Twilight zone.

I was with you up to the point you said "...and may be able to induce a voltage spike across the antenna."

In order to produce a meaningful effect (a current), the voltage induced in the "receiving wire" has to be part of a closed circuit. The antenna is an open circuit. No current flows through the antenna wire.

Moreover, the antenna is followed by a circuit (filter) that is tuned to allow ONLY very high frequencies (in the MHz range) to pass. The spike doesn't count as a very high frequency. It is one event. Not to mention that car alarms work with hopping frequency remotes to protect the car from thieves that could "record" the signal for unlocking the car. Furthermore, there are many decoupling capacitors on the power supply inside the alarm. They should do just that. Shield the circuits from any spike transferred directly through the alarm from the car electric circuit. What happened in your case was that some capacitors are old (20 years!) and they have lost the capacity to filter the power line as good as before. I can guarantee that if you re-cap the alarm, it will work flawlessly another 20 years.

Therefore, I'd say that unless the inner circuits of the alarm are directly exposed to a very powerful, variable and high frequency magnetic flux, nothing will "trigger a specific part of the circuit" in a parasitic way by means of induction.

Edited 23 March, 20215 yr by RicardoM

13 hours ago, RicardoM said:

I can guarantee that if you re-cap the alarm, it will work flawlessly another 20 years.

 

Obviously if problem reappears I will recap the alarm there is no arguing that. 

But the antenna isn't really an open circuit but rather a closed one capacitvely coupled to the rest of the circuit. As for the impulse. 

Asuming it's a Dirac function that impulse is the base of white noise so it does indeed contain high frequencies even in terms of mhz. That, coupled with the fact that my capacitors could very well be leaky thus not decoupling effectively which I won't argue with, could lead to voltage spike in parts of the circuit. 

With an oscilloscope probe dangling in the air you can pick up switches turning on and off why would not a senstive circuit be triggered by that. As for the frequency hoping to avoid code jacking that is not my point. It's that it triggers something when it should not. And given that used to happen only at the point of maximum current draw in the system I was lead to that assumption. In any case I hope it doesn't happen again but if it does I know the cure