I have a 199 chevy s10 4x4. the starter will engage in the run position and stay engaged even when the engine starts. .I've replaced the starter and checked wiring at started. replaced ignition switch(electrical ) and switched relays in power distribution box. i've also moved wiring harness around under hood and under dash. Any ideas????

check the ignition switch as it may be keeping the starter circuit engaged after it is released back to run position

-the wires are not correctly re-installed for the starter.
- u may need wiring diagram
Did you take pictures before you took it apart, or did you tag wires before unconnecting them.

Is the engine turning over with the key in the start position? If the engine is not spinning over and you hear a whining noise, the starter is not engaging the flywheel. If the engine is turning over with the starter engaged, the engine has most likely jumped time that creates low or no compression which results in the whining noise.

You don't say whether the starter is continuously cranking the engine or just spinning continuously or whether after starting the engine the starter switches off but remains engaged? Lack of detail makes it difficult to give anything but a general answer.

Certainly the starter should be removed and checked for damage and correct operation. The pinion might be tight on the helix, the solenoid could be tight, the return spring broken or the arm pivot seized. If the starter has been engaged while the engine has run at speed or for more than a few minutes the overrun clutch should be replaced.

In any case where the starter has remained engaged the ignition/starter switch and the drive mechanism in the steering column lock should be inspected for correct operation.

its one of two faults a starter fault or a switch fault,,
test the switch by hooking a wire to the small terminil on the starter and hooking a 12 volt light bulb up to it,,,now turn the key,,this bulb will light up,,,only for the time the key is held in the start up poshion if it stayes light up when you have let go the key the key switch if faulty

solenoid may be sticking. Have someone start the car, if it sticks tap the solenoid with screw driver handle. If that stops the starter, replace the solenoid. If it were me, I wouldn't bother testing it and just replace solenoid.

solenoid on starter is sticking and needs to be replaced or repaired

The starter relay on the fender is the most common cause of this condition. ,It is on passenger side feder in the engine compartment, it is round and the positive cable of the battery goes to it,

the problem is the crank shaft position sensor ! I just replaced it on my wifes and it runs like new....