[yorkie_chris]

You say it disengages?

Does that mean it goes "zz zz-nowt"? Thats the motor itself not having power.
Put power to the starter motor directly from a jumplead on its contact under the rubber boot, that'll show if the starter motor's ok.

If it goes zz zz zzzzzzzzzzzz then thats the starter spinning but the starter clutch isnt engaging, and its the starter clutch you want to check out.