After I was done with mine a friend asked me if I used diode protected relays. I never even heard of such a thing. So, I searched the internet for information about diode protected relays for radiator fans. Despite the incredible amount of "information" I found, I don't think I know anything more that I did a month ago. I trust your opinion on this. Do you know if relays with diodes are needed?
Relays with diodes are more flexible regarding the switch that tells the relay to turn on, and are preferred when available. The winding part of the relay acts just like a miniature ignition coil when connected to a mechanical switch (which acts like a set of distributor points) or a transistor. When the mechanical switch is opened, the magnetic field in the relay winding collapses, causing a high voltage spike across the switch (just like a set of points will arc if the condenser isn't there). The diode, connected across the windings, lets the relay winding current recirculate through the windings, causing the magnetic field to collapse slower, reducing or eliminating the arc. This protects the contacts of the control switch (the fan temperature switch, for example), or the transistor in an electronic fan controller.
At a minimum, the relays should have a resistor across the relay windings (to allow a recirculation path), but a diode is certainly preferred.
Is there a "schematic" of the relay internals printed on the housings of the relays you're using?