Had a similar problem on a friend's old Caddy last year. It wasn't until I was standing underneath while he worked the key the cause revealed itself and I saw a tiny spark. Cleaning and refitting the power cable and starter motor connection to/from the solenoid did the trick.
Mostly such problems are caused by worn or sticking motor brushes, assuming the supply and ground are ok, and it is best when a problem is experienced to dismantle the motor and take a look. Often, if there is access, the problem can be further recognised by striking the starter sharply with a hammer in the vicinity of the brushgear while someone is holding the key in the start position. If that causes the engine to crank or causes more clicking the problem is likely to be the brushgear.
The starter won't receive current from the solenoid until the solenoid is fully "home". The ground path of the pull-in winding of the solenoid is via the motor brushes and if insufficient current flows there will only be partial solenoid travel aided by the hold-in winding which is not powerful enough for full engagement alone.
The internal contacts of the solenoid rarely fail but the possibility should not be ruled out if no other cause is apparent.
After the motor has been examined it is wise to bench-test it before refitting.
Answers & Comments
Had a similar problem on a friend's old Caddy last year. It wasn't until I was standing underneath while he worked the key the cause revealed itself and I saw a tiny spark. Cleaning and refitting the power cable and starter motor connection to/from the solenoid did the trick.
Mostly such problems are caused by worn or sticking motor brushes, assuming the supply and ground are ok, and it is best when a problem is experienced to dismantle the motor and take a look. Often, if there is access, the problem can be further recognised by striking the starter sharply with a hammer in the vicinity of the brushgear while someone is holding the key in the start position. If that causes the engine to crank or causes more clicking the problem is likely to be the brushgear.
The starter won't receive current from the solenoid until the solenoid is fully "home". The ground path of the pull-in winding of the solenoid is via the motor brushes and if insufficient current flows there will only be partial solenoid travel aided by the hold-in winding which is not powerful enough for full engagement alone.
The internal contacts of the solenoid rarely fail but the possibility should not be ruled out if no other cause is apparent.
After the motor has been examined it is wise to bench-test it before refitting.