Digital voltmeter ammeter wrong reading
It seems you have something close to this type of V/A meter, 
https://www.amazon.com/Paddsun-Digital-Voltmeter-Ammeter-Meter/dp/B01LSGP1UI 

in which case the proper wiring connections are included on the left side of the amazon page.  

Notably the thin red and black wires are the modules power inputs, while the yellow is the voltage sense contact. The wiring method depends on if you want the meter powered from the source power or the output power.