YJ Heating System Explained

Chris

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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:

Wrangler%20-%20Duct.jpg


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:

Duct%20-%20off.jpg


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.

Duct%20-%20defrost.jpg


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.

Duct%20-%20heat.jpg


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.

Duct%20-%20blend%20heat.jpg


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.

Duct%20-%20cool%20air.jpg


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.
 
Brother your post just saved me about half of what hair I have left. Simply said, restore with stuck cables. AWESOME HELP !!!!!!!!
 
Great post! Thanks so much for the explanation. I'm a new YJ owner and just starting to learn what I've got so that I know how to fix it when it breaks.
 
Just found this forum!
Please advise:
My heater blows all the time. The high position doesn't change anything. It's all the same speed and constant. Any suggestions? I pulled the blower fuse but that just stops my a/c from working.
Thanks in advance.
 
You might check to make sure the cables are connected from the heater to the controller. I had a couple that were frayed and broke which made the controls basically useless.
 
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Are there any diagrams for which ports on the vacuum switch the green and black lines should be connected to? Found the black line off and the green connected to the center port. I hooked the black line up to one of the outside ports, but not sure if it the correct one. Not home right now, but could post a picture later once I get home.

Note: 88 YJ 4.2l Sahara
 
Here is a picture online I found from one of the forums. Maybe this will help. The black one has to go one one of those two outside ports I would think. Since its a vacuum it should be ok if you try it on either one. One of them should work
20170917_171726.jpg
 
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Great Post. But when I switch to defrost, is the floor vent supposed to close thereby forcing all the warm air to the screen? And if I switch to not defrost should the screen vents close 100% forcing hot air down into the floor vent? Mine comes out of both even when I move the selector from window to floor? Also, my floor vent looks like something should be attached to it? Thanks!

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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:

View attachment 115540

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:

View attachment 115541

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.

View attachment 115542

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.

View attachment 115543

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.

View attachment 115544

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.

View attachment 115545

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.
So when I switch from defrost to heat, what would cause the heat to blow only on the driver side and nothing on the passenger side floor?
 
So when I switch from defrost to heat, what would cause the heat to blow only on the driver side and nothing on the passenger side floor?
There should be heat coming from both sides, but much stronger on the driver side as that is the direction the air naturally flows coming from the box and the splitter's duct for driver is a lot larger than passenger. The duct that routes air to driver and passenger is just a bolt on piece that hooks in with one screw. It should be fairly simple to evaluate what's wrong.
 
SOOO, as I am working on my new 1999 jeep tj and adding mods and new features... as an idiot I dropped a Bezel screw down the vent thinking no problem I will just grab it with a magnetic stick.. we being hot, tired, frustrated and grumpy I dropped the 2 1/2 ft magnetic stick down it as well.. Guess its in there forever now? no way to get it out? and of so HOW? Everything still works great but will it develop problems later down the line?? any advise is welcome.. telling me what a kucklehead I am is welcome as well.. ugh
 
SOOO, as I am working on my new 1999 jeep tj and adding mods and new features... as an idiot I dropped a Bezel screw down the vent thinking no problem I will just grab it with a magnetic stick.. we being hot, tired, frustrated and grumpy I dropped the 2 1/2 ft magnetic stick down it as well.. Guess its in there forever now? no way to get it out? and of so HOW? Everything still works great but will it develop problems later down the line?? any advise is welcome.. telling me what a kucklehead I am is welcome as well.. ugh
You're on the YJ forum, you'll want to migrate over to the TJ forum to get more details about maybe getting the screw and magnet pulled back out of the vent.
 
There should be heat coming from both sides, but much stronger on the driver side as that is the direction the air naturally flows coming from the box and the splitter's duct for driver is a lot larger than passenger. The duct that routes air to driver and passenger is just a bolt on piece that hooks in with one screw. It should be fairly simple to evaluate what's wrong.
Late in on this one. I had the same issue with questionable floor flow and only after folks helped me in here figure out a relates cable connection, did I find out that there is supposed to be a plastic flow box screwed to the bottom of the air box which directs air to the driver footwell and passenger footwell. It's biased to the driver side by design, but once that was purchased and fitted, it made a huge difference to the footwell heat and was more channelled and powerful. This forum is awesome for tips like that.