If you are testing the cable, you should see ~19.5V between the inner sleeve (positive) and outer sleeve (negative). There is usually no measurable voltage on the center ID pin relative to ground.
The laptop will not power on at all (newer models) or will run extremely slow (older models). Dell laptops require the data signal to enable the main power rail. 3 Wire Dell Laptop Charger Wire Diagram
If you are splicing a cut cable: