The 121°C thermal fuse in my shopvac motor got burned out. I think I might have had a shop towel draped over the motor vent and it overheated.

I bought a replacement 121°C thermal fuse and soldered it in place of the old one. The old thermal fuse was crimped onto inline to the copper wire. I didn't have any splice connectors or a crimp tool, so I used my soldering gun. I put an alligator clip between the solder joint and the fuse itself to act as a heat sink. After I soldered it on, I tested with a meter and it showed the circuit was continuous, so good to go.

Put it back together and viola, the shopvac worked! ...for awhile.

After about 15 minutes of vacuuming up some light sawdust, the shopvac quit. I haven't taken it apart but I suspect the thermal fuse burned out again (luckily I bought 5 fuses).

By soldering in the replacement fuse instead of crimping it on, is it more than likely that I damaged the thermal fuse and lowered it's breaking point to below 121°C?

 

yes, don't solder a thermal fuse. Some even come with crimp terminals.

 

OK, I replaced the soldered in thermal fuse (it had blown, as suspected) with a crimped on thermal fuse.

 

Well, that didn't make a difference.

The thermal fuse that I crimped on (no solder) just blew after about the same amount of use, about 10 minutes.

Is there a difference in types of in-line thermal fuses that is causing the replacements to blow? I figured "121°C" is the same no matter what.

The replacement thermal fuse that I used is labeled:
SEFUSE
SF119E
121°C
10A
JET250V
C1162

The original thermal fuse (as best I can read the markings) was labeled:
MICROTEMP
??GCQU (can't read this exactly)
121

 

The thermal fuse may be doing its job so you need to look for the cause.

 

The motor overheated. The windings have probably been slightly compromised.

The thermal fuse is more of a liability protection item for the manufacturer. They don't want the motor to overheat and melt/burn the plastic housing.

You could try a higher current device like.....
G5A01121C - Thermal Cutoff Fuse, MICROTEMP Series, 16 A, 250 V, 121 °C

Or try the next higher temperature like... 128°C. Adds about 15°F to the operating temp.
Don't shorten the leads to the fuse. Make sure the crimps are tight or they will generate heat and blow the fuse. Keep the fuse further from the motor.

 

Motor not being cooled by the fan.
Bearing tight/worn out.
Winding to winding short.

Does the motor come up to full speed?
Does it smell bad after it was run?