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Old 11-11-2010, 11:38 PM   #1
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YJ Heating System Explained

I get asked a lot of questions about the YJ heating system and I've noticed a lot of posts on the forums about heater related issues. In fact, most people know very little about what exactly goes on inside the heating plenum when they slide those dash levers for heat or fresh air.
And because of this, diagnosing their own heater related problems can be difficult. So I've written up a tutorial on the basic heating and air functions of the Wrangler YJ.
So for those of you who have never been inside the plenum and want to know what's going on in there, read on:

Here is the basic layout of the YJ heating and fresh air ducts. For the most part the entire system is cable controlled except for the right side Cowl Gate which is operated by a vacuum motor. The vacuum is supplied by the engine and is switched automatically by the dash control panel. All other functions of the system are operated by cables controlled by the dash control panel. Each cable is directly connected to, and controls one function. All of the cables (except the Blend Gate cable) operate in one of two positions at all times to either open or close their respective gates.
The Blend Gate cable is the only multi-position cable and is used to mix warm and cold air within the plenum. (More on that later)
The blower motor is located directly behind and below the passenger vent (as shown) and pulls air directly from the cowl vent at all times while in operation. The blower motor operates only when the dash control is in the Defrost and Heat positions. It's speed is directly controlled by the fan speed switch on the dash control panel.
The Heater Core is located just to the left of the Blower Motor and has engine coolant circulating through it at all times while the engine is running.

Now lets take a look at airflow:

Here we have a picture of the heating system in the "Off" position. Notice the position of the dash control sliders. In this position, all Gates are closed, and the Blower Motor is not running. The Heater Core is always hot while the engine is running but no air is moving through the system.


Now we'll take a look at Defrost. In this position, vacuum is supplied to the vacuum motor which opens the right Cowl Gate.
This allows fresh air from outside to enter the system through the Cowl Vent and travel down to the blower motor. The passenger Vent Gate is closed directing the air towards the heater core. Notice the temperature slider has been moved over to the warm setting. This fully opens the Blend Gate allowing air to pass through the Heater core. The upper slider in the Defrost position opens the Defroster Gate directing the warm air upwards to the windshield dash vents. In this position the blower motor is running.


Moving the slider to the Heat position changes only one thing. The Defrost Gate closes directing air downward and out through the floor vents located above the transmission hump. The passenger vent gate is still closed and the blower motor is still running.


Moving the temperature slider to the center position moves the Blend Gate exactly halfway. This splits the cold airflow directing half through the heater core and bypassing the other half under the core. The bypassed cold air and the warm air from the heater core remix later and become moderate before being directed out of the system by the Defrost Gate.


Moving the slider into the Vent position does a number of things all at once. It shuts off the blower motor. It opens the passenger Vent Gate for the first time. And it opens the left Cowl Gate for the first time. The Defroster Gate remains unchanged and the Blend Gate (although it can be moved) serves no purpose since there is no airflow past the passenger Vent Gate. When this gate opens it blocks 99% of the airflow into the blower motor and the rest of the system.
The left Cowl Gate (the most difficult to see since it's deep within the dash above and behind the speedometer) allows fresh air directly into the cab bypassing the entire system.
It should also be noted that the right Cowl Gate defaults to the open position when the vacuum motor, which controls it, looses vacuum. This happens hours after the engine is shut down regardless of the dash slider positions. This typically is not a problem even in the rain as long as the Cowl drain tube remains clear. Should the drain tube become clogged, the Cowl can partially fill with water which can seep past both Cowl Gates. The Cowl drain tube is located within the engine compartment to the right of the battery.

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Old 11-12-2010, 12:20 AM   #2
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so why does my YJ's heating system sometimes blow cold air (mostly at higher speeds like the highway) when my heater is on and working well ... its like sudden moments of cold air, seemingly coming out of the Vent.. It then diminishes the heat in the cab, bit annoying for my passengers I might add.

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Old 11-12-2010, 10:42 AM   #3
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Awesome write up i might add.
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Old 11-12-2010, 11:02 AM   #4
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the vent door may not be all the way closed. they tend to hang up. i closed mine by hand to eliminate the same thing you just described.
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Old 11-12-2010, 11:58 PM   #5
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Quote:
Originally Posted by Nsoares3 View Post
so why does my YJ's heating system sometimes blow cold air (mostly at higher speeds like the highway) when my heater is on and working well ... its like sudden moments of cold air, seemingly coming out of the Vent.. It then diminishes the heat in the cab, bit annoying for my passengers I might add.
Your heat comes from the fluid circulating from the pump through the engine to the heater core. Going down the highway in cool weather the thermostat will stay closed until the fluid in the engine warms beyond the 195 degrees and opens the thermostat. When the thermostat opens it lets cold fluid from the radiator into the system and through your heater core and your heat drops off. The colder the weather the more dramatic the drop in heat. If you have the properly working thermostat and your cooling system is operating correctly you can try to block off some of the air going to the radiator when its really cold out and it may help regulate the system better.
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Old 11-13-2010, 12:33 AM   #6
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In the heat position, air blows out the floor vents and the defrost vents.....at least mine does.

