Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
What would be a good substitute for a knock sensor for testing? Would my incandescent test light be a good substitute to connect one end to the 5V knock sensor power from the pcm and the other end to ground to see how the engine runs? Or would a 5k ohm resistor be a good substitute? For this particular engine, the 5v knock sensor being pulled down to 2.4V is considered a good reading.
What car/engine exactly do you refer to? Knock sensors typically are piezo elements, generating an AC voltage when they detect engine knock. The voltages you mention may we'll be bias feeds only, just to check circuit integrity. Manipulating them may result in circuit fault codes but not in the pcm changing the actual ignition timing.
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Thanks. It's a 2001 Nissan Frontier v6 with a supercharger. It has an occasional knock sensor circuit code. I verified it has 5v unplugged, and 2.4v plugged in at idle. Service data says that 2.4v is good. The only thing I see visually is the plastic cover on the wire is brittle and the wire itself is exposed near the knock sensor. I know that is supposed to be a shielded wire, and with the bare wire exposed, I'm not sure what effect that missing shielding may be having. I may be off in my thinking, but my thought was to remove the knock sensor and substitute the load with something like my test light and see if the code comes back or not.
Unfortunately, I don't know if there's a good substitute for a knock sensor, other than a good knock sensor. :silly: It may not say so in the service info, but I suspect that most manufacturers run an intrusive test for knock sensor performance. They'll change the fuel or timing to produce knock, and watch the knock sensor to report accordingly.
Plus, knock sensors usually have relatively high resistance values. Hundreds or thousands of ohms. A test light is likely has too little resistance to get the desired 2.4-2.5V on the knock sensor circuit.
I know that is supposed to be a shielded wire, and with the bare wire exposed, I'm not sure what effect that missing shielding may be having.
That exposed wire may be the shielding, with an insulated signal wire inside it. Though if the outer sheathing has degraded the inner insulation may also be bad.............
Ohming out the knock sensor will probably tell you what resistance you need for a substitute. Or if you have a DMM with a mA shunt you could ground the signal and calculate the resistance inside the ECM. Then use that for R1 in a voltage divider calculation.
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