amp and vol measures are TWO different things.
AMP in IN LINE with power kables
VOLT is parallel to the "Battery"
you need to CONNECT the two THICK kabels going today to the AMP meter DIRECKTLY
- then your enegine will start as befor
for the VOLT meter besides a ground connection somewhere in the dash you need to find a Bat + somewhere / well best a direct wire from Bat+ to meter..as otherwise you get may be too low readings. The best is to use the BAT+ plus AT THE AMP Metter today. so when you connect the two AMP kabels add a thin wire to it as VOLT + connection. That will do perfectly..