We had a 2006 VW Caddy 1.9TDI dropped off with running problems.
- From cold the engine would start and run fine after warming up it would start to stutter and then cut out – without being able to re-start
- Leaving the engine to cool down would let you start the engine again
- Customer informed us a new crankshaft and camshaft sensor have been fitted
- Scanning the engine ECU gave us a fault code off – 16705 – Engine speed sensor (G28): Implausible Signal
We started by looking at the crankshaft position sensor (G28) – These are a 3 wire Hall effect sensor.
- We first checked for an earth to sensor this was found to be good and very low resistance back to a good earth point
- Next we checked for a voltage to the sensor – We found a very stable +5 volts
With the basic checks done we connected the Picoscope to the output off the crankshaft sensor (G28) (Red trace in the photo) I was also taking a reading from the Camshaft sensor G40 (Blue trace), I also connected my Fluke voltmeter to the +5 volts off the sensor and the earth cable into the earth for the sensor, Started the engine and took some readings
With the engine running we kept taking the readings – as the engine warmed up we could see the crankshaft sensor (G28) signal started to break down
When this happened we could see the 5 volt input voltage started to drop off –
Next step was to remove the earth from the sensor plug and connect this to the battery – with the earth on the battery the + 5 volts was still unstable, the problem is with +5 volt feed.
After stripping the main engine loom open we found the wiring loom had chafed by the gearbox and upon further inspection we could see if had been fitted incorrectly and guessing the gearbox must have been removed at some point. We repaired the broken wire, refitted the loom correctly, cleared the codes and everything was all ok.