Great write up by the way.
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Old 11-13-2010, 04:57 AM   #7
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This helped a lot!
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Old 11-13-2010, 05:58 AM   #8
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Nice writeup! Very helpful.
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Old 11-13-2010, 07:08 AM   #9
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So when u put it on vent the door you can see under the passanger side dash is supposed to open? Mine doesnt. I have to open that and close it by hand.
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Old 11-13-2010, 07:29 AM   #10
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Awesome explanation! I just posted a question about the R cowl gate about a week ago and didn't get any solid answers. Thank you so much!
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Old 11-13-2010, 07:45 AM   #11
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beaver, yes it should.
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Old 11-15-2010, 07:42 PM   #12
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great writeup!!
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Old 12-01-2010, 12:38 AM   #13
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so good write up, but i have a question, how "loud" is the heater when on low. because i cant hear/feel any air but when i put it on high its like an airplane. Is this situation normal?
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Old 12-01-2010, 02:59 AM   #14
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Stick it please.
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Old 12-01-2010, 03:42 PM   #15
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Let me ask, on the 92 4.0 does the heater control panel in the dash have a light? Is it supposed to be illuminated? Mine isnt and it drives me crazy. I am planning to put a light on it if it does not have one. Someone let me know.
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Old 12-01-2010, 03:51 PM   #16
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its supposed to
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Old 12-01-2010, 03:58 PM   #17
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ok, do you know where the bulb is located? little hard to each underneath since the previous owner has obstructions as in cig lighter box and what not.
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Old 12-01-2010, 04:01 PM   #18
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Umm not off hand, I will try to look on one of the controls tonight when I get home
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Old 12-01-2010, 04:09 PM   #19
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thanks. I was just back there as far as i could go with a mirror and a pen light. I did not see any placement for a bulb housing. I even jammed the pen light in the back and pointed forward just to see if the signs lit up. nothing.
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Old 12-01-2010, 04:14 PM   #20
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Hmmmm, odd. I will find out asap and letcha know
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Old 12-01-2010, 04:43 PM   #21
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Well i just took apart the dash since i had time.....NO LIGHT!! not even a socket hole or anything. WTF!.....i am going to have to make one, or just continue to use the controls blinded at night. I have grown accustomed thus far.
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Old 12-01-2010, 04:51 PM   #22
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Mine has a light and light socket area right behind the right side between the 2 levers

the bulb is burned out... but its there
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Old 12-01-2010, 05:25 PM   #23
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Very right side of the assembly there should be a half inch hole with a twist in socket
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Old 12-01-2010, 10:08 PM   #24
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i am telling you guys. I removed the complete assembly. There is no hole for a light, less it is an aftermarket piece. but i am positive!!!!
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Old 12-02-2010, 08:56 AM   #25
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I looked at 3 in dashs I have off last night and they all have a hole on the right side...one of them is broken off a lil bit, but its there. I will try to get a pic so you can see where its at.
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Old 12-02-2010, 10:28 AM   #26
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That was a very awsome description of the heating system with good explainations but I have a question exclusively about the islander model with the 4.2l . I seem to have only gotten warm heat at any time of year and even on high my motor sounds like it's on low compared to most other vehicles. I'm almost positive it doesn't have a clogged heater core and am kinda stumped. Is it directly related to the motor running so slow or were islanders made for warmer climates. I live in new Hampshire
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Old 12-02-2010, 10:43 AM   #27
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I have no light or socket on my 95 either. Could there be two different types of dashes?
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Old 12-02-2010, 04:20 PM   #28
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^ see!! lol....i knew i was not the only person with this issue.
I am debating to install a small blue led on the side facing toward the controls to match my radio or not. Not sure yet. I am somewhat lazy to do so these days. Plus had damn squirrels living in the shops attic that i had to fight and get rid of all day today amongst work. Im beat!
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Old 12-02-2010, 09:44 PM   #29
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Ok, first great write up...
My question is does anyone know where i can pick up either the cable that controls the temp and for the vent / floor / defrost?
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Old 12-03-2010, 12:12 AM   #30
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Great writeup and pics!! Thanks!
I just finished working on mine, I added a bolt through the 'vent flap' to keep it closed during the winter weather, it just will not stay closed.
I also worked on the tubing that connects the upper defrost assembly. It was missing, my son forgot to put it in when he installed my new heater core that I removed.
I hope that is all I will need to do to it this winter
BTW I also have the lite on the right side of the controls on my '87.

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