Having a peculiar issue with the max6675 and grounds? Answered
So first off, this is an application for use in a car. I have ensured that all of my grounds are good, and the power is clean, so I know these are not the issue, at least not on the car side of things.
I am trying to get an electric fan controlled by an atmega328 (through a relay), which I have successfully achieved on bench tests using an alternative heat source. It works reliably until I apply the circuit in the car. When I install it in the car with my thermocouple on the radiator pipe for the temperature reference I get all sorts of temperatures that are not the correct one. Its either very high, or very low. I have tried multiple thermocouples (of similar style) with no changes. If my thermocouple is not making contact with any grounded portion of the car (for example, the engine) it reads fine, so I know that my issue lies somewhere in there. An easy solution would be to simply mount the sensor in a coupler in the radiator hose (thus isolating it as best as I can) but I would rather fix the issue than apply a bandaid. I've tried searching for other people having a similar issue but haven't had any luck.
One peculiar thing to note is that if I have the ground disconnected, and the thermocouple touching a ground, the system receives power, so I am assuming that the junction makes contact with the outer case of the thermocouple.
My circuit is nothing too elaborate (I can upload a schematic if need be); I have a DC/DC adjustable converter to get 5v for the micro and 6675 power, which does not have an isolated ground, an atmega328, an OLED display, some tact buttons, and a max6675 development board. There are other passives as well but those should be minor.
Does anyone have any idea what may be going on with this? I'm stumped. I should be able to provide any info you may need.
PS: this is the development board and thermocouple that I am using: