Blower fan keeps burning plug

Hi.

I have a 2010 peterbilt 387.
My driver installed a new 4 speed switch for the blower fan motor.
We did this cus function 1 to 3 quit working, but function 4 still works fine.
....

Now we are back at the same problem after about 2 month of no issues.
1 to 3 not working but nr 4 works.

Fuses dont go out either....
The complete unit costs about $500 but it's only the switch that's not working.

Any advise would be great.

Hello,

2 possible causes the switch gets burned: excessive current draw or excessive resistance/bad contacts. Measure the current draw and compare to specs. or a known good one. Look at the connectors at the blower side, do some voltage drop tests. Check if the blower motor spins freely or if it's binding. Often times the resistor unit - if present and the blower is not pwm controlled - is cooled by the airstream, any debris on it or restricted airflow to it will allow the temperature to rise.

To me it looks like the contacts in the plug have been loose and arcing, if it had been exceesive current getting pulled by thr blower motor I would expect the fast speed to be worst affected.

Odds are the blower motor resister when you loose speeds 1-3 and have 4. There are resistors in the blower motor resistor speed control. When they go bad you loose the lower speeds. There is no resistor on 4 speed. The cicuit will pull 20 amps on high speed you will want to hook a scope up to it and check the waveform for potential issues with blower motor as well