Disconnect the coolant temp sensor used by the ECM, the fans should turn on within a few seconds.
Also, disconnect the pressure sensor in the high pressure A/C line and short the connector pins with a paper clip or piece of wire. The fans should turn on almost instantly.
If both fail and you are positive the relays are good, one more thing to check is the fusible link for the fans located in the area shown in the following picture:
If the link is also good, it is almost certainly the ECM.
96 T/A, Black/Graphite Leather, T-Tops, 6 speed, 10 speaker Delco/Bose "Monsoon"
Original owner, 187k miles and counting.
Total engine rebuild at 187 K miles.
Complete AIR delete
Bored .030 over.
Comp cams Xtreme Energy LT1 266HR-14 street cam.
lift I .500 E .510 duration I 266 E 276