How do i fix sewing machine tension?

The hook pulls the top thread half way around the bobbin case and then the slack arm pulls it the rest of the way around. The slack arm is in the upper part of the machine barely visible on newer machines, but you do hook the thread thru it while threading the machine. If your thread doesn't go thru the tension discs or the tension is very low, then the slack arm has nothing to pull against, like a tug of war where the guys on the other end of the rope let go!
Try pulling on the upper thread before the needle and be sure the presser foot is down (up releases the tension). Adjust the tension and you should feel a difference.

Does the thread actually break? The upper thread sensor is normally connected to the check spring that is part of the top thread tension assembly. I would check or have a Viking tech check the tension assembly to see if something is causing the sensor to stop working. It might be a bit of thread or something else.

You sound like a very experienced seamstress. Are you inquiring about a serger or a sewing machine. Work with one tension at a time. Is it the upper thread that is loose when sewing on a sewing machine. The bobbin controls the upper thread and visa versa. Set the tension dial on the sewing machine to zero and work from there slowly. Perhaps the tension discs are not controlling the thread any longer. I hope this helps. Kate

Check your top tension dial setting is on 5, as this sounds like the thread is not in tension fully so is looping up around the rotary hook and not being held back by the tension device. Is the pressure foot lowered (this turns the tension on) fully and is the thread through the tension discs correctly. Lower the pressure foot and pull the top thread from below the tension dial, it should feel firm to pull, if it comes out easy, its not undertension, device could be faulty and machine will need to be serviced.

Also, check that the bobbin case is fully inserted into the rotary hook in the correct position, bobbin is in and rotating the right way and thread is into the tension spring fully.

If you have the manual pull it out, and carefully check the threading up instructions and follow on your machine - it is probably one of these things causing the jamming.

Sounds like you have the upper threading wrong, or you have the upper tension set too loose, set it to 5.

Please make sure your upper thread is threaded correctly, more importantly it sounds like there may be a build up of lint or dirt in between the upper tension. Prise the discs apart and remove any lint and make sure when you thread the machine that the needle thread goes right in between the tension discs and does not just sit on the outside of them. Hope this helps you.

rethread the needle as the thread has no tension

bunching up on the bottom means that the upper is not threaded correctly - no upper tension.

hi! there wind thread on bobbins winder 4 times by your hand rotate clockwise,insert in shaft bobiins winder on top of your machine near hand wheel push latch to engage loop thread on disc winder tension start your machine on pedal foot automatically winder disengage if already filled up of thead pull this winder and insert to bobbins casing insert thread to slot and pull out winder should rotate clockwise as shown.insert on shuttle hook casing and close cover needle plate.next thread your upper

threading tension as shown on red threading from spool pull threading and loop to dial tension threading and insert thread to lever needle and to needle hole tip. pull tread under the needle foot and feeder tooth.rotate hand wheel counter clockwise to hook up thread bobbibs and appear at top of needle plate,teo thread shoul appear,set dial tension to # 3 and stich to # 8 range to # 12 depend on fabric and layer of fabrics and set on straight stich sewing,let needle on upper position insert fabric for trial sewiing.put down foot needle lever. manually by hand rotate for assurance needle is free to sew if obtruction on loop bobbins thread occurs adjust all tension gradually then try again until free from sewing w/o problem star sewing.