If so, and if it has a duplexer unit fitted, can you tell me if the duplexer fan should spin at power on?

Our recently acquired 4100 works in itself, but every time I put the duplexer on, it puts the paper into the unit and jams solid...so I do wonder if the duplexer has failed and motors aren't running to pull the paper through.

Anyone got any ideas for me? It's an interesting unit, but it's beginning to really annoy the hell out of me.