Discussion and questions related to the course EFI Wiring Fundamentals
question regarding ratio metric sensors, just say a 5v sensor if we didn't have exactly a 5 v supply how would it effect the signal output voltage? for example a common rail fuel pressure sensor is a 5v sensor everything working correctly we should see around 1.5v at idle, if we had 4.5v supply that would change output signal voltage then the ecu would think we have low fuel pressure etc. is there a certain way to determine how our voltage drop is effecting out signal voltage? and is there a certain amount of voltage drop allowable we can have before we start to see problems ?
This signal is affected ratiometrically -- so if you were 10% low on the supply voltage, then the signal would be 10% low. Typically power supplies are within more like 3-5%, so this is often good enough.
Now on a better ECU (for example MoTeC M1 series), when you configure the input you can specify if the sensor is ratiometric or absolute voltage, and the ECU compares the calibration voltage to the actual power supply voltage and makes an adjustment. An example of an absolute voltage sensor is one that can accept 10-20 V input, and always produces a 1 - 5V output.