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.