Diagnosis and Part Selection Advice:
How did you know this part needed replacement? Why did you choose a certain brand/part number?
You know you need a new blower motor resistor when your heating/AC fan works on its highest setting only. The blower motor resistor is actually a resistor network, usually made up of two to four resistors. It achieves the lower fan speeds by introducing more resistance into the circuit supplying voltage to the fan blower, making it rotate slower. This is also why heat builds up in the resistor network, and over time causes failure. It features multiple pins, in the case of my car five pins, to get the voltage to flow through more or fewer resistors, depending on the fan switch setting. This helps to identify what you're looking for while your back is folded in half and your head is up inside your dashboard.
I chose the genuine AC/Delco part because it was least expensive, and I had more confidence it would be the direct replacement for my Oldsmobile with no compatibility issues. However, I've heard that some of the aftermarket parts are actually more reliable, so use your own judgment.
What came in the box? Were additional nuts, bolts, sealant, etc. needed for the repair?
The box contained the resistor itself, nothing more. It is already mounted on a connector (for my car, male) that should interface perfectly with your wiring harness (female for my car). Disconnect the old one, snap the new one on...it should be that simple. (See the next section before you misconstrue "simple" to mean "straightforward".) Some people also caution that you should clean the connector on the wiring harness as best you can...moisture buildup can cause a bit of corrosion over time...and use a bit of dielectric grease when you connect the new part to prevent moisture at this joint. You can get dielectric grease at the auto parts store easily and cheaply enough, but my resistor worked when I connected it to test it...so I didn't disconnect it again to apply the goo, I just skipped this step.
Ok, that's the electrical connection...as for mounting it, you typically have two screws. Two come out, replace the part, two go back in. No additional parts provided or needed, as long as you don't lose your screws. Again, simple doesn't always mean easy. This part mounts inside the cabin at the firewall, very near the blower motor, so that the air flow from the motor itself can provide some kind of cooling for the resistor. (Remember when we said above this part will get hot? If the electricity isn't going to be passed on to the motor itself to spin the fan at top speed, it's going to build up in the resistor network as heat.) The resistors are mounted on a small plate that probably acts to some degree as a heat sink, in addition to holding the resistors and circuit traces themselves, but having it next to the air circulation from the blower also helps keep it cool.
I'm told some older model cars have the resistor mounted in the engine compartment on the firewall and water cooled, but I have no personal knowledge of this. My Oldsmobile Alero (a GM N-body product, see also Pontiac Grand Am) uses the air cooling method just described.
Repair / Installation tips:
Special tools needed? Have to remove other parts to reach this one? Any left-hand threads, sharp edges, messy fluids or other pitfalls to avoid?
Clear your evening. If lightning strikes and you get done in time to watch the "Talladega Nights" DVD for the 8th time or "Trick My Truck" on CMT, so much the better...but plan on a couple or three hours at the least. If it's the coldest week of winter, which it was when mine failed, do this in the garage with a space heater nearby. Move the other car out, you'll need plenty of room. You'll be poking your head up underneath the dashboard on the passenger side of the car, so it is helpful if you remove the passenger seat. I didn't, but it's a real backache if you don't, and I'm told it's easy enough to do with about four bolts and five minutes. I wish I'd known that before I started. Good idea not to have the car running, by the way. A lot of folks advise disconnecting at least one of the battery terminals also whenever you're doing electrical work on the car.
You need your socket set. A ratchet handle with a U-joint at the drive is helpful. Have a couple extensions and your socket adapter handy. A screwdriver-style handle for the sockets helps too. Get your shortest screwdriver with a blade (minus) tip, not phillips (plus). Besides this, bring whatever drink you prefer to keep you warm or cool, depending on the season, and your most colorful language. (It is NOT recommended to attempt this repair with small children milling about the area.)
Dashboards vary of course, but to get to my blower resistor you must take off the modesty panel that hides all the wiring. The right side of this panel (to the right of the courtesy lamp) just swings out of the way on a plastic hinge, after you untuck it from the other side. It does not come all the way off, but it doesn't need to. After this, the other (larger) section removes with three screws. (This panel must be reinstalled correctly, because molded into it was part of the ductwork that routes air up to the defrost vent on that side. Make note of how it fits before you take it out.) Lay all this carefully aside where you can find them later, especially the screws. (I threw the panel in the back seat, and put the screws right underneath where I was laying so I could feel that they were still there.)
