Heater Core Replacement

New and old heater cores.

Need To Get Warm Again?

Over time, most CJs will need to have their heating system overhauled.  This page will give you a peek at what's inside the "black box" of the CJ's heater system, and shows how to replace the heater core - which is not so conveniently located.   The project is fairly straight-forward and should provide you with a little more warmth!

How It All Works...

The Jeep's air vent and heater system is rather simple.  Fresh air comes into the Jeep system through the vent on the cowling in front of the windshield.  Once inside the cowling intake, the air goes through one of two separate channels: the driver's side fresh air intake or the main air intake. 

Any water that comes into the cowling intake will drain through the hole at the very bottom of the intake and exits through the engine side of the lower firewall.  If you look inside the engine bay, you'll see a large diameter rubber hose pointing down.   Sometimes, this hose has a folded end which sort of serves as a one-way valve - if it's still on the end of the hose!

Fresh Air Circuit.  The fresh air circuit works for both the driver and passenger.  The driver's side fresh air intake is on the left side of the cowling intake and is covered by a small flap, which is normally closed.  When the flap is opened (AIR knob pulled all the way out  - Vent, second step), fresh air goes through this opening and directly enters the cab below the steering wheel. 

View looking down the heater duct main intake, with the passenger side fresh air flap open.

Fresh air also goes via the main air intake into the passenger side of the Jeep and then out the opening above the fan motor in the heater intake.  The flap above the fan motor opens inward and directs most of the flow out before it reaches the fan motor.

Heat Circuit.  The heat circuit has several variations ranging in temperature and also includes the window defrost/defogger system.  

Heater duct intake, open-view.

This circuit starts by collecting fresh air through the main air intake and directing all of the air over to the passenger side of the Jeep.  In this case, all the air flows past the fan motor.

View of the fan-motor and rear duct drain.

Below the fan motor is another small drain on the lower back side of the heater duct work assembly.  This works as a secondary drain (for the main air intake) or for any coolant that may become present in the system (due to an unexpected failure of the heater core).

View of air-flow through Normal mode.

In the normal setting (with all the dash knobs in) the heat flap is closed (located below the heater core, next to the nut-driver with the yellow handle).   The air passes through the fan blades, under the heater core, and out to the vent into the cab (on the lower-right side of this photo).

View of Heater flap all the way open.

With the heater setting (HEAT knob pulled all the way out) the heat flap is all the way open directing all the air flow through the heater core.  Adjusting the amount the HEAT knob is pulled out adjusts the amount of air that flows either above the flap (through the heater core) or below the flap.

View of Defrost setting.

In the defroster setting (HEAT knob pulled out, and DEFROST knob also pulled out), you can see that the second flap (right side of photo) drops down and re-directs all of the heated air upwards through another duct that connects with the windshield frame vents.  Not having the HEAT knob out will only direct cooler fresh air up to the windshield.

Replacing The Heater Core...

Because there are several different versions of heating systems used in the CJs, most after-market repair manuals kind of gloss over the particulars of replacing the heater core.  They do provide an exploding diagram of the heater duct work, and that view can come in handy when trying to assemble all the parts once you've removed it from the Jeep.  Here are a few more details that may help you take on this chore.

Step 1

According to the repair manual, the first step is to disconnect the negative battery cable so you don't run the risk of shorting out something.   Since you will be working under the hood, this is possible.  However, the only electrical part associated with this project is the heater fan motor.

Step 2 The next step is to drain the top part of your cooling system.  You don't need to drain the whole system as the heater hoses run off the top of the overall system.  About 2-quarts is all that the repair manuals recommended.  However, I found the quick way to do this was to have an empty coolant jug on hand and once the heater hoses were disconnected (Step 3) at the fire wall, I just let the coolant drain from there into the jug.
Location of the two heater-hoses at the firewall (near the battery). This somewhat fuzzy photo shows where the two heater hoses are located on the firewall.  If you also note (very carefully) you can barely see a small nut just above the top hose... remember this one in Step 6!
Step 4 Back inside the cab, remove the looped ends of the three heater control cables from the rods on the heater duct work.  Two are right in front, the third is over by the passenger side between the intake duct and the body of the Jeep.  There are little metal-toothed washers that secure these looped ends to the rods, be careful not to lose them.  Also remove the heater vent deflector (located above the center hump) and the flexible defroster duct (on the top).
View of removed intake duct. Now remove the intake duct (located on the passenger side) that covers the fan.  This unit is held to the front of the heater duct assembly with sheet-metal screws.  It is very possible that you'll tear the rubber gasket on the top side of this unit (mine was glued down).  If so, it is not a big deal.  Just be careful not to thrash this gasket any more than you have to!
Backside view of heater duct assembly. Now you're ready to remove the heater duct assembly.  The back of the heater duct assembly has four threaded studs that extend through the firewall.  Each stud is held with a small nut (which are all the same size and are located in the engine bay).  You'll also need to detach the heater motor components (the electrical plug fitting and the small drain hose).  The four nuts are located as follows:

#1 behind the engine valve cover, #2 above the two heater hoses, #3 behind the battery (in the upper corner), and #4 below the heater motor and to the right of the small drain.

