One of the second-tier issues with Louie, the 1972 2002tii that I bought last February in Louisville and road-tripped home, was the heater box. It was as non-functional as a heater box could be and still occupy physical space in the car: The heater core leaked, the blower fan was dead, the defroster vent was stuck in the off position, and the footwell vent was stuck in the on position. It was about as useful as a cinder block.

    First, a shout-out to my Facebook friend Delia Wolfe. Delia and I have an online relationship characterized by good-natured but slightly antagonistic ribbing. Last summer, Delia posted that she was about to have to rebuild the heater box in a customer’s car. I breezily chimed in that doing a heater box wasn’t really that big of a deal. Then, while I was still in Louisville sorting out Louie and discovered its leaking heater core, I posted with some degree of stridency that I was not going to rebuild the heater box as part of the car’s on-site sort-out. “I thought you said it wasn’t a big deal,” Delia cheerfully reminded me. Ah, what are Facebook friends for? :^)

    I kid. Both are right; it isn’t really that big of a deal (unless you have an air-conditioned car), but it is a pain. There’s a very thorough heater-box how-to on bmw2002faq, so I’m not going to go over every point in a blow-by-blow fashion; I’ll just list some of the things that stood out.

    The heater box in a 2002 is pretty simple. Antifreeze flow to the heater core is enabled or disabled via a cable-actuated cut-off valve. The valve is, mercifully, on the outside of the box; it is one of the few things than can be changed without removing the box from the car. In contrast, although the blower motor can be seen from under the windshield cowl, in order to replace it, the box has to come out and be opened up. Cable-actuated flaps open and close the flow of fresh air through the box and channel the air to the defroster vents, to the footwells, or to both.

    Common problems include the blower motor dying, the foam crumbling off the flaps and creating constant air leaks even in the closed positon (cold in the winter, and hot air working against the air conditioning in the summer), the flaps jamming up so they don’t work at all, the Bowden cables breaking, and rodent intrusion.

    Ah, yes: our little furry friends. The mice must think the footwell vents and defroster tubes are tailor-made for them, like those gerbil runs you see in pet stores. Mice crawl through these, get inside the heater box, and set up shop for long-term occupancy, inviting their rodent friends, having rodent raves, the whole shebang. If you’re lucky, the evidence is just leaves and acorns; if you’re less than lucky, it’s urine and turds. If you’ve got a negative karmic balance, there are bodies whose olfactory funk can be off the charts.

    But even without bodies, the smell can be truly astonishing. Thirty years ago I had a ’74 200tii. I bought it in the dead of winter up in Ipswich, Massachusetts. It was freezing when I drove it home. I turned on the heat, and the smell that came out would’ve gagged a maggot. “Oh my god make it stop make it stop MAKE IT STOP!” was my immediate reaction. I shut off the heat and opened the windows until the stench dissipated. I was afraid to ever turn it back on, but later that winter, on another brutally cold day, I was freezing my butt off and thought, “how bad could it have been, really?” I turned the heat on, and—“Oh my god make it stop make it stop MAKE IT STOP!” When I finally yanked the box and took it apart, I was prepared to perform some pagan act upon the mouse bodies, and was stunned when I found only what looked like clean nesting material.

    When removing Louie’s heater box, I began seeing evidence of mouse infestation as soon as I removed the center console, so I strongly suspected the worst.

    Note that if you have an air-conditioned car, the a/c evaporator assembly is blocking the heater box, so it has to come out first. Therefore, linking the two repairs is crucial to maintaining your automotive sanity. In fact, the last heater box I did was when I was resurrecting Kugel’s air-conditioning. The evaporator assembly had to come out to clean it and replace the expansion valve, and the heater box had the traditional dead blower motor and deteriorated flap foam, so the two repairs went together like ham and eggs. Or urine and turds.

