There definitely is a relay in the circuit. I checked the cooling fan relay for
my 91 Civic, no A/C, a few weeks ago.

http://www.honda.co.uk/owner/ConcertoManual/62sk301/16-77.pdf , page 1, shows
about where it is. Mine is under the hood in the car's right, front corner, just
forward and maybe a little below the level of the of the air filter in its box,
IIRC. Trace the wires from the fan's motor, and you should come to it. It's not
hard to find. Kind of a pain to disconnect for testing, but maybe that's just my
inexperience with electrical connectors. It's do-able even by the neophyte.

The site above also has instructions on how to test the relay (page 4).

Note: The 1990 Concerto engine is identical or nearly so to the 1990 Civic
engine.

www.autozone.com reproduces what's in my Chilton's and mentions the relay, but
gives no other information. If you want to see it, click on the "free repair
guides" link on the right and put in your car's info. It's under "engine &
engine overhaul, "engine mechanical," "cooling fan."

Updates are welcome.

Good luck.

I think the relay is in the underhood fuse box.

There is also a "timer" in the circuit, located under the dash near the
steering post (on an 84 Civic anyway).
It's a black, plastic box about 3.5" sq. by about 1" thick. A bit pricey!