In some examples, an alternator control device includes interface configured to receive a voltage signal from an engine control device, pull-down circuitry, and a switch electrically connected between the interface and the pull-down circuitry. The alternator control device further includes processing circuitry configured to determine that the voltage signal is higher than a threshold voltage level and to activate the switch to electrically connect the interface to the pull-down circuitry in response to determining that the voltage signal is higher than the threshold voltage level. The processing circuitry is further configured to determine that, at least a threshold time duration after activating the switch, the voltage signal is higher than the threshold voltage level and to refrain from delivering the excitation current based on determining that, at least the threshold time duration after activating the switch, the voltage signal is higher than a threshold voltage level.