I've got two knock sensors in my hands and after thorough checking I can't say if any of them is OK. I did everything like Bentley says. Voltmeter set to AC, tapping with some iron stuff. Not a volt or even milivolt. Perhaps my voltmeter is not so good because at VAC scale the smallest sensitivity is 200V which of course doesn't mean than any smaller can't be measured. What am I doing wrong? No continuity even for a moment. Both of them are dead? Or should I tap it in special way?