MY slider fan switch only works on high. I am assuming that I have to replace the control unit.... I found one at the junk yard still in the 1994 850 sedan volvo. when trying to remove it there are three black cables connected to the back of the unit. does anyone know if those three cables disconnect from the back of the unit or do I have to find where they connect elsewhere???? Thanks so much Bob

 

If it only works on high then the blower resistor might be bad.

 

can i confirm somehow that its the blower resistor, or just change it and se??
also is this something I can replace myself? can I get a used reliable resistor or buy new? Bob

 

Usually when you only have high fan speed only it is the resistor. High fan speed bypasses the resistor. You might be able to find a used one but not sure.
You should be able to handle the swap.