Tajima 4 head 291 error

now i think is more clear.
when the machine run 14 move up and down and hit 16
when 14 hit 16 close an electrical contact.

if the machine stop for false upper thead breakage means 14 is all the time in contact with 16(contact ON) because:
spring(14) force is to big
under 12 is rust
adjust spring force and clean the rust

if the machine DON T stop for upper thred breakage means 14 never make contact with 16(spring-14 force is to small) or 14 is mechanical in contact with16,but between 14 and 16 is fabric dust,temporary spray,etc.and electrical the contact is OFF
adjust spring force and clean 14 and 16

I agree with George, it's almost always the spring to contact plate,so what I do is isolate 3 heads,sew on just one, go through one needle at a time until the error occurs,then unscrew the check plate just above the main tension assembly it's held on with a small silver Philips head screw. When you have this of look at the little tongue the part that the spring rests against when machine is at rest,I polish this with smooth emery cloth,lay cloth flat on table,press tongue flat down and move from side to side until oxidation is removed and there is an even shine ,then replace and set Philips screw in center of slot,then loosen 4 screws holding on the tension head .
on the back of the tension giving trouble you will see a small black slotted screw, Loosen this off and remove the assembly, I spray the pin and spring with wd40 then rub it all clean, also I jam a rag into the hole where the tension drops in and polish the brass section inside, then reassemble making sure the tension assembly is all the way down and adjust the check spring to medium response then tighten little black screw. I have my 10 year old does this when I am doing a full service, so once you've done one you just continue through each one..good luck

for upper thread breakage the LED on tensione base is ON-red light
for UNDER thread breakage the LED flash red,but many,many times the panel display the same error 291.
the error for under thread breakage is 292 or 293

1.press F3 key until nr.2 is diplayed-UTC parameter
turn jog dial(the whils on panel right upper side) and select -(minus)
press SET.
now the under thread sensors are OFF.
make test.

2.let under thread sensors OFF
press F2 key until nr.5 is displayed-TD parameter.
turn jog dial and select max. value.(to turn OFF the sensors select -(minus)
press SET
make test

what model is your Tajima?

generaly for all Tajima machines with this kind of sistem for upper thread breakage(spring):

above the thread check spring is an plate.
between this plate and the spring is an electrical contact.
when the machine run this contact is ON-OFF,ON-OFF...and give this information to tension base card.
in case of thread breakage the spring don t move,cntact is ON and the machine stop.
if the spring force is to big and thread tension to lose the spring don t move and you will have false thread breakage.
to adjust spring force put an -screwdriver in the midle of button and turn right or left to increase or decrease spring force.
to be sure if the spring is the problem or not:

adjust the spring force.
take off the plate(above spring) and clean contact surface,and the back side plate surface
clean the metal part on tension base(where you put the plate fixing screw)
clean spring contact surface.
make test.
try for one head,one neadle first.

if not the spring is the problem:
clean with dry contact cleaner(spray)and compress air:
tennsion base card connector,cable connectors between tension base card and the back side of the machine.
cable from tension base goes in to an joint card.
clean joint card connectors.

if don t work maibe is necessary to make machine reconfiguration or to install again software.(considering all cards OK)
but about this tommorow, now is to late(1 AM)

is an seting for thread breakage stitches number.
i don t remember if is at F1,F2 or F3
find TD parameter,turn jog dial and set to maximum.press SET
make test.
if the machine has old software,max. value is 2
if is newer is 9

i forgot
the spring is fixed by button shaft.
take off button assy.
if in the tension base shaft hole is rust,also you will have false thread breakage.
clean the rust.

sorry if my english is not OK
if some needles work ok and some not i am sure not the cards is the problem.
is contact between spring and plate above.
clean first with compress air spring and around.
adjust the spring force
if in case of thread breakage the spring don t hit the plate above the machine don t stop