Perhaps needs to say more
These DC circuits are typically a high point 12-14 V and a low point 0 V
Speaking of battery the 12 V is on the + and 0 V on the minus -
The frame ground if everything is OK Is the same as - on the battery.
But if there is rust .lose connections . paint on the frame .. old wires and so on .. The frame ground is not as good as the Battery minus ground.
The ignition with + from the battery and - from the battery gives correct perfect physics .and error sources are taken out.
The + cannot be connected direct all the time ..because it can fry the coils and ignition. But I use it for fault finding .Just disconnect the + when
it is tested.
But Minus can as far as I know.
You did find a fault in the electronics perhaps it solves it and frame Ground is good and solid.