Pulsed ground signal into positive output

soulessshadow said:

What would you add to the circuit to protect it from the possibility? Given that it's a race car application and has a kill switch so that scenario of the battery being disconnected while the engine running is a likely possibility

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The only way to protect your circuit from load dump spikes is to design for them as a normal circuit condition. In other words, your circuit should be ok with a voltage level at the normal 12 - 14V but also at voltages up to 60V and survive with voltages down to -60V. The 60V condition is short lived so there are some things you can do to take advantage of that fact. For example, you could take advantage of the transient rating of resistors and the transient thermal impedance of silicon power components. But the voltage is inescapable. The parts of your circuit which will be exposed to it must be able to handle it without question.

I am not sure what will happen to your output connected to the coil trigger. My guess is that the spike would not happen at that node but others on here might know about these interfaces. It is not impossible for a spike to appear on that node due to protection diodes etc.

In your case I would suggest something to limit the voltage of the supply, a series diode and linear regulator rated for automotive load dumps and modify the output drive to be a current source or sink with a series blocking diode to protect from reverse polarity load dumps.

For the input, one solution is to divide the input down such that a load dump does not take the input voltage to a dangerous level but then you will also need to detect much smaller input voltage swings under normal operating conditions. An alternative is to use a transient voltage suppressor (TVS) which is basically a big ass zener diode with remarkably low leakage up to near the knee voltage and a massive thermal mass to go with the massive current handling capability to form an effective clamp. Add to that a suitable series resistance in the input (rated to take the huge power pulse) and you should have a bullet proof circuit.

For a surge rated resistor have a look at the data sheet for this:
https://www.digikey.com.au/en/products/detail/bourns-inc/CMP0805-FX-1000ELF/12333612

There are several other suppliers for similar parts and the parts in the link are not the most impressive performers I have seen. There are 0603 resistors with a peak power handling up to 1kW (for very short pulses of course) but to do that without the resistance value drifting all over the place is just mind blowing. These parts in 0805 will take a 40W pulse for 1ms without appreciable drift.

It is all a bit vague and hand wavy I know but I hope that helps you.