Cooling Fan Relay Diagnosis
Now for the relay, or relays. Some vehicles will have a high-speed relay and a low-speed relay. Turn on the ignition switch, and using your wiring diagram, locate the wire for the ground side of the relay coil. When you ground this wire, you should hear the relay click and the fan(s) should come on. If it does, then you know the wiring up to, and including the relay, are good. Now we know the problem is between the CTS and the relay.

If the fan doesn't come on, we need to continue with the relay and it's wiring. There are two current feeds, one for the relay coil and the other for the cooling fan(s). Using your wiring diagram, locate these feed wires and probe them with a test light. With the ignition key on, there should be power at both wires. If you have power to one and not the other, you have an open in the wire from the fuse to the relay.

If the fan doesn't come on, we need to continue with the relay and it's wiring. There are two current feeds, one for the relay coil and the other for the cooling fan(s). Using your wiring diagram, locate these feed wires and probe them with a test light. With the ignition key on, there should be power at both wires. If you have power to one and not the other, you have an open in the wire from the fuse to the relay.

On systems that have computer control using a sensor signal, you can make a similar test of the relay current feed terminals with a test light and the ignition key on.

If the current feeds are good, ground the relay's switch terminal (the one with the wire that goes to the coolant switch). If there's a sensor and the switch terminal wire goes to the PCM, unplug the wire before grounding the terminal. The relay should click and operate the fans. If it doesn't, replace the relay.

You may have trouble doing this with relays that plug into an under hood relay "center." Unplug the relay; turn on the ignition and with your test light probe the two current feed terminals in the relay center. If they pass (turning on the test light), make up short jumper wires to connect to the unplugged relay and one long jumper (that you run to an electrical ground) for the switch terminal. If the relay still doesn't work (ignition on), replace the relay. If the relay does click, probe the output terminal to the fan motor with a grounded test light. If the light goes on, the problem is in the wiring from relay to fan motor.

Your diagnosis may point to the PCM. It's rare, but a computer failure can be responsible without a Check Engine Light or DTC. Perhaps just a single driver has blown, so the PCM itself seems to be performing normally. In addition, in a number of cars, particularly GM models, the PCM turns off the fans if the vehicle is cruising 40 to 45 mph or higher. This strategy relies on the vehicle speed sensor, which may be misbehaving. If your speedometer is way off, or not working at all, it's something to consider if the relay works when grounded.