What do you mean with \ "when I turn it on only \" directly feed the fan with battery cables?
It is possible that what is failing to you is the thermocontact of the electro -ventilator, it is located on the side of the radiator and it comes two or more cables so that when the water brushes the 100ºC the current pass is established through the thermocontact.
To check it, it will be enough to bridge the cables with the contact on, if everything else is fine, you will cause an immediate start of the electro -seasons while you are doing the test, as soon as you release the cables, the electros will stop. (Be careful not to be the manicure with the fading blades).
Another although less likely would be the failure in the engine thermostat, the thermostat closes the water outlet to the radiator while the engine has not reached its working temperature. If the thermostat has broken, it is closed and does not let the water pass to the radiator, causing engine heating, the electros do not jump because the radiator is cold.
To check this anomaly, it will be enough to start the engine and put the hand on the radiator panel, while the engine is in the heating phase, the panel will be at room temperature, once the engine has heated, the radiator panel will begin to heat up until a time comes when it burns when touching it. (about 80ºC or 90ºC). After a few minutes, the elects should be launched to cool the water.
All the best.