My Pfaff sewing machine has started to loop the top thread underneath. Tried fixing the tension on the bobbin thread as they state in the manual, but the problem still persists. Any suggestions?

increase the needle thead tension

Check this link, it is threading diagrams, hope it can help you.

nothing to do with bobbins
A lot to do with insufficient needle thread tension

not sure if this will help you...Check Your Tension Settings. Looping underneath the fabric usually occurs when there is not enough tension in the needle thread to pull the bobbin thread. Sometimesthread buching can be caused by something as simple as the tension dial being inadvertently turned.

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Look very closely at your sample stitching and determine which thread (top or bobbin) is looping and which is breaking. That will narrow down where the problem exists.

A top thread that loops and makes thread "barfs" under the fabric is generally an upper thread tension problem.
Try: Remove the top thread. ALWAYS RAISE the presser foot and rethread from the beginning. Set the upper tension to the midway point (probably will need to tweak this tension depending on your machine).

Sewing Lesson 10 How to Fix Tension on Your Sewing Machine

Understanding Thread Tension Threads

Sewing Machine Tension

Hello, If the thread is loose on the bottom then it is the tension on the top. If the thread is loose on top then it is underneath.

This is usualy a thread tension problem. Adjust the upper thread tension first. If tightening the upper thread tension doesn't work, then adjust the bobbin holder (lower thread tension) check with the user manual for your specific machine for the correct procedure.

If the machine is now threaded, lift the presser foot.

Now, pull about 10" of thread through the needle.

The next time you thread the machine, lift the presser foot first.

When the presser foot is up, the tension control opens and allows the thread to "seat" into the control, then when the foot is down the control closes to the indicated setting.

When threading the machine with the foot is down, thread does not enter the tension control and when you start to sew, there is no tension on the thread.

No tension = loosey, goosey, loopy stitches underneath.

Because the lack of tension on the top thread, the bobbin thread cannot lock a proper stitch.

Rule of thumb.. Loops on the bottom, problem on the top. Loops on the top, problems on the bottom.

Set your upper tension at 4, and rethread the machine from the top. Just before you thread the needle, but the presser foot down. If the thread pulls right thru easily, then the thread is not thru the tension wheel right. Because, if you can't pull the thread thru with the presser foot down, you have it threaded right.

I don't have the same brand as yours, but on my machine there is a little screw on the bobbin case that you tighten to increase the bobbin tension. If you've got one, turn it 1/8 to 1/4 turn at a time.