I have a 2007 Mazda RX8 and am getting it ready for the hot summer weather. Put a new a/c compressor in only to find that it stays running whether or not the switch is on inside the car. This lead to the discovery that the previous owner had hardwired the compressor to stay on by taking its relay out of the picture and grounding the wire to the car frame. I also found that the ground wire not only is spliced to the a/c compressor relay, but the cooling fan as well. My question is why do this? What is the benefit or what would they have been trying to accomplish besides burning out the compressor? Can I do away with the ground wire and just have my compressor run as normal?
My suggestion is, take it out and see why it isn't working. I'll venture to assume there is another reason they had done this ... a reason which only makes sense in the mind of an amature. More than likely, one of the pressure switches has gone bad and they needed a way to turn it on. As you suggested, this is a really bad idea to run the compressor all the time, as this is not how it was designed to run. I'm surprised they didn't cause an issue with over pressurizing something, causing it to blow an o-ring somewhere. Once you get the ground removed, you'll have to figure out exactly what wiring is supposed to be in place. Once that is done, I'm sure you'll have to figure out why it won't kick itself on. Good luck.
Given the extreme heat produced by the renesis engine and the instant death an overheat can cause to the RX-8, a lot of people have a lot of mods to prevent overheating. I encourage you to give this a google, there are some smart and some very scatterbrained ideas out there.
Why do this? Controlling when the radiator fans (there is indeed 2 of them) kick on is a common modification to keep things cool. My guess would be that the previous owner was attempting to force activiation of the fans, and didn't know a whole lot about wiring. That or the A/C button was broken and they thought it easier to wire it to always be on rather than fix the A/C button. What could you possibly gain from running the AC compressor all the time? I'd chalk this up to an uninformed but good intentioned "repair"
Can I do away with the ground wire and just have my compressor run as normal? Uhm. Is the rest of the fuse box still there? It would be easiest to scrap all that wiring and put the proper fuses in. If that is not an option, I don't think removing the ground would cause it to operate properly. It has already bypassed the switch. Unfortunately, I think you need to dig this out and rewire it properly.
From the diagrams below, we can see that fan number 1 is connected to the PCM, as is the A/C. I've read some forum posts, but cannot confirm at this time, that one of the fans is run specifically with the AC on (as Paulster2 alluded too in his comment). Perhaps this is actually just a substitution of a ground wire for a relay? Obviously, that would never work. But you may be able to just wire in new relays to this setup without having to go digging for wires.
I also wouldn't recommend leaving it like that. With the compresser always on, you are going to burn it out quicker, you'll be down on power, and a few 9K RPM runs will probably toast it. Honestly i'm surprised it lasted for... any amount of time.
Here is the relevant wiring diagrams and their source