Diodes across coil
Relay and solenoid coils generate a negative spike across themselves when they are switched off. A diode across the coil with the band-end towards the (+) supply will become forward biased (turns ON) to 'short out' the spike generated as the magnetic field around the coil collapses.
Aero electric explains it pretty well here:
http://www.aeroelectric.com/articles/spikecatcher.pdf
Bob discusses several transient suppression ideas in this one and arrives at the diode across the coil. This may be a bit misleading as ALL coils should have a diode across them first. If you need to suppress arcs across contacts etc, that's a separate issue. His other schematics also show which terminals are which on the contactors and how the diodes hook up. The 'simple electrical schematic' in the RV7 preview plans also shows where to put them.
Note how with a 12V coil, he shows a -300 volt spike. That's about right and may actually be a bit low. I scoped one for grins at lunch and my battery contactor generated -500V across the coil when it let go (without a diode). It ALSO injected much of that into the BUS terminal. Even with internal protection, imagine how delighted your EFIS will be to have this presented on its power lines
Vans and lots of others sell ready made diode assys with install directions. Or you can roll your own. I used some 3Amp 1KV ones that I had around. Something like a 1N5408 will set you back a whoppin' 41 cents at Mouser and will handle about anything you throw at it. Put em right at the contactors keeping leads as short as possible.
1N5408 at Mouser
However you decide to do it, diodes across these (and any other DC coils) should always be included.
BTW: be careful when measuring voltage across large coils. A client of mine once called to say that every time he tried to measure the coil voltage on a (larger) 24V contactor, his DVM blew up, actually 3 of them did. Scoping it out revealed a 2500volt spike when the coil was shut off. Smoked the protection circuits on his el cheapo DVMs. He later added diodes across the coils and bought better meters.
And of course, I am sure that you NEVER would tell anyone to hold onto that no-diode 12V coil terminal while you cycled the master switch.. would you..
Have fun,
John