So a little background of the problem before I jump into this. The heater fan in a 98 Ford Explorer stopped working completely for my friend. He bought a replacement fan and all was good for about two weeks. Then the fan stopped working on all speeds except "high." He bought a replacement resistor pack from an aftermarket company. I won't mention any names but its a major auto parts store. All was well for about a week and then the same thing happened again. So I get a call from my friend a little bit annoyed and confused about the whole problem. We go through the possibilities because surely it wasn't the resistor pack, that was just replaced... it could be a bad connection, bad relay, bad switch, blown fuse...etc. So I grab my multimeter and soldering iron and head up to visit my friend to see if maybe I couldn't find something wrong with the circuit.
All fuses that pertain to the circuit. Check
Voltage and Ground at the motor. Check.
Relay click. Check
Switch check. Check
So we decided that maybe the plug-in that goes to the resistor pack was corroded just enough to block current from going through it. I replaced that, solder and shrink tube, (the only way to make wire connections in my opinion) and plugged it in with tons of dielectric grease on it expecting it to fix the problem. Well it didn't hurt, but it didn't fix it.
Now starting to scratch my own head, I decided to pull the old resistor pack he had replaced out of the box to see what actually went wrong. Normally I'd be looking for a broken connection or a burnt up coil. This one is the resistor pack you see in the photo with my multimeter in the background. It looks relatively in good condition...so what gives? Continuity check time just for good measure. The way this circuit works is all on the ground side. 12 volts is always present at the motor, and the switch in your dash provides the ground. On speeds 1-3 the ground is re-routed through the resistor pack and on high speed it bypasses the resistor pack and the motor gets full voltage. So effectively, the lower the voltage, the lower the speed. The way the resistor pack does this is by shortening or lengthening the circuit through the coils to add or subtract the amount of resistance it takes to get the speed you want. So naturally, three speeds, four pins. One for each speed and one that heads to the motor. I start my continuity check across each coil and everything is fine until I reach the final pin. I check my meter "beeeep"...then check again...nothing. Then I realize there had been a cylindrical bullet shaped thing I didn't recognize stopping the current from passing through. Finally! Something wrong! So what is it?
Well if you haven't come across one before, as I hadn't, its a thermal fuse. Odds are it has a few numbers on it and a rating. (Thanks google) Here's another tid bit. The original Ford part that I was looking at had a Thermal Fuse with a 121 degrees C. The replacement part had a fuse with the rating 113 degrees C.
So we have something wrong. Now how to fix it? Well in this case, it was like 2am and nothing was going to happen. You could risk it and jumper the terminals but I wouldn't recommend it. After all, they put the fuse there to prevent something. I'm guessing a fire in your heater box... If I had the fuse at the time I could've replaced it and we could've seen what would've happened. The next morning we went to a different auto parts store and got the same part, this time with the correct fuse rating. Replaced the part and everything was good until...about two months or so later. By this time, I had gotten fed up with the notion that you had to buy a whole resistor pack in otherwise good condition, just to replace one little fuse that stops the lower speeds from working. I had ordered some fuses and gave it my shot at replacing them. I've read where people have replaced this said fuse with success but I couldn't find any, step by step process or pictures of how they did it.
Step 1: Order the Correct Fuse
Step 2: Remove Old Fuse
Step 3: Continue Removal
Step 4: Tin
Step 5: Solder the Crimps
Step 6: Add the Fuse
Step 7: Rinse and Repeat
Step 8: Another Shot
So why do these fuses blow in an otherwise normal circuit? Why wouldn't the main circuit fuse blow instead? Well that's the question I have and the only logical answer I have is: Heat. Heat either caused by poor airflow across the resistor pack resulting in poor cooling, or heat caused by a higher amperage draw from a worn out motor with poor lubricated bearings/bushings or worn brushes. If the motor was serviceable, that would be the next route I would take. Seeing as it isn't, and this fixed the problem very cheaply with spare resistor packs resulting from the idea that the more practice, the better, then this is the last I'm going to worry about it. That is unless the problem occurs again and again with increasing frequency. Then it almost has to be a problem with a faulty motor I would think.
That's all for now. I tried to be as detailed as possible. I hope this instructable is enjoyable and helps someone troubleshooting with the same predicament.