We present single-photon spectroscopy in molecular hydrogen starting from the metastable c3Πu− state to a number of triplet nd-Rydberg states (v=0–4, n=12–20). Using fast beam spectroscopy both the autoionization channel and the predissociation channel are quantified, field free, as well as with small electric fields. Coupling with the i3Πg state is assumed to be responsible for field-free predissociation of the v=0 Rydberg levels. The stronger observed predissociation channel of the v=1 Rydberg levels is due to the nonadiabatic interaction with the h3Σg+ state in combination with l mixing due to an external electric field. No direct evidence is found for possible electric field induced predissociation of the gerade Rydberg states by low lying ungerade states. The competition between autoionization and predissociation is discussed in terms of possible consequences for dissociative recombination involving low energy electron collisions with the H2+ molecular ion.