voltage divider ground problem

Hello all!

I have a 12V power source, which powers a arduino as well as a mosfet that controls a 12v computer fan.
I wanted to measure the voltage after the mosfet in relation to my pwm signal.
I created a voltage divider on the mosftet output using 100K (+ side) and 47K resistors (- side) of which the output runs to an analog pin on the arduino.
When the output of this divider is not connected to the arduino, i get what i expect, about 3.8v when the output of the mosfet is 12v.
However, when connected to the arduino analog pin, the voltage soars as the mosfet output voltage goes down. I think I’m lucky that i haven’t fried it.
I am obviously not understanding how the ground is working through the arduino.

Can someone lend a hand?
Thanks everyone for you time!

Is the -ve side also connected to the arduino's ground?

I believe in one of my "experiments" to figure out what was going on, i tied the - mosfet output side to the arduino ground pin. I wish I could remember, but it was very late last night. Next time I will have to record my steps.

Would that normally be the correct way to do this?

Thanks for your reply!

As you said:-

100K (+ side) and 47K resistors (- side)

with the junction between the two resistors going to the analogue input AND the -ve of your 12V going to the arduino's ground.

Hi, yes, misunderstood what you meant.
+12v --100k––47k–ground

I will post a full wiring diagram tomorrow since I have a low pass filter on the pwm and some other things, I dont think they are affecting the system, but obviously i’m missing something.