Thank you for the question.
Q:Is it normal that the FMB 120 is so inaccurate? What is the point of setting the Trigger External Voltage in I / O if the measurements are not real?
A:If one uses external voltage as a trigger to indicate whether the car has ignition (which is what this parameter is meant for), the voltage jumps by 2V(12V vs 14V) and ~0.2V(claimed by your measurements) accuracy should be more than enough for the application.
On the otherhand AIN is usually used for fuel sensors and such, here the accuracy is important to know exact fuel level, therefore it is designed with higher accuracy in mind.
Q:How to measure the voltage in the car installation?
A:If external voltage parameter is enabled in the configuration, you can check it by using:
Please find additional information here: https://wiki.teltonika.lt/view/FMA120
If you have any further questions don't hesitate to ask. Alternatively, upvote the answer if it provided a solution to your question.
Have a great day. :)