An interesting app note from Littelfuse about automotive circuit protection but also discusses about where transients came from in an automotive setup. Link here (PDF)
The designers of automotive electronics face many technical challenges during the system design process, including designing methods of protection against a variety of electrical hazards. The three major sources of electrical hazards in these systems are electrostatic discharge (ESD), lightning, and switching loads in power electronics circuits. Overcoming transient surges that can harm the vehicle’s electronics is one of the biggest challenges of the design process.
Protecting automotive electronics includes eliminating transient surges that can damage the control units, infotainment electronics, sensors, fuel injectors, valves, motors, 12/24/42/48V powertrains and hydrolytic controllers, etc.