The time-dependent electric Aharonov-Bohm effect, where shielded potentials vary while electrons are in transit, offers a unique test of the Lorenz condition $partial_mu A^mu=0$. We introduce a phenomenological coupling $kappa$ between the scalar $partial_mu A^mu$ and the electron. In the field-free interior of shielded cylinders, $partial_mu A^mu = dot{Phi}/c^2$, giving an additional phase $Deltaphi_{lorenzScalar} = -kappa[Delta V(T)-Delta V(0)]$. For sinusoidal modulation this yields a $1-cos(omega T)$ term orthogonal to the standard $sin(omega T)$ Aharonov-Bohm phase. We propose a realistic electron interferometer, analyse fringe-field systematics, and project a sensitivity $|kappa|sim10^{-6},text{V}^{-1}$, the first such probe. A null result would confirm the scalar mode's decoupling from matter; a positive signal would uncover a new electromagnetic degree of freedom.