The problem we see at Karmakanix is that the crankshaft position sensor can momentarily quit functioning while reading the slots in the wheel on the crankshaft. This causes the ignition coils to quit, the spark cuts out, and the engine dies. Sometimes this also happens when trying to start the car. In some cases, a diagnostic trouble code (DTC) is generated in the engine control unit (ECU), and the check engine light comes on. Strangely, in some cases the check engine light never comes on. In many of those instances, we can catch the bad crankshaft position sensor by checking the electrical resistance. But sometimes there is no test that conclusively condemns the sensor, not even an oscilloscope voltage trace. Well, sometimes with an oscilloscope trace. We should see an even sine wave with one larger glitch that corresponds to the one bigger gap on the crankshaft sensor wheel. We just have to catch it misbehaving. We believe that the sensor signal strength may be diminished when the engine stays hot for some undetermined period. We could no doubt catch that with an oscilloscope if a Karmakanix technician happened to be riding with you and was hooked up while you were driving and the problem occurred. Get the picture? There is a long history of this exact problem dating back to the 1990 Audi V8 Quattro.
Most times when the crank sensor fails and the engine dies but won’t start again, there is a code present for the crankshaft position sensor in the ECU. And most of those times, the check engine light is either on, or came on for some part of your drive. Sometimes when the engine either lost power momentarily, or died but restarted, and if there is no code, then the guessing game begins. Broken down, but no DTC, normally means either the sensor went bunk, or fuel delivery stopped. There is no code for “Fuel Pump Failed”, and the issue can also be a relay or wiring to the fuel pump. Nothing beats a dead car with a bad crankshaft position sensor, MIL lit up, and a positive DTC for ease of diagnosis. We don’t always get that.
So without perfect evidence of this problem, sometimes we just have to replace the crankshaft position sensor and see if the problem ever happens again. Frankly, our hit ratio is about 4 out of 5. Generally the sensors are not too hard to replace, taking from 1/2 hour to 1 1/2 hours. Most of the sensors cost between $50 and $150, with a few Audi flagship exceptions. Compared to the unquantifiable costs of breaking down, and the potential hazards involved, this is not a bad scenario. We do our best to communicate with the customer about the level of ultimate certainty, and hope they understand that this is our best judgement call. Remember, we are on your side.