    There aren’t that many things attached to the heater box. You remove the console, under-dash trim panels, and the glovebox, remove the two heater hoses from under the windshield cowl, undo the clamps to the defroster vents, pull the wires off, pry off the faceplates at the cable levers and undo the two Phillips screws at both sides, undo the 10-mm nuts on either side of the heater box, and then pull the box out.

    But it is a bit tricky. You’d swear there’s not enough room; you wind up having to pull the box backward, then rotate it 90 degrees so that the fan is facing forward. Because this points the heater core inlet and outlet down, you need to first block them off, or the core will spill antifreeze inside the car. Fortunately, there are heater-box block-off caps sold at any auto-parts store.

    In the photo below, the box has been rotated. You can’t see the fan; it’s on the front side of the box. But you can see the heater valve on the right side of the box. Note that it’s now facing down instead of forward.

    After you rotate the box, you slide the whole thing to the right. As you’re doing this, you carefully guide the cables with their attached handles out.

    Once the box is out, you can either replace it or rebuild it. Occasionally, rebuilt ones and good used ones show up on eBay, but be aware that there are differences in both the wiring and in the cable levers (the early levers and the screw-hole spacing of their holder is different from that of the later levers that have small lights to illuminate their functions). Most people elect to rebuild the one they have, but if yours needs everything—and it probably does—if a compatible used or rebuilt one is available, it’s worth considering.

    Taking the box apart and rebuilding it is well-described in the 2002FAQ article. You first need to disconnect the heater valve and its cable, then drill out the rivets holding the lid on, at which point you can then pull the lid and front off the box. With the lid off, you can immediately see if there has been rodent infestation. Mine was laughable; although I didn’t find any bodies inside, it was probably a good thing that the box was completely non-functional. That way I couldn’t have turned it on even if I’d been desperate.

    I already knew that I had a leaking heater core, but still I put the old one in the sink and filled it with water so that I could see the leak with my own eyes. It was immediately obvious.

    I put out the word that I needed a replacement heater core. A local 2002 guy offered to trade one for a copy of my electrical book. Everyone went home happy.

    If you look on 2002FAQ, you can find people who sell a heater-box rebuild kit with grommets and pre-cut foam, but I just buy the parts from McMaster-Carr (www.mcmaster.com, the best web site in the known universe). Part number 8694K127 is a 50-foot strip of adhesive-backed 1¼" wide by 3/16" thick weather and abrasion-resistant foam for about fifteen bucks, enough to do a lifetime of 2002 heater-box flaps. Part number 9307K11 is a bag of 50 rubber grommets for a 5/16" hole, $2.67. There are a few other parts you may need as well; see the article on the FAQ.

    In order to change the grommets on the linkage for the flaps, you have to drill out the rivets holding the flap mechanism in place. You then take everything apart, clean it, put the new grommets in place, sand or Scotch-Brite all traces of the old foam off the flaps, stick on the new flap foam, lube and install everything, and trim the foam to make the flaps close as completely as possible. If you want to re-paint or re-plate the interior components so they look like new, knock yourself out, but from a functional standpoint, the flaps just need to be clean enough that the adhesive foam will stick, and the linkage mechanism just needs to move freely with minimal play.

    The fan motor deserves its own separate article. Much of the information can be found on 2002FAQ. There are early (small body) and late (large body) motors. Nowadays, the replacement parts use motors that are the same size, with the later large-body part nothing more than the small-body motor wrapped in a plastic sleeve.

    There are several issues to be aware of regarding the fan motor. The motor is available with the fan on it (part number 64 11 1 356 500 for the later large-body motor and fan), but it’s expensive that way, about $300. It’s much cheaper to buy just the motor, which, fortunately, cross-references to an evaporator-fan motor for a Porsche 911. Use Bosch part number 0130007081 for the earlier small-body one without the sleeve and 0130007002 for a later large-body one with the sleeve. It’s only about $75.

