My '05 Z85 LS just had this problem (but the connector and pins were not burnt), so I bought the replacement resistor. Being the inquisitive type, I decided to investigate (reverse-engineer, if you will) this part. What I found was the following:
The thermal cutoff is connected from pin 'C'
on the connector to the common point of three wire-wound resistors. I think it is a 240-degree C cutoff (just a guess, based on the part markings).
The lowest resistance winding goes to pin 'B'
, and corresponds to motor speed '3'. This resistance should be 0.20 ohms
The medium resistance winding (0.92 ohms
) goes to pin 'A'
The high resistance winding (2.77 ohms
) goes to pin 'D'
The resistances above correspond to the markings on the front of the resistor assembly. The actual measurements I got were slightly higher (by about 0.3 ohms), most likely due to lead resistance and the accuracy of my ohmmeter (I have an old Fluke 75 that hasn't been calibrated in a while). At any rate, an ohmmeter will show 'OL'
if the thermal fuse or one or more of the resistors fail. A good unit will give resistance readings close to the values above when the ohmmeter is connected between pin 'C' and the other pins. I tried to attach a PDF file of the schematic I drew, but I was unable to do so.
Tomorrow, I plan to install the replacement in my truck and take voltage and/or current readings at each speed setting, so that I can try and resolve this problem once and for all. That is, if anyone is interested. I see from these forums that many of you have experienced this problem, and some have replaced the blower motor resistor several times. That points to a possible design flaw, either in the connector, the power handling capability of the resistors (i.e. wattage), or the blower motor windings themselves. I'm not sure that I can come up with a solution, but I'm willing to try.
I'll post my results if anyone is interested.
It is my understanding that Chevrolet put out a TSB after an investigation of this problem, but I don't have access to it currently (I have ordered it).