Starter motor misbehaviour

I have a 98 XJ, straight 6, auto. For the last 6 months, on start up, sometimes if it doesnt fire straight away, it just engages and then whirrs away. Its now got worse and won't turn over the motor. Popped the starter out today and checked it over, cleaned the solenoid and bench tested and all appears to work on the bench
Could it be the bendix clutch? It sounds like it engages and then starts to whirr.If I short the small terminal to the thick pos cable connected to the solenoid in situ it does the same thing.The battery is good and new, even tried jumping it but that makes no difference. Help!

If your cables are good, connections clean and tight, then I would suspect the bendix.

Try a voltage drop test on your cables: http://www.aa1car.com/library/voltage_drop_testing.htm

If it is the original starter and your symptoms seem to be bendix related, replace the starter, or have it rebuilt.

Brushes may be stuck,

Only other thing to check are the flywheel teeth. Look thru the starter opening and turn the motor by hand.

Thanks guys!All sorted! managed to source a bendix and for good measure a solenoid this morning and rebuilt the starter and now its perfect.Parts are not very easy to come by here, so I consider myself lucky managing to find them.