I'm fairly new to the Arduino, but I am quickly learning the ropes. However, I have run into a problem that I can't figure out. I am using my arduino to give readouts on the sensors in my pre OBDII vehicle (1988). Specifically oil temperature, oil pressure, coolant temperature, and battery voltage. The problem is that the oil temperature and coolant temperature are single pole thermistors.
Originally power came into the terminal on top, and the sensor was grounded through the block/head. I have figured out a way to read them by powering the pole and using a ring terminal with both ground and analog input wires crimped into it. When I bench tested both sensors with this circuit I got accurate readings, however, when I screwed both into the engine I get some interesting results.
When I power the arduino over usb from my laptop I seem to get correct readings, but both coolant and oil temps are the exact same values when they shouldn't be. Now comes the really weird part; when I power the arduino over usb from a 12v to 5v converter wired into the car, on the LCD screen the output for both oil/water temps is -495F, and never changes.
I am thinking it is a grounding issue or something of the sort because when I put my multimeter in beep mode and touch the leads to each of the ground wires going to the temp sensors it beeps. The same happens for the analog input wires. The only idea I have come up with is that I need to somehow isolate those circuits but I am not sure how.