoke, incomplete DPF regeneration or a fault message for AdBlue and emission systems. However, a stored fault code is not automatic proof that the sensor is defective. Corrosion on connector pins, oil ingress in wiring harnesses, worn-through insulation, poor earth points, leaking charge air pipes and contaminated measuring ports can cause similar fault symptoms.

First read out fault codes, freeze-frame data and live values using suitable diagnostic equipment. Then compare the measured value with the reference value when the engine is cold, at idle speed and under load. For NTC temperature sensors, check the resistance-temperature curve with a digital multimeter; an open circuit or short circuit generally produces an implausible fixed temperature value. For three-pin pressure sensors, measure the 5 V reference voltage, earth integrity and signal voltage under pressure. Check inductive crankshaft and camshaft sensors with an oscilloscope for amplitude, signal waveform and missing teeth. A multimeter alone cannot detect a disturbed pulse pattern.

Replace sensors only after checking the wiring, connectors, supply voltage, earth and mechanical cause. Do not clean affected measuring elements on exhaust gas temperature, NOx and differential pressure sensors with aggressive products: replace damaged components and also check the measuring lines for condensation, soot build-up or heat damage. Fit pressure sensors with the correct O-rings and tightening torques to prevent false air, coolant leakage or damaged plastic housings.

After installation, fault memories must be cleared, adaptation values or learning procedures carried out where specified, and live data checked during a test drive. Braem can provide technical support in identifying the correct sensor and assessing fault symptoms.