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Jeep XJ/MJ Heater Core Replacement

Posted in How To: Engine on October 18, 2016
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If you haven’t figured it out yet, we’re about to cover the replacement of a heater core. Our subject is a ’89 Jeep MJ Comanche, but the steps are basically the same for an XJ Cherokee of similar vintage. This Comanche had a rough life, and it was time to bring it back to its former glory. The heater core was shot, but as Jeep stopped making the original part some time ago, Crown Automotive had stepped up to the plate and made a replacement. Crown has been building quality replacement parts for Jeep vehicles for decades, even going so far as to use OE part numbers to make finding and purchasing them even easier.

A minor amount of knuckle skin was shed, and a few not-safe-for-work words were said, but we got through it. While doing so, we also attempted some of the so-called “quick fix” ways to replace the heater core in an XJ or MJ. Often, the quick fixes drew us in because we wanted to get back on the trail. Unfortunately, though, some short cuts can lead to more garage time than if it was done by the book. Here’s what we learned.

First, we attempted to take the box apart without removing the dash at all. All of the HVAC boxes used in the XJ and MJ Jeeps are two piece, so internal parts can be replaced, but some are a allegedly a design that allows you to remove the bottom portion of the box without taking the dash assembly out. We learned this was a no-go. Next, we tried doing a partial pull of the dash, unbolting it, leaving most wires connected, and positioning it out of the way so the heater box could be removed worked with some finagling, but this took longer than having taken the few extra steps to completely remove the entire dash.

As tedious as it was, pulling the dash was not that difficult. Lots of small hardware held it in, lots of wires snake through it, but it is entirely doable. The process took about four hours, with a short lunch break in the middle. Follow along as we go through the installation of the new heater core.

Once you do get the HVAC box pulled, separated from the dash assembly, and you have removed it from the vehicle, we suggest you place it on a steady surface to work on it.
Due to the leaking heater core, the hoses were previously removed and connected together. There was no more leak inside the cab, but that also meant no more heat inside the cab. Drain the coolant and dispose of it properly.
If your vehicle has air conditioning, the Freon needs to be professionally evacuated. Most shops with A/C repair experience can do this for a nominal fee. Use a flare wrench on the nut to avoid deforming or destroying it. Having a large wrench to pull against on the block helps tremendously.
Removing the dash is not difficult, just tedious. Take your time and document where things go and everything will work out fine. It will seem like there are a million screws, nuts, and bolts. Be sure to save them all. You will have to disassemble and remove the instrument cluster, switch panels, headlight switch, clock, radio, HVAC control panel, and the 12V outlet in the process. An extra set of hands is helpful when removing it, and make sure to unplug and pull the wires through the dash first.
With the dash out of the way, the HVAC unit is now in full view. Set the wiring aside and remove the defrost vent ducting.
The front of the center console is attached to the HVAC box and must be removed. There is no shortcut for this.
Pull the shifter boot by pushing on the plastic ring inside of it and leveraging it up. Automatic models have a specific notch to pry up on. Also, pull the bezel around the transfer case shifter, if equipped.
Remove the rear two screws under the console lid. The emergency brake on a Comanche is on the floor. On Cherokees you will need to wiggle around the handle in the center.
Near the shifters are screws that hold the console to the floor. Inside the console are two more, as well as two below the emergency brake handle area. Once those are all removed, the console can be removed from the vehicle.
The HVAC box is held to the firewall with five studs coming from the box. Three come through around the blower motor and two are behind the cylinder head.
A common issue with the studs from the HVAC is that they spin while trying to remove the nut. Using ratchet wrenches and a pair of locking pliers on the end to pull against helps.
The two HVAC studs behind the cylinder head can be difficult to reach. Use extensions and swivel sockets to remove the nuts.
Have a friend help hold the backside of the box while removing it to make sure the A/C block and the studs do not hang up on the firewall.
The entire HVAC box is now free and clear and can be removed.
Years of use in California and Nevada deserts let silt and dirt everywhere behind and in the HVAC box. Now is the time to use an air hose and blow it all out.
Disconnect the vacuum hoses from the actuators, and remove the metal rings holding the actuator levers to the flap rods. If done carefully, the metal rings can be reused.
Remove what seems like 100 (well, maybe not that many) screws holding the top panel to the main body of the box. There are screws both on top and on the side.
If the heater core has never been replaced before, there will be a band wrapped around the unit. Some of these bands are plastic; some are metal. Cut it and then remove the top panel. There are alignment posts and a metal tab, so working this top panel off takes some finagling.
The heater core is now in full view. Partially pull the firewall gasket off and the heater core tubes out.
Inside the bottom of the HVAC box, the heater core has a tab with a screw holding it down. Near the end of the tubes, there is a screw holding the core to the box as well.
Our heater core has a rubber gasket around it that had deteriorated. The gasket was not reused during install.
Always compare new to old parts. The new Crown Automotive heater core (PN 56000049) was modeled after the larger factory unit that our Jeep did not have. The larger capacity core should help increase the heater system output.
The new Crown unit had the correct tabs to bolt directly into the HVAC box, in exactly the same location as the old core.
When putting the top lid back on, make sure the fresh air vent flap is properly seated.
Fit the lid back onto the box. Extra hands from a friend made this much easier. Portions of the lid may need to be partially and temporarily deformed to fit it back in the exact right spot. Reinstall all the screws removed.
Reinstall the actuator levers to the rods (we were able to reuse the metal retainer rings). Hook up all the vacuum hoses on the actuators, too.
Now is a good time to clean up the threads of the studs on the box. A dab of silicone was used when rethreading the studs to the box to help make sure they stay in place.
Reinstall the box to the firewall and attach the vertical support. Having a friend on the opposite side helps to align the box and work around any hang-ups.
Attach the heater hoses. On this install, the original heater valve was removed. The valve is a common leak and failure point, though bypassing it does send warm coolant to the heater core constantly. The flaps inside the HVAC box are relied upon to keep that warm air from entering the cab when the heater is not turned on. Fill the system with coolant and check for leaks. Better to find leaks now than after you have reinstalled the dash.
Reinstall the dash in full reverse of disassembly. Dealing with the dash is the hardest part of the entire process. As long as you take notes (shooting some disassembly photos as you go too) of where everything goes, it will all come back together fairly easy.

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