You didnt do anything wrong. The stock gauge is labeled as 75 amp but it aint. More like 30 amp. It reacts wildly to the oscillating voltage regulator. This is the reason people use the shunts.
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.