As the weather got colder the summer after my engine rebuild, I noticed that the heater in my Wagoneer wasn’t blowing very much air (even with the heater fan on high). I figured that, after 25 years, my heater box was most likely clogged with leaves and debris. Removing the blower fan from the housing in the engine bay (it’s held in place with six or so sheet metal screws) and inserting a mirror through the hole proved this to be the case. I set about to remove the blower fan and heater box from the firewall so I could clean things out.

First step is to disconnect the fan motor wires and air deflector cable from the housings. The blower fan housing is held to the firewall with two nuts on the engine side of the firewall and two nuts on the interior side of the firewall. Next, drain at least two quarts of coolant from the radiator (I drained nearly a gallon) and then disconnect the two heater hoses from the heater core ports on the top of the heater box. The hoses have different diameters, so don’t worry about confusing which one goes where.

Those interior nuts cannot be accessed with the air conditioning unit in place under the dash. This unit doesn’t need to be removed — just lowered several inches. There is one nut on a stud on the firewall that supports the back side of the A/C unit, and ten or so sheet metal screws that attach the front side of the A/C unit to the bottom of the dash. All of these must be removed so that the A/C unit can be lowered to the floor to expose the heater box and blower fan housing nuts. Note that none of the A/C hoses need to be disconnected. If your FSJ doesn’t have A/C, then count yourself lucky this time.

As viewed from under the dash, the two right-most studs/nuts belong to the blower fan housing, and the other two belong to the heater box. Two of these four studs are double-nutted, but I can’t remember now which two they are. Probably one each on the blower housing and heater box.

Once you’ve removed the two interior nuts and the two exterior nuts, the blower housing should come off the engine side of the firewall easily. The fan housing is connected to the heater box with some non-adhesive foam sealant, and it should peel away easily. The heater box is removed from the firewall the same way — two nuts on each side of the firewall. A U-joint socket extension comes in handy for the lower bolt nearest the exhaust manifold. Speaking of the exhaust, interference from the heater box makes working on the passenger side exhaust components difficult. If there’s any exhaust work you’ve been wanting to do, it’ll be much easier with the heater box is removed.

In order to properly clean the heater box, you’ll have to dismantle it. There are a dozen or more sheet metal screws that hold the outer halves together, with an adhesive sealant between them that requires a small flat-head screwdriver to pry the halves apart. Sandwiched between the halves is a flat plate to which the heater core is bolted. You should now be able to adequately clean any debris from inside the heater box. Mine was clogged with an unbelievable amount of leaves, pine needles, and maple “helicopters.” Since you’ve got it torn apart anyway, this would be a good time to have a radiator shop check the condition of your heater core. Dean’s Radiator & Driveshaft in Lincoln, NE, checked and cleaned mine and straightened the mangled hose ports for just $10.

After 25 years, the foam that coats the air deflector in my heater box had severely deteriorated and was mostly missing. This foam provides a mostly-air-tight seal when the door is positioned at either extreme, so it’s important to have it there. Removed the V-shaped baffle that holds it in place, then removed all the old foam and replaced it with strips of foam weatherstrip across both sides of the door. This door will be exposed to severe temperature extremes, so I glued the weatherstrip in place rather than trusting the adhesive that it came with.

Now would be a good time to apply a fresh coat of paint to the outside of your heater box and blower fan housing if you feel so inclined. I didn’t bother, but I’m now wishing I would have.

I used adhesive silicone caulking to reassemble the sheet metal portions of the heater box, including the V-shaped piece that holds the air deflector door in place. I also used some caulking on the firewall surface when I bolted the heater box and blower fan housing back in place. Reassembly is straightforward as long as you remember how you took everything apart. The bolts and brackets behind the A/C unit caused me the most trouble, but YMMV.

The heater blew plenty warm in -10 F temps this week, so it seems to be working well so far.

In order to keep this from happening again, I removed the grate over the vent intake on top of the cowl (between the windshield and hood) and cut a piece of screen to fit under the grate. I used screen with 1/8″ squares. This should be small enough to prevent most objects from getting into the vents, but still large enough to prevent water from freezing solid across the screen and preventing air flow.

Although I managed to get the grate off the Wagoneer with the windshield wipers in place, I had to removed them in order to reinstall the grate. Be sure to mark the wiper arm locations before you remove them so you know how to align the arms upon reassembly. The wiper arms just mount to splined shafts, so there are about two dozen positions where they could be installed.

The grate is held in place with screws along the forward edge just under the hood line and with tabs along the rear edge that insert into slots below the windshield trim. Be careful not to irreparably mangle the washer fluid squirters when you removed them from the grate. Also be sure to cut holes in your screen for these squirters. While I had the grate off, I alternately used my vacuum and air compressor to clean all the leaves out of the fresh air vents that exit into the cab by the front passengers’ ankles. I couldn’t remove the grates on those vents, but I think I got pretty much all the crud out. It helps to move the vent cables in and out while you’re doing it.

While I had the coolant drained from the radiator, decided to have a closer look at some minor leaks from various points in my original 2-core radiator. I yanked the radiator and took it to Dean’s along with the heater core. He explained how radiators were constructed and why the particular leaks in my radiator (along the rear bottom edge of the upper tank and at the base of the lower radiator hose nozzle) would be difficult to fix. He said he could band-aid the radiator back together, but it would never last as long as my newly-rebuilt engine and transmission. I opted to buy a new 3-core radiator from him for about $200. I also had Dean rotate the overflow nozzle by the radiator cap to point to the driver side so I could install an overflow bottle from my ’78 parts Wag (the radiator from that Wag was much worse off than my ’77). I debated ordering one from Radiator.Com, but this was Wednesday of Thanksgiving week, and I really didn’t want my Wag sitting immobile all weekend. I haven’t really stressed the cooling system yet, but it’s working great so far.

Until they’re sorted, some photos are available here.