Ok, coil resistance is ~4 ohms, voltage is about ~12, so the coil needs to bleed off about ~3 amps. The diagnostic literature says the coil should draw ~3.5 amps, figure at least 4 amps to be safe. So should a 6 amp 400 volt diode do the job?
Certainly, with a margin of safety. As "Backyard Mechanic" points out: Diode current ratings are for CONSTANT current. The 1 amp 1N40xx series can handle 30 amps of PEAK current. The diode operates milliseconds each time the coil is shut off. So if you find diode rated at 2 or 3 amps at 200v, it should be more than enough. Certainly don't need a 6 amper.
Would that "snub" the collapsing field to protect the rest of the car's circuitry? I am assuming that more is not necessarily better but at least not detrimental.
Certainly, with a margin of safety.
NOTE: The voltage spike from the disconnected coil will try to rise infinitely to maintain the coil current, until some current path is found. It could be through a suppression diode, a resistor, a capacitor, an ionized spark gap between the relay points, or a breakdown of insulation in the coil, wire harness or controller circuit board. Without protection the usual path is the relay point gap. Controller boards are built with protection against voltage spikes, but sometimes one gets through.
Actually, because your existing diode is shorted and not open, it is still providing voltage suppression by acting as a resistor. The problem is that it draws unwanted extra current during coil engagement, and it may eventually open up.
Good luck.
Rodan.