Pressure sensor

Pressure sensors convert the pressure of compressed air, refrigerant or hydraulic fluid into electrical readings. The relevant control unit uses these signals for system control, monitoring and fault detection. An incorrect reading can disrupt system operation or trigger false pressure warnings. On MAN and Mercedes trucks, buses and vans, diagnosis must therefore distinguish between a faulty pressure transducer, an electrical problem and genuinely insufficient system pressure.

Depending on the vehicle equipment, these pressure sensors may be found in the following systems, among others:

1. Compressed air supply: measure the stored pressure in air reservoirs for pressure monitoring, warnings and air supply control.
2. Braking system: measure pressure in the brake circuits for monitoring and electronic brake control.
3. Air suspension: measure pressure in the air springs to support load determination and suspension control.
4. Air conditioning: measure refrigerant pressure for compressor control and protection and fan control.
5. Transmission: depending on the version, measure hydraulic or pneumatic pressure to monitor and control gearshift and clutch functions.
6. Auxiliary hydraulic systems, such as a tail lift, tipper or loader crane: measure operating pressure for control, load monitoring and protection, where the system is equipped for this.
7. Direct tyre pressure monitoring system: measure tyre pressure and report pressure loss to the monitoring system.

There are absolute pressure sensors, gauge pressure sensors and differential pressure sensors. Absolute pressure sensors measure relative to a vacuum; gauge pressure sensors use ambient pressure as a reference. Differential pressure sensors monitor, for example, the pressure drop across a filter in a hydraulic system. Piezoresistive sensing elements and ceramic measuring cells are common, depending on the application and design. A pressure switch with a fixed switching point is not a substitute for a proportional pressure sensor.

On MAN and Mercedes vehicles, connections, pressure ranges and signal characteristics vary by system and version. On Sprinter and TGE models too, the correct choice depends on the vehicle equipment and system configuration. Parts must always be identified using the chassis number. Visual comparison or selection based solely on the model name is never sufficient.

In addition to the part number, check the nominal operating pressure, overpressure resistance, temperature range, measurement accuracy and permissible zero offset. The thread, sealing seat, insertion depth and connector keying must match. Stainless steel diaphragms and elastomer seals must be suitable for the medium. Many analogue versions operate with a 5 V supply voltage and an output signal of approximately 0.5–4.5 V, but this is not a universal standard; digital variants require a different measurement method.

Typical faults include zero drift, temperature-related signal deviations, diaphragm damage caused by pressure spikes and intermittent contact caused by vibration. Moisture in connectors, corrosion or broken wires can sometimes cause the same fault codes. With differential pressure sensors, blocked, split or incorrectly connected sensing lines can make a serviceable sensor appear faulty.

1. Read out fault codes and live data. Record the system status, load and temperature when the fault occurs. Compare requested and measured pressure where the vehicle diagnostics support this.
2. Check connectors, the wiring harness, pressure passages and any sensing hoses. With the circuit safely depressurised, compare the sensor reading with the specified reference value. With equal pressure at both sensing ports, check the differential pressure reading against the manufacturer's tolerance.
3. Measure the supply voltage and signal with a high-impedance multimeter using suitable test adapters. Check the voltage drop across the sensor earth connection with the sensor connected. A shared 5 V reference can be disrupted by another faulty sensor.
4. Compare hydraulic or compressed air pressure with a calibrated pressure gauge at the specified test point. Where permitted, use a regulated pressure source to test at several pressure points, including rising and falling pressure, to assess linearity and hysteresis.
5. Use an oscilloscope to detect brief signal dropouts during a wiring wiggle test. Assess digital signals only in accordance with the applicable protocol and the manufacturer's data.

Depressurise the relevant circuit in accordance with the manufacturer's procedure before opening any connections. Dangerous residual pressure may remain in compressed air and hydraulic systems: secure the vehicle against rolling and support air-suspended vehicle components or raised loads in accordance with the manufacturer's instructions. Refrigerant must only be recovered from air conditioning systems by authorised personnel using suitable equipment. Have genuinely low system pressure investigated first; replacing a sensor will not fix a faulty pressure supply or a leak.

Fit the specified seal, clean the seat and observe the tightening torque. Do not use sealant unless explicitly permitted. Also repair damaged wiring or clear blocked sensing hoses. After replacement, carry out zero-point calibration if required, clear fault codes and check pressure readings and leak-tightness at operating temperature and under load. This prevents a temporary fault-code fix from becoming a recurring workshop complaint.