You have to hook it to an ignition signal, the factory tach gets a refinded voltage that won't drive an aftermarket one
You are realizing that you are connecting a unknown something or the other to a system [car] that has more computer/ electronics than what went to the moon!!
My advice is to toss that thing. WHY would you risk blowing up your ECU?
This is how to search in Google, anything on the next line is entered PRECISELY!
"Dragon Gauge" tachometer
Explanation anything inside double quotes is treated as one term
Maybe you can be so lucky to find some sucker that hooked that thing to his Ford and blew his ECU and now is crying about it
I am totally amassed at the number of people that report 'battery problems' and throw parts at that problem but yet cannot take voltage reading with a ~$5.00 Harbor Freight mutimeter