Just got me an overdrive

Hi all.

I've bought a Faiery overdrive for my Series III.
When I turn the output shaft, by hand, it only rotates a lit bit and then it blocks. Should it be possible to rotate by hand or could it be that something is broken?

Also, what should I check for before fitting it ? All the cog wheels seem fine and there's no lint to be seen.

Thanks,
Manuel

not sure what happens if its in neutral

I cant remember but heres the manual if need be

http://www.landroverweb.com/Pdf-files/Maunals/Fairey_Overdrive.pdf


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Thanks for the answers.
I haven't tried it in neutral. I opened the top plate and used a wrench to move the selector from in to out (not sure which is which), but didn't realise there was a neutral point.
The manual doesn't mention anything that can help me decide if the resistance is expectable or not

Help ?