Mazda Repair Questions? Ask Mechanics for Answers ASAP
It's the job of the PCM to send the ground signal to the fan relay's and turn them on when the car gets up to temp and they have to come on. It should also turn one on when you turn on the a/c and the compressor engages.
Since the issue is not consistent, I would first try disconnecting the PCM and re connecting, it's quite possible there is a bad connection at that point or a loose wire that is sometimes not making contact. Since the fans do work at times it seems the computer is turning them on when it's suppose to.
If this doesn't help, then it's likely that you will have to trace the wiring harness from the computer to the fan relays.
Please let me know if there is any more I can do to help you from here.
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The relays ground via the pcm and that sends power to the cooling fans.
I would be lying if I told you that both fans should come on, to be honest I don't know, but the condenser fan should cycle with the a/c unless the engine is very hot, then both fans would kick on. So, it seems likely they should both default to on when the coolant temp sensor is unplugged.
Also, Check the box itself for a bad connection. With the age it's possible there is some corrosion inside the box where the harness goes in. Maybe you have a loose wire in there causing some grief.
I agree, it's going to take some patience to locate the problem, but also seems to point to a poor contact. It could be a poor contact inside the computer, but honestly I would be thinking wiring first. Since you see the temp increase I would assume the ECT is working properly as well.