[vettestr]

Boris,
Seems we have some who do not see need for diode but I don't agree with statement spikes are "are mostly a figment of the imagination" but like Indy mentioned they don't hurt anything. Let me toss in my 2 cents for what it's worth and do what works for you.

When you have a load on the relay that generates a flux field like on a motor running your cooling fans it collapses to throw feedback at the relay contact/s. A load like driving lights does not have mass that is still turning like the armature in a motor, alternator or ? so does not have the same spike potential but contacts do arc in the relay when cycled on/off, all these things lead to eventual failure. All relays with or without diodes or load types will fail sooner or later so just a matter of cost and benefit with things like reliability and amount of cycles expected as points to consider. There was an article in the links of the cooling fan thread that I thought explained this well, it is on my other computer but will try and dig it out.

Driving lights are higher amperage draws but the number of times it will turn on/off in a year of driving plus no circuit feedback ..... I would not drive to parts house to get a diode protected relay if I had proper rated relay in my hand. If installing a high load cooling fan like the Lincoln fans drawing 40 plus amps plus considering how many times it will turn on and off through the t-stat in a year of driving I would only install a diode or a relay with a diode built in. The non protected relay would work for sure but I want to include as much reliability on this device as possible.