Operating parameters complicate a simple fault By Damien Coleman, Snap-on Diagnostic Software Specialist
Vehicle fault: Volkswagen Polo – Engine management light illuminated and glow plug light flashing Damien describes how a relatively simple fault became slightly more complicated to rectify when a component with different operating parameters was fitted. He provides a guide on how to resolve the issue quickly and effectively by further investigation and testing, using a TRITON-D8 and a Multimeter from Snap-on A 2010 Volkswagen Polo with a 1.6 litre common rail diesel injection engine (engine code CAYC) was reported to have both an engine management light and glow plug warning light illuminated. A diagnostic scan tool was connected to the vehicle’s data link connector and the following error code was stored in the Electronic Control Module: P2455 – Diesel particulate filter pressure sensor, short to positive or open circuit. This fault code can be caused by a number of factors: • Short circuit in the wiring between the sensor supply voltage (5 volts) and the signal wire • Open circuit on the ground circuit of the sensor • Faulty exhaust gas differential pressure sensor.
The simplified diagram in Figure 1 shows the circuit layout. The signal wire (pin 3) has a voltage of 5 volts applied to the sensor signal circuit from the engine control module. The sensor is a piezo-resistive measuring cell with integrated evaluation electronics. When the differential pressure is low, the voltage is ‘pulled’ closer to ground potential, as the differential pressure across the filter increases the output voltage increases proportionally. The engine control module uses this voltage signal to determine the soot loading of the Diesel Particulate Filter. Increased soot loading will cause an increase in exhaust gas pressure upstream of the filter. This is referred to as closed loop monitoring. As can be seen from the fault code, a number of electrical tests must be conducted to pinpoint the actual fault. After the initial voltage test at the sensor signal wire (4.9 volts), the next test was to validate the ground circuit. The ground for the differential pressure sensor is shared with the turbocharger boost pressure sensor and the transmission neutral position sensor, so a fault here was highly unlikely as there were no other fault codes stored in the Engine ECM, however the circuit was tested and proved to be in good condition. The wiring was also tested for a short circuit between the supply and signal wires and no fault was found here either. This proved the fault was within the DPF pressure sensor.
A volt meter was connected to the signal wire of the sensor and a voltage of 4.9 Volts was measured.
CASE STUDY
Figure 2: Sensor construction
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Figure 1: Component pin assignment: Pin 1: 5 Volt supply Pin 2: Ground Pin 3: Signal output
