How do I fix the A/C not working with a Power-FC on a Mazda RX-7 FC?

This is a notorious issue when running a Power-FC on an older RX-7. While the community documentation for this specific quirk heavily references the FD3S, the root cause and the fix are exactly the same for your FC.

Difficulty: Moderate (Requires basic 12V wiring and splicing)

The Root Cause

If your system has freon, the high and low pressures are equalized (meaning the compressor isn't running), and your engine RPM increases when you press the A/C button, your compressor clutch isn't engaging ¹.

The issue is that the Power-FC is much more sensitive to ground resistance than the stock ECU. As the climate control fan switch and chassis wiring age, they provide a "fuzzy" or high-resistance ground signal. The stock ECU tolerates this, but the Power-FC does not, causing it to ignore the A/C request—especially on fan speeds 3 and 4 ⁴.

The Fix: The Relay Mod

While some owners have tried soldering diodes behind the climate control panel or cleaning the fan switch contacts, community consensus is that the most reliable, permanent fix is a relay modification developed by Freddie Hebert ¹. This bypasses the aging wiring and feeds the Power-FC the crisp, heavy-duty ground it needs to trigger the compressor.

Because you are working on an FC, I don't have the verified ECU pinout or wire color for your specific A/C request wire (the FD uses wire 1E, which is violet). Do not guess this wire—check your specific Power-FC and FC chassis wiring diagrams to identify the A/C request signal wire going to the ECU.

Once you have identified the correct A/C request wire at the ECU, here is the standard procedure to wire in a standard 4-prong or 5-prong Bosch-style 12V/40A relay:

1. Cut the A/C request wire about 2 inches back from the Power-FC connector.
2. Pin 86 (Relay): Connect this to the A/C request wire on the harness side (coming from the dash).
3. Pin 85 (Relay): Connect this to a switched Ignition 12V+ source.
4. Pin 87 (Relay): Connect this to the A/C request wire on the Power-FC side (the 2-inch pigtail you left on the ECU).
5. Pin 30 (Relay): Route this straight to a clean, bare-metal chassis ground.

How it works: When you turn the A/C on, the weak ground from your aging fan switch triggers the relay. The relay then snaps shut, sending a perfect, zero-resistance chassis ground directly into the Power-FC. This allows the ECU to turn on the compressor, retain the Idle Speed Control (ISC) bump, and still cut the A/C off at wide-open throttle ¹.

If you want to avoid cutting your factory harness, you can wire this relay into a patch harness instead.