You need to check your GROUNDS on the car. Remove the ground cable that runs from the battery to the chassis and clean it. Check the engine to chassis ground strap. Chech the POSITIVE lead that feeds the cabin.

You should see more like 13.75 volts @3000rpm

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My Truck.. 2007 DODGE, 5.9 Cummins, 6spd stick, 4X4. My car..1977 240D, OTHER WHEELS...1955 VW Oval window bug, European Delivery (Holland) with a 1700cc, 2 barrel, Porsche drum brakes. 1939 WILLYS Pick-up. 1967 Triumph 200cc Tiger Cub. 1976 Honda 550F 4cyl Motor Cycle.