    Then you need to deal with the fan blades. The motor’s shaft has a metric 6-mm (0.236") diameter. I looked but could never find a source of fans with a metric bore, but fans with a ¼" (0.25") bore are plentiful. Years ago, I wrote an article for the FAQ about using aftermarket 0.25" fan blades. It’s cross-posted on the Bentley website here. It was an interesting problem to look at, but the more straightforward thing to do is to re-use the fan blades from the original motor. The problem is, the plastic bushing in the center of the fan blades is press-fit onto the motor’s shaft, and it’s devilishly tight.

    Fortunately, a few years back, our own Mike Self turned me on to the trick you need to know to re-use the fan blades: Sacrifice the old fan motor to save the blades. Clamp the old motor in a vise. There’s just enough room to get a hacksaw between the motor and the blades. Saw off the shaft, then balance the fan on a vise or even a ratchet socket and, using a hammer and a drift, punch the remaining part of the shaft through the plastic bushing.

    Figuring out how far to push the fan into the shaft is a matter of trial and error. If it’s not pushed down far enough, the fan blades will hit the main heater flap as it’s opened and closed. But if it’s pushed too far down, the blades will hit the underside of the heater-box lid. Basically, you want the fan pushed onto the shaft as far as it’ll go without hitting the underside of the lid. Tap it on a little at a time, test-fit the motor on the lid, and check.

    There’s been some back-and-forth over whether the polarity of the Porsche blower motor is reverse that of the original one. Jim Gerock wrote a few years back on the FAQ that the polarity is reversed on the Porsche motor, and that the leads thus need to be reversed or the motor will suck instead of blow. However, I have not found that to be the case. Be sure to check for yourself before you button things up.

    The heater core is originally wrapped in foam along the front and sides. If you want, you can look on 2002FAQ for photos of what it should look like to be “correct,” but the real issue is to make sure it’s secure and not banging around inside. I found that the same foam I was using to line the flaps, when cut along its long axis, was about the right thickness to secure the core without causing the plastic of the case to bulge.

    When most people re-assemble the heater box, they don’t re-rivet it; instead, they replace the rivets with small screws. It’s a good idea to use flat-head screws and countersink the holes so the screw heads will sit flush with the top rim of the box and not impede sealing against the underside of the firewall. Be certain to test the functionality of all cables and flaps before buttoning up the box for good. By all means, you should run the fan motor by hooking it up to a battery and put the flaps through their paces to make sure nothing hits the fan blades. The heater flap is the one in the danger zone; you’ll cry if you put it all together and install it only to have it sound like a chainsaw.

    Regarding sealing, 3M Strip Caulk (part number 08578) works well, and is more impervious to moisture than the original foam gasket.

    Some people repair cracks in the outside of the box with JB-Weld, then sand the repair and paint the box so it looks like new. Personally, I am not going to restore a heater box. Never. Ever. Nuh-uh. Not going to happen. And I have never seen an ad for a 2002 that said “restored heater box.” But if returning your heater box to that early 1970s gloss makes you happy, who am I to interfere with your dream?

    They say “assembly is the reverse of disassembly,” but it helps to see the sequence. Be sure not to wedge or bang the back of the fan or the heater valve against the inside of the firewall as you maneuver the box into position, as it’s very easy to crack the plastic.

    It’s ironic that Louie is home in Boston, and it’s spring, and now is when I have a working heater box. But even heat notwithstanding, it is awfully nice to have the flaps work to both open and close the defroster and footwell vents.

    See, Delia, no big deal, right?

    (Next week, probably the windshield gasket.)—Rob Siegel

    Rob’s first book, Memoirs of a Hack Mechanic, and his new book, The Hack Mechanic Guide to European Automotive Electrical Systems, are available through Bentley Publishers, Amazon, ECS Tuning, and Bavarian Autosport—or you can get personally inscribed copies through Rob’s website: www.robsiegel.com And stay tuned for Rob’s upcoming book, Ran When Parked: How I Resurrected A Decade-Dead 1972 BMW 2002tii And Road-Tripped It A Thousand Miles Back Home—And How You Can, Too.