Should both radiator fans on a Mazda RX-7 FD be running all the time?

Yes, when the cooling system is triggered on your FD RX-7, both radiator fans should operate simultaneously and at the exact same speed (e.g., both on low or both on high) ¹. If the water temperature reaches the trigger point or you switch on the A/C, both fans must spin together.

However, if by "all the time" you mean the fans run constantly—even when the engine is dead cold or the ignition key is completely removed—this is a malfunction that will quickly drain your battery. This constant running is typically caused by a faulty relay or a damaged fan relay harness backfeeding power ⁵.

Troubleshooting (Difficulty: Easy to Moderate):

• If only one fan is running: Check the stationary fan to ensure the blades aren't physically jammed. If the fan spins freely by hand, inspect the wiring going to the fan motor, or test the motor itself to see if it has failed ¹.
• If both fans run non-stop (even when cold/off): Inspect the fan relays mounted on the passenger-side wheel well. Pull the relays to see if the fans cut off, and inspect the inside of the relay harness plugs. Look for trashed wiring, bare wires touching each other, or damaged clips. Cutting out the damaged clips and repinning the wires will usually solve the backfeeding issue ⁵.