1997 Dodge Truck Does 97 2500 Cummins Loose Battery Power While Driving Bought

Could have bad batteries. Measure the voltage with the engine off, then again with it running. With it off you should find 12.6 volts if they're fully charged. At 12.2 volts they are good but discharged. If you find it closer to 11 volts, disconnect one of them and measure each one individually. 11 volts indicates a shorted cell and that battery must be replaced.

With the engine running, the voltage must be between 13.75 and 14.75 volts. If it is low, measure the voltages on the back of the alternator with the engine running. If the voltage on the fat output wire is different than across the batteries, look for a blown large bolted-in fuse in the under-hood fuse box. If the voltage is low there too, measure on the two smaller wires on the back of the alternator. One must have full battery voltage. The other one must have less but not 0 volts. If it has 0 volts, the brushes are open, (worn), and either they or the alternator must be replaced. If you find exactly the same voltage on both wires, there is a break in the circuit after that point, either in the wire, a corroded connector terminal, or the voltage regulator itself inside the Engine Computer.