My speedometer cable was broken, I changed it + the cable that goes to the clock.

Old one was connected to some kind of electric sensor that god broken:



The mechanic took it off and putted a cable from a 75SX punto without a sensor straight to the gearbox.
What is that kind of sensor? would that cause anything?

Looks like a custom hall-effect sensor, used to measure speed digitally. These are usually used in aftermarket fuel consumption/trip/taxi computers and cruise control systems. Do you have any of those installed?

It's not part of the cars original electronics, so if you don't have any device of that sort installed, don't worry about the sensor.

I know the ELX/90 version came with it + a mechanical cable connected to it.
When I changed the speedometer cable I took the unit off because the plastic is broken and couldn't connect to the cable.

The gauge works now, but EML light is on.
I guess it's a part of the GM ECU that couldn't work without it.

Oops, you are right. I checked, this is an original speed sensor after all. If the speedometer is cable operated, the ecu may be using this sensor to count speed and mileage.

The EML might be about that sensor not being registered. You might need to get a new one to keep the light off. If the old sensor gave no EML, it might be only broken mechanically, and maybe theres a way to fit the cable still?

This is the old cable that I've replaced:



It was connected to this bit:



Is it the inside of the sensor?
It looks different

It could be, try if you can fit the axle inside the sensor. Sorry I can't be of much help as I have never seen that sensor personally.