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My Toyota SFT39 (RS2000 3D Series, 2012) sewing machine has suddenly started accelerating at top speed no matter how gentle I am on the foot pedal. I opened up the foot pedal and can’t see anything wrong.

Can anyone help me troubleshoot?

It was sewing fine before this and has never gone this fast, unless of course I put firm downward pressure on the pedal.

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    Maybe post photos? Pedal (open and closed), connector between pedal and the machine (I expect that the pedal's cord is detachable)... Also, the cable of the pedal. You should inspect the entire length of the cable, to check if there is any obvious sign of internal degradation. Worst case: the problem is inside the machine itslef, and the best solution in that case is taking it to a specialized service. – virolino Apr 8 at 5:23
  • Please have a look at this question. Maybe there is a similar dial on your pedal that got adjusted accidently. – Elmy Apr 8 at 7:00
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    I fixed a Bernina foot pedal in which the metal strip that actuates the speed control (and a further microswitch to completely stop the machine) was bent to the point that it affected behaviour. If you're troubleshooting, beware, as there's live mains in the pedal, exposed at soon as you get the cover off – Chris H Apr 9 at 10:38
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This could be caused by a faulty capacitor in the machine.

See glossary item on this website:

Electrical sewing machines have a component inside the motor of the machine called a capacitor. This helps eliminate electrical noise from the motor and prevents the electrical noise from going back into your homes electrical grid. When capacitors age they can split open and leak electrolytical fluid, which renders them faulty. Once this happens, the motor of the machine will at times start running by itself. The best course of action for when this happens is to take it in to get it looked at by a qualified sewing machine technician.

from: https://sewingmachinecentre.nz/pages/glossary#Running-On

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While I don't have this particular machine -

Most of these universal motors use electronic speed controls that use a device called either a TRIAC or SCR, that regulate the motor speed by chopping the mains frequency (i.e. turning on and off rapidly 100 or 120 times per second). The usual failure mode is a short circuit, which will produce the symptoms of "only runs at full speed".
These electronic speed controls also use a filter with capacitors and inductors, and it is possible that a shorted filter capacitor will also produce the same fault, as mentioned in another answer. The capacitors used are known as "X2 class safety capacitors"; they have an internal mechanism that is supposed to clear faults by blowing internal fuses, however this mechanism may not work with low current loads (like sewing machine motors).

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