I've searched but there are to many threads to review.

I'm finished with my rebuild and hooking up the ECU and have two plugs that I have no idea where they go.

This first pic has a single wire black connector on it and is coming off of the engine harness. I check the old harness and a previous owner had cut off the plug and wired it to the ground that connects to the ECU mounting bracket.

The second pic has the exact same plug, black and single wire, coming off the other harness. Any help is greatly appreciated.

 

i think one of them is for the cooling fan diagnostic and plugs into a little black box, i will try to look tomorrow at my wiring

 

That single wire black connector is a diagnostic port to test the fans. If you ground it for 30 seconds, then shut the car off and unground it, the fans will run for 10 minutes. Some people have hooked this up to a switch and call it the "fan mod". It normally doesn't get connected anywhere. It was part of the jumper harness and fan relay box mazda added in one of the cooling recalls. New engine harnesses have the additional connectors wired in so you can discard the jumper harness.

 

Excellent info Rotary Experiment. After searching more I did find that one was for diagnostics on the fans but couldn't figure out why I now have two with the new harness. Thanks for the piece of mind.

Steve