Front view of removed heater duct assembly. Pulling the duct assembly out can be kind of tricky.  Because the four studs are extending through the firewall, and because the back of the motor also extends through the firewall, the whole unit needs to be pulled slightly forward first, then down.
Removal of the back cover. Now the back side of the assembly can be removed.  First, remove the fan motor assembly.  There are several small screws that hold the backing plate to the plastic duct.  The first time I replaced my core, I discovered some of the screws were rusted or broken.  This will be a good time to replace any that need to be.   Two similar screws also hold the heater core to the backing plate and need to be removed.  Once all the screws are removed, pry the back cover off the assembly.
Step 9 Once everything is disassembled, all the parts can be cleaned.   Compressed air works good to remove all the dust that accumulates inside this unit.   Also take note of how the two internal flaps work (see details above) and how their rubber seals fit inside.
Silicon the back cover! Now you can reassemble the duct unit.  Place the new heater core into the plastic duct and note that the flap seal is correctly in place.  I recommend placing a silicon bead along the lower edges of the duct unit (look closely at the photo).   Figure if the heater core springs a leak, where will all that coolant go?
Setting the defroster-flap pin through the back. Start attaching the back cover onto the plastic duct unit.  It is easiest to start from the small end and work towards the larger side.  Make sure the defroster-flap pin is through the hole on the back (next to my finger).
Checking the heater-flap pin is through the back. Also make sure the heater-flap pin is through the back.  Once both pins are through the back, and the back is centered on the plastic duct unit, all of the screws can be installed.  You may want to attach the two screws that hold the heater core to the back first - to make sure it doesn't move during this assembly step.   Also, check that the silicon has formed a complete seal on the lower edge and clean-off any excess silicon.  Double-check that the flaps still work by moving the control arms.
Step 13 The fan motor can now be attached to the backing plate.  Note the motor mounting holes can be attached in a variety of ways.  The correct way is with the electric plug on the lower right side (as viewed from the back - see photo in Step 6 again).  This will help tremendously when trying to fit the duct assembly back under the dash!
Step 14 The rest of the assembly is just a backwards repeat of Step 6 through Step 1.  Don't forget to attach all the nuts on the firewall, and the looped cable ends.   Also, be careful when attaching the heater hose ends to the brass heater core ends!   Excessive force of pressing the hoses onto these ends can break the solder, causing you to have to start all over again.  Once everything is attached, top off the coolant level and reconnect the negative battery cable (if you removed it).

Helpful Tips!

Helpful toolbox item:  a 5-8ths-inch heater hose bypass. If you don't think you're going to be needing to replace your heater core anytime soon, do yourself a favor the next time you're in the auto parts store and pick up one of these plastic 5/8ths-inch bypass fittings and a couple of hose clamps.  This way, if your heater core does later form a leak, you can remove the two hose ends from the core (at the firewall) and connect them together with this plastic by-pass fitting.   Also, in the event one of your heater hoses gets a hole in it, you can cut the hose in half at the hole, and reconnect each end to the fitting with the hose clamps.

Outside view of windshield frame seal - a common leak source!

One of the common sources of leaks in your Jeep is from the windshield frame/cowling seal.  The outside portion of this seal has a lip that faces upwards.   The next photo below will show were the water will travel if the upward facing lip of this seal is not silicone-sealed to the frame...
View of defroster holes in cowling. Once water comes through the seal (above) it can travel down the defroster access holes in the seal (shown here with the windshield folded down).
Rotted old defroster duct. Right above your in-dash radio is the flexible defroster duct hose!   Here is what the original duct hose looked like in my CJ.  It was made from a paper-based material that had long since rotted (cracked).  Any water that makes it this far will enter the cab right above your radio...

Fuzzy shot behind the dash - showing the top of the defroster duct hose.

For less than $15 you can order a new flexible defroster duct hose which are now made from a plastic material.  If water does go this far, it should make it into the heater duct work, which has another drain down below the fan motor.