sharp mx-4070N folds paper when making a 2 sided copy

[driving lots of miles]

We have a Sharp MX-4070N that works fine when making a single sided copy. When making a 2 sided copy the paper has a fold before kicking out to the output tray. We have the machine in the shop and the customer has a loaner. We have a working machine have been swapping out parts from but have not solved the issue. The paper folds before it reaches the transfer belt when printing the second side and the paper is not printed where it is folded.
Firmware is up to date. We have swapped the drive motors, the side door, fuser, Transfer belt unit, 2nd transfer, paper path modules above the fuser, paper path checks out ok from the door exit area to the transfer belt.
I am currently out of ideas; any help is appreciated.

[AAce]

try checking the shift motor or if not do a copy and open the front cover (Before reaching the transfer belt) to jam the paper and check were did the fold occur.

[bilyahn]

Check to see if there are any teeth missing on the reversing gate at the entry of the duplex unit. Had this cause paper folds before.

Good luck

Bil

Oops never mind, if you swapped out the side door that would of checked the gate!!

[driving lots of miles]

On further inspection it seems to be the upper ADU transport roller on the right door that the timing seems to be off. It seems to be starting after the paper arrives. As i have switched doors it must be in one of the boards? any further Ideas appreciated

[mansart]

found this in my notes.....

xx50, xx70,xx51, xx71 duplex jamming
Friday,July 26, 2019
1:39PM
paper z folds induplex
passes 8.5*11
jams on longer paper
check connectors onPCU board, ribbon cable from right side door

[Hansen88]

I had one that tat had duplex issues that replacing the right side door fixed. On mine I put the old one back on and swapped the board that the ribbon cables connect to in the door and that fixed mine.

[driving lots of miles]

found this in my notes.....

xx50, xx70,xx51, xx71 duplex jamming
Friday,July 26, 2019
1:39PM
paper z folds induplex
passes 8.5*11
jams on longer paper
check connectors onPCU board, ribbon cable from right side door

I checked the ribbon connectors on the PCU PWB and with barley a touch one of the ribbon connectors came off the board. So I will order a new board and hopefully have solved my problem. Thanks for all the help.