I have a 1996 Camaro 3.8L with a rebuild engine. The cooling fans don't come on and the overheats. I replaced the temperature switch. The relays are good. It appears that the ECM sends ground to the relay coils to pull in, but this doesn't happen.

Has anyone ever seen an ECM not work on just some channels? I can jump ground to the ECM harness on the DRK/GRN (slow) or DRK/BLUE (fast) and the relays pull in and the fans run, so its not the wiring.

 

Have you put a scan tool on it to see what the PCM thinks the coolant temperature is?

Could be that the PCM is not reading the coolant temperature properly. Check that before throwing more parts at it.