Customer has installed spring assist system on a “used” trailer.  After winding the spring the door opens and closes but the cable on one side “jumps” off of the cable spool, pulley, or drum.

Things to check – cable anchor brackets are the same distance down on both sides of the door.

Suggested fix – Close the door and secure it.  Go inside the trailer and loosen the set screw on the cable drum that the cable is coming off of.  IMPORTANT  –  BE SURE THE DOOR IS CLOSED BEFORE YOU MESS WITH THE CABLE DRUM.  LOOSEN THE SET SCREWS ON THE CABLE DRUM NOT THE WOUND SPRING OR YOU MAY BE HURT.  After loosening the set screws wind the cable up on the drum until it is finger tight.  Tighten the set screws.  Open and close the door a couple of times to set the cable into the grooves of the drum.  This should correct the problem.

If it does not – either the door is racked or the rear hoop frame of the trailer is racked.  Both drums are the same diameter and they move on a common shaft at the same speed.  Since both cables are the same length, the “take up” is the same on both sides.  The only variable is the squareness of the door and trailer frames.  Racking of frame components is not a situation M-3 can correct with the spring system.