We report the ionization-detected absorption spectra of autoionizing Rydberg states converging to the (010) vibrational level of HCO+. Sharp second-photon resonances appear in transitions from first-photon-prepared originating states that have total angular momentum (less-spin) from N'=0 to 5, selected from the Sigma(+) and Sigma(-) components of the (010) band of the 3p pi(2)Pi Rydberg state. We systematically compare spectra in order to characterize observed resonances in terms of the good total angular momentum quantum number, N. Rydberg analysis establishes the convergence of series to detailed cation-core rotational quantum numbers, N+. Observed series are found to fit well with simulations employing a limited set of constant quantum defects (delta=1.062, 0.794, 0.606, 0.253, 0.015, 0.002, -0.027 and -0.076). The strengths of observed transitions as a function of initial and final total angular momentum provide a purely experimental indication of the appropriate assignment of the approximately good orbital angular momentum quantum number, l, for each series. Interactions between Rydberg orbital and core rotational angular momentum are found to conform with a coupling case intermediate between Hund's cases (b) and (d). Splitting patterns further assign certain features according to predominant case (b) composition.