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Have you ever started your car up in the morning only to realize that your heater fan doesn't work on any of the slower settings but does work on high speed?  Well, my friend did.  In, fact, this occurrence happened to him not once, not twice, but three times.  I'm not completely sure of the construction of different car manufacturers' resistor packs, but odds are, if you own a Ford, this may apply to you.  This is the basis for this instructable.  

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

The part I ordered is from NTE Electronics.  It is the correct 121 degree C value and it comes with little crimps and instructions.  The part itself cost roughly $1.xx...I ordered 6 with shipping for around $10.  A new resistor pack at this time cost somewhere from $16-20 from aftermarket suppliers.  Like I said earlier, I won't mention names...but they rhyme with "crappa" and "autotone"...  I never did check with original Ford parts, but I'm sure it would've been twice that.  Bare in mind I had 3 resistor packs to try it on and I could easily screw one of them up haha.  So you could say I'm potentially saving my friend quite a bit of money here.

Step 2: Remove Old Fuse

Now the instructions say to use the crimps and only make mechanical connections.  By no means, do you solder this guy on...as tempting as it may be.  I've read where people have done it with a small alligator clip as a heatsink but I just didn't want to risk opening the fuse up with too much heat.  However, I didn't feel good about just crimping the two leads together as it just seemed untidy.  So, this is my spin on it.  Use a die grinder or equivalent to remove the original crimps down to where the lead breaks in two and the fuse will fall off.

Step 3: Continue Removal

Use the pointy end of the grinder bit to just carefully drill the rest of the material out and take a bit of the galvanized surface down to clean metal.  I suppose you could use a small drill bit for this step too.  I found it quicker to just use the die grinder.

Step 4: Tin

Heat up your soldering iron and tin the crap out of the surface.  My goal was to add just enough solder to fill the holes over and provide a nice shiny surface.  I know it doesn't look like it in the picture but its kind of blurry and any dark spots you see are flux residue.  I used lots of flux to ensure good solder flow.

Step 5: Solder the Crimps

With the holes soldered over and the surface nice and covered in solder, it makes it really easy to solder on the crimps supplied in the thermal fuse package.  Just set the crimp on the area.  I used a bit of flux just to make sure and then heat with iron and add a bit of solder and you got it held in place.  Again, the dark spots are flux residue.  I assure you there are good solid solder joints there.  When I'm all finished I use a remover to clean it up.  Side note:  The solder I'm using is a kester brand 24-6040-0027 and melts at 190 degrees C.  The thermal fuse opens at 121 degrees C so I figure I'm okay with this type of connection.

Step 6: Add the Fuse

Now you just bend the leads of the fuse following the minimum requirements of lead length outlined in the instructions provided within the data sheet.  Insert the fuse into the crimp barrels and crimp them in.  Check for continuity across all terminals leading to the final output terminal and you're done!

Step 7: Rinse and Repeat

If you have multiple resistor packs to do, I would suggest just knocking them out while you're doing it.  It only took me a matter of 15 min from start to finish.  The longest waits are waiting for your package of thermal fuses to arrive, and waiting for your soldering iron to heat up.

Step 8: Another Shot

Just another shot of the finished product.  It may look a bit sketchy but I assure you this method works and works well.  The reason I did it this way is because I can easily desolder the old crimps and discard the old fuse.  Resoldering the replacement fuse would be a breeze.  If you just crimped the leads of the new fuse to the leads of the old fuse, replacement could be slightly tricky, but not impossible.  Personally I'm just more comfortable with soldering.  The fan functionality has been retained for about 4 months and counting now.  I can only attribute it to the higher quality thermal fuse.

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. 
<p>Good article thanks. I replaced it 3X and was starting to think like you but did not know what that part was. Is there a part number for the thermal fuse? There is one other thing that I did. I replaced the cabin air filter this time and was it a clogged mess! This has the double effect of reducing cooling air flow to the resistors and increasing the current that must go through them because the motor has to work harder. More current means more power dissipation with less cooling air flow and you have a recipe for heat rise. So check the air filter if there ever is a next time. you will breathe easier in more ways than one. Thanks again.</p>
<p>Good, helpful article. I didn't know what that thermal fuse gizmo was, either. That was the problem, thanks! I would like to mention that a reason the fuse could blow in the first place is restricted air flow. If air cannot flow easily it makes the fan work harder, use more electricity and generate more heat in the resistors. In my case, I had a rodent nest in the blower box. I recommend removing the blower assembly and checking for restrictions, or your new fuse just might go again.</p>
<p>Hi All,</p><p>I have wagon r 2009 model lxi. Whenver i run it around 20Km and in case if it stops or i remove key and start again, engine feels very hard and either hardly starts or dont starts. My battery is new. what could be the issue? even mechanic failed to find same. It cranks too slow and doesn't start, if i drive it a bit longer.</p><p>https://www.youtube.com/watch?v=irgvh_UJHMo</p>
So i eotk in a shop. We have had issues chassing these types of things. A bad resistor can cook a fan and a bad fan can cook a resistor. At done point you chase back and forth until you use high tech (and a pain to use) equipment to prove a failing fan ehen resistor is shot or you guess and do both. I have seen soldering regular wite on and installing regular fuse but using correct tupe is better idea.
<p>Very good job. Thankyou. Am going to do this. $30. for my resistor assy. vs. $1.50!!</p>
<p>Thanks! Glad you liked it! When you get yours done you should post an &quot;I Made it&quot; comment back here so I can see it. I should edit this Instructable and add a section that says &quot;Update - It still works so far.&quot; I keep asking my friend every so often if it still works and he says it works great! I believe I did the mod in Jan 2014. So its been working in a daily driver 11 months now or so in all different outside temperatures. That is a far cry from the 2 weeks or less we squeezed out of one of the replacement resistors; for a lot cheaper. I have a feeling it will keep working for a long time too because of a better quality thermal fuse. If you have any problems feel free to let me know! Good luck!</p>
<p>Nice to know!</p>

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