A quick way to test the IRCM is to attach a code reader and run a KOEO test...the fan should hit both speeds in very rapid succession, then turn off.
As well, be sure to disconnect the harness at the fan, and jumper 12v hot to both speeds as a sanity check on the motor itself. I had a fan issue and thought it was the IRCM but it turned out to be the connector at the fan.
This is a way to test the IRCM circuits on car (example '90 SC):
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Breaking the circut on wire #14 turns on low speed fan. Gounding wire #17 turns on power to the high speed fan motor windings. The fan should always be started in low speed (high pulls too many amps) - when the high speed is turned on, my testing showed that lo is still being fed current as well.
IRCM/CCRM Pin Outs (Main Fan Relays)
Pins 1/2 (black/yellow and black/orange) - Power to HIGH speed fan windings
Pins 6/7 (2 blue wires) - Power to LOW speed fan windings
Pin 13 (red) - 12V Ignition Hot (splice with 3A fuse if using external relays for fan)
Pin 14 (tan/orange) - Signal from EEC to IRCM for low speed fan
Pin 17 (green) - Signal from EEC to IRCM for high speed fan