By Damien Coleman
A 2008 Audi A6 2.0L PD compression ignition engine with an engine code of BRD had an intermittent no-start condition when the engine was up to operating temperature.
A Snap-on VERUS Pro car diagnostic scan tool was connected to the vehicle via the 16-pin data link connector and the following fault code was retrieved:
- P0341: Camshaft sensor implausible signal or short to power
The camshaft sensor on this particular engine is fitted behind the upper timing belt cover and the phonic wheel is part of the camshaft sprocket. As the vehicle recently had its timing belt replaced, the actual static valve timing was validated. However it was later discovered that the initial fault was occurring prior to the timing belt replacement.
The key to diagnosing this fault was this subcode:
- Implausible signal or short to power
The camshaft sensor on this vehicle has a three-pin connector with the following pin assignments:
- Red/white wire: 5V supply
- Violet/blue wire: Signal
- Red/violet: Ground
The engine control module supplies a 5 volt and ground circuit to the sensor and also a 5 volt reference to the signal wire which is grounded when a tooth on the phonic wheel passes the sensor.
The magnetic resistance of the sensor is altered when a tooth is in close proximately to the sensor. This results in a transistor being switched on which connects the 5 volt signal reference to ground.
With an understanding of the system layout as shown above, it is now apparent that a fault with the camshaft sensor or a fault in the ground circuit will result in this subcode:
- Implausible signal or short to power
An oscilloscope was connected to the camshaft sensor and the crankshaft sensor (as a point of reference). The waveform below was captured while the vehicle was operating normally:
- Yellow channel: Camshaft position sensor signal
- Green channel: Camshaft position sensor ground
- Blue channel: Camshaft position sensor supply
- Red channel: Crankshaft position sensor
As the issue with this vehicle was only evident when the engine was up to operating temperature and a quick re-start was attempted, the vehicle was allowed to warm-up.
After a number of engine shutdown and engine crank attempts the vehicle finally failed to start. The following waveform was captured when this condition occurred:
The scope setup was identical to the previous waveform but this time the fault can be observed.
The signal reference from the engine control module is present but not being pulled to ground. As the scope is connected to the ground circuit the control module ground can be validated.
This allowed the diagnosis of a faulty camshaft position sensor to be made, and the camshaft sensor was replaced which meant removal of the timing belt again. The fault code was erased and the repair was validated.
Date posted: 8 November 2016