October 2021

OCTOBER 2021 AFTERMARKET 31 Fig.1 Fig.2 Fig.3 Fig.4 Fig.5 to be in good condition. The wiring was also tested for a short circuit between the supply and signal wires which, no fault was found here either. This proved the fault was in the within the DPF pressure sensor. New fault The diagram as seen in Fig.2 illustrates a simplified sensor layout, with a piezo-resistive bridge circuit connected to an operational amplifier. The bridge is two series circuits connected in parallel. Normally three resistors within the bridge circuit will have the same resistance value with one resistor having a variable resistance. With zero pressure applied to the circuit, all resistance values are equal so the output from the operational amplifier is zero (V+ = V-). When a pressure change occurs in the sensor the bridge will become unbalanced and a voltage will be output (Vout) to the sensor evaluation circuit. A new sensor was sourced and fitted, a volt meter connected to the signal wire displayed 0.5 volts with the key on, engine off. With the engine idling the voltage remained at 0.5 Volts and on a road tested increased to 0.9 Volts under wide open throttle, a new fault was now present in the engine management system: P2453 - Diesel particulate filter pressure sensor, signal implausible. The graph as seen in Fig.3 outlines the normal expected voltage output with respect to exhaust gas differential pressure. This code would initially clear but return once the ignition key was cycled (key on, engine off). This was an indication the engine control module detected an error with the signal voltage under this condition. Any mechanical faults could be ruled out as the engine did not need to be running for the fault code to return. A basic setting for the DPF sensor was carried out using a diagnostic scantool, however this procedure failed; See Fig.4. Next step The next step in the diagnostic process was to manipulate the signal output voltage using a potentiometer. The voltage was altered in steps of 100 millivolts and the trouble code erased at each increment. The trouble code returned each time until a voltage of 1 volt was reached. The code would not return when the ignition was cycled. This indicated the expected key on, engine off voltage from the sensor. Further investigation into the part supplied was carried out and the issue was found with the supplier of the part. Although the part was ordered from the vehicle identification number there was a different part number supplied. This can be normal procedure as parts can often be superseded. It was later found that the part supplied for vehicles fitted with a Siemens engine control module, the part number of this sensor was 281 006 005. The sensor originally fitted to the vehicle was 281 006 083 and these sensors are fitted to engines with a Continential engine control module. With the incorrect sensor fitted the engine control module interpreted the signal voltage as a pressure of – 15 kPa, hence the implausibility fault code. The graph as seen in Fig.5 outlines the difference in voltage output between both pressure sensors. The fault code was cleared and the vehicle preformed as expected with the correct DPF Pressure sensor fitted. diagnostics.snapon.co.uk