I've owned my car since 2013 and the A/C has never worked properly. The A/C would only engage with the fan speed selected to 1, occasionally work intermittently on fan speed 3, and never work on settings 2 or 4. I decided it was time to fix it for good.

https://www.rx7club.com/power-fc-for...ition-1151025/
https://www.rx7club.com/power-fc-for...ng-fix-997129/

There are a few threads about doing the relay mod for A/C, but information is sparse on how to do it on a Series 7/8 FD. So I made a handy diagram. This should also work with other standalone ECUs if you're having the same problem.





Be sure to label your stuff before wiring it in. Part of the reason I waited so long to do this is because I hate having to hack up my ECU harness. But with the proper labels I can at least avoid having a mess of random mystery wires.

On a series 7/8 FD3S, 4V is the wire you want to cut and rewire as shown above. Keep in mind that this image is from the perspective of looking at the wire receptacles on the ECU. If you're looking towards the plug side, everything will be mirrored.

 

To see if your A/C switch is working or not, go to the sensor/switch page on the PFC commander, select the A/C button ON and turn on the fan. Observe what happens to the A/C status indication on the commander screen. The engine does not have to be running while you do this, you only need to turn the key to the "ON" position.

Before wiring in the relay, I tested the voltage at the 4V ECU pin at different fan speed settings with the A/C button on. These are the results with the ECU turned on but engine not running:

Fan off: 11.02V
Fan 1: 1.72V
Fan 2: 2.57V
Fan 3: 2.49V
Fan 4: 2.49V

These values are obviously way off from an ideal ground, which is why the PFC was not interpreting the A/C request properly.

After wiring in the relay, my A/C now works flawlessly. Before, it only worked on fan speed 1.

 

What size fuse did you use? I was going to take the B+ ignition on from the room loom so this would have a fuse to protect it