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!
I was working on t CAT C15 engine that had a code for the oil pressure sensor voltage high. I check the pins and I have 5v supply, ground and 12v on the signal with the sensor unplugged. I ground the signal and the ECM respond with voltage low code so I replace the sensor the it fixed the problem. My question is, why does the signal wire goes to 12v with sensor unplugged? I was expecting zero volts. Is that an example of bias voltage, because the 12v would not light up my test light either.
Definitely sounds like a case of bias voltage to me. That kind of detection logic is exactly the same thing we see on passenger cars and light duty trucks.
Was there an associated gauge or data PID? Might have been fun to install a variable resistor and see if you could manipulate the signal into something rational.
