Peterbilt 379 blower motor 1995

Ohms law is V (volts) = I (amps) x R (resistance) so I = V/R Let's say V is constant at 12 volts. I = 12/R so the higher the resistance the lower the amps and the slower the fan runs. Different resistance for each fan speed.
Regarding the black butt connectors. I was going to say that someone tried to heat them but looks more like burnt from the inside. High resistance due to a poor connection will cause higher heat and the burn marks. Or a fan going bad drawing too much (more than normal but not enough to blow the fuse or breaker) could have caused the connections to get hot resulting in the burn marks.
Look closely at the Kuject box under Feature. The 4th bullet point states 'solderless'. Bullet point 5 states in part: ...'hot melt adhesive and SOLDER inside'....
LOL

 

Check amp draw on the motor during operation. Any resistance to airflow thru the system(blocked heater core or EVAP core) will also increase amperage. Even the factory connector will get hot and melt and a single good hard splice will usually suffice but soldered connection is better.

 

Ohms for resistance is checked with power off, disconnected. Otherwise will blow the fuse or fry the meter. Voltage is checked in parallel. Amps in series. Amps usually checked with an inductive clamp such as on a load tester or the newer type capacitance testers for an alternator. If using a meter say for alternator amps you would have to disconnect a wire from the alternator and one meter probe would go on where the wire was disconnected (stud) and the other on the end of the disconnected wire. The meter probes are between so that amps runs through the meter.

 

With a clamp meter, they sell cheap ones at Harbor Freight.

 

Looks like my cab blower motor. Had a short on the fan side. Cut the Truck side by mistake. Yes I butchered my own wiring. Also tried to use a bunk blower instead by swapping the plug ends. Bunk blower turn’s Opposite way. Explains the different plug. Lol. Swapped wires, but squirrel cage is still opposite. Finally got it fixed right. Have since replaced it with my spare. What happens is the motor gets worn and spins slower. Problems aggravated by built up dust. The wires then get hot. You can check this and that. Easy way, feel the wires. Hot - not good. Lol. Replace fan, and notice how much better it works. I’ve got to do mine now. Wires are pretty hot, and fans blowing very little. Tip - Don’t spray the motor with any WD or especially silicone spray. Did that on my first Truck, had it spinning good again, till it burst into flames. Almost set my Truck on fire!! Good times!!