Once this is out of the way, the real fun begins. The blower motor is behind your glove box, partially obstructed by an electronic relay box. (Someone told me this box makes your power locks and windows work, but I have no idea.) The blower motor relay is behind the blower motor, jammed up against the firewall. Disconnect the wiring harness from the resistor and the motor; tuck it out of the way. You should not have to take the blower motor out to get to the resistor, but if it's in your way, go ahead. Be careful to move your head out from under the dash...the blower motor is heavy, and if it falls on your forehead, you'll have an embarrassing mark to explain to people for about a month. (Actually, you'd have to remove the relay box for it to fall on you, never fear.) The resistor mounts with two screws. Take these out.
Sounds simple enough, but this takes you about an hour, because you've discovered the clips that keep the wiring harness mated to the connectors on the components (this is why you needed the short screwdriver, to pry these off...hopefully without breaking them in the process, like I did). And, you've now realized that it is impossible to get a tool close enough to the bolts to do you much good. After much tugging and grunting, budging things the best I could with the socket attached to alternately the ratchet handle and the screwdriver handle, I finally got mine out by using just the socket itself connected to the adapter. Holding only these in my bare hand, I finally unscrewed the bolt next to the firewall (one was closer to the wall than the other). It was during this step that the colorful language would have really helped, but my daughter's grandpa was in the garage and he's a deacon.
They say that there will be burn or scorch marks on the resistor if it's bad. Mine didn't have these marks, but more about that later. (There's some debate that my resistor was the problem, but let's get yours installed first.) Anyway, look for these marks, they confirm that you're on the right track.
Put the new resistor in place and reinstall the two screws. This should go quicker than taking them out. If you can only tighten them hand tight, but you can't wiggle the thing once it's in, that is sufficient. I was screwing into plastic mounts on my car, and you don't want to overtighten here anyway. Crack those and you've got an even bigger problem. You go that far, and these instructions won't help you...it's on to improvising with a big bottle of Gorilla Glue. Reconnect the wiring harness to the resistor and motor. That part goes MUCH quicker; snap snap. (Unless you're going to clean the connectors and do the dielectric grease step...if so, now is the time, before you reconnect.)
If you disconnected the battery, or pulled fuses to your HVAC circuits...whatever safety precautions you felt necessary...put all that back together and test your fan speed settings. If they all work, put the modesty panel back in carefully, swing the other side into place and reinsert the little tab that keeps it tucked under, and you're done! Congratulations. You have something to brag about come poker night. But if this doesn't work...
Consider that it may have been a fuse to begin with. On my car, the fuses are found in three places: On either side of the dash board, inside a little panel on either side only accessible if the door on that side is open; and a block of fuses under the hood. In the fusebox by the passenger door, there's a 20A fuse for HVAC. Under the hood, there's a 30A fuse and a relay. Do all these test out as good? Chances are they do if your presenting symptom was high speed blower but no lower speeds. Moving on...
Could be the fan switch itself. Internet research indicates that it's not uncommon for the switch to go bad, although I've not had that problem on this car. (It did happen on my '93 Plymouth, a long time ago. Can't give you instructions for a fix on that, it was under warranty and I hauled it back to the dealer, to give his service department that particular headache.)
My presenting problem was that all blower speeds worked intermittently (no speed was totally dead), but it was much slower than normal at all speeds. After replacing the resistor didn't help, I had to go ahead and replace the blower motor anyway. I didn't notice scorch marks on the old resistor when it came out, but I still installed the new one, since I had it in my hand. I wonder now if the old resistor is still ok...but after the instructions I gave you on switching them, I didn't reinstall it to find out. (Do you blame me???)
Anyway, with a little careful diagnosis to begin with, I should have known I needed a motor, not a resistor. However, two fun-filled, knuckle-busting weeks and about $80 later counting parts and tools, I have heat in the car again. On these four-degree days, it still all seems worth it. Happy repairing!!