flyback/snubber diode in relays

Since I buy the damn things in 200 batches (4x 1N5404 and 5 x 1N4004 on one E46 BMW) yes. I solder them across the relay base on EVERY alarm output. I'm taking no chances. Seen the bit on the 5901 where it says use diodes? You have to with some Euro locking systems. I've even had Euro GM and others where if you add a BT phone kit or aftermarket head unit and trigger it from the ACC, you can't lock the doors, residual voltage to the ignition switch tells the processor the ignition is on, it refuses to lock the doors, so diode required.

sequoia2006 wrote: From what I understand is that the collapse of the magnetic field would cause induced voltage greater than 12V+ so current will flow toward the 12V+

It's the same as an ignition coil. The coil tries to keep the current flowing, or fight the change (as a capacitor does for voltage). That reaction is a voltage. (Direction - every direction.) The spike at the coil is present at any point connected to it (noting that the worst spikes probably occurs because of the break - hence one end is floating....) But like a lighting front it collapses due to path impedance (resistance, capacitance, inductance) etc. If not, some component will absorb the spike - eg, a battery, a transistor, a CPU. And with luck, that component will survive the spike. Hence the above answers. Good auto accessories expect the harsh environment and thus handle (say) 8-16V supplies with 200-400V spikes, and inputs/outputs incorporate noise filters. Low quality devices do not. Hence we might place quenchers across relays to prevent damage. I don't, but I don't have CPUs and OEM electronics.