When o engage the starter I only hear the motor running, what is the reason?

Sounds like the starter drive is not engaging. The starter drive on the starter is not engaging into the flywheel. You would be able to hear the motor on the starter it is just that the drive gear on the end of the starter is not coming out to the flywheel .

Sounds like the solenoid has gone on your starter motor, this is the device that throws the gear out of the starter motor to engage the flywheel to crank over the engine. Remove starter motor and bench test to confirm, how to do this is: starter motor removed and in vice, earth the body of motor and where the thickest input wire was is main 12v add a 12v to this, and the small input wire just tap 12v to it and see your motor work if not working then replace starter motor, it should throw gear out and spin

several reasons
the teeth on the ring gear were damaged from continuous grinding during pinion engagement or engagement of the starter when the engine is running causing a burring of the teeth that altered the mesh ability and jammed the ring gear and pinion
metal in the teeth of the ring gear that causes a jam in rotation
starter motor not engaging before rotating
wrong no of teeth on replacement starter pinion or starter motor

If this is an old starter then it is going out. It is not engaging with the fly wheel and that is the "running sound" you are hearing. It is also the reason that it is not starting the vehicle for you. The shaft is not coming out and engaging all of the time. Long and short of it is replace the starter and all should be good

The starter solenoid mounted on top of the starter has two functions, as the key is operated the solenoid slides the starter pinion to engage the flywheel, THEN operates heavy duty contacts to power the motor. The solenoid is not engaging the pinion.

not enough voltage?
Did you try jumping?

Hi without seeing (hearing) it sounds like the clutch in the bendix is faulty, as the solenoid will engage the bendix at the same time as connecting feed to the starter motor.
New starter required or a good strip down and rebuild on the solenoid and bendix.
What condition is the ring gear in by the way (Starter Motor removed). David

Sounds like either the starter motor has jammed or your battery is down on power so unable to engage the starter motor solenoid. Cures: 1.Hit starter motor with hammer using short sharp shocks - this often frees the stuck starter motor engaging cog. 2.Put car in gear and attempt to rock the car back and forth. 3. Jump start using stronger battery. 4.Charge battery - you might want to check that your alternator is charging the battery, the easiest way is to use a multimeter on the 0-20volt DC scale and see that the voltage is higher than the battery in a non running state. You should see a reading of around 13-14 volts. Obviously you will need the motor running to check this.Lastly if possible try bump starting.

Without hearing the sound it makes - it's kind of hard to diagnose.

The engine starting process consists of turning the key to start, the starter motor's rotor moving forward to engage the engine's flywheel, and then the starter motor 's "rotor" spinning until you move the key from "start" to "run". You can hear the starter motor spinning - but one usually can't hear the rotor moving forward to engage the flywheel. As a result, you hear the odd spinning sound - which is the starter rotor spinning. The engine never actually turns over because the rotor never engages the fly wheel. If this is the case, then it is very likely that your starter needs to be checked and / or replaced.

Good luck!

Sounds like the starter solenoid or starter motor not engageing. Remove the complete starter motor assembly and take it to a shop to have it tested. Also when you remove the starter from engine look at the flywheel teeth to see if they are sheared off causing a no start condition. Hope this will help you.