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YJ Heating System Explained

301K views 276 replies 137 participants last post by  WSYJ 
#1 ·
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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#220 ·
Hi guys, I am new here (was in the facebook group), I did not read all 11 pages of posts and apologize if the question has been put up already.
so I have a 1993 wrangler YJ and my heating system works fine. when I switch in on, though the middle position or the medium blowing strength does not work, its either max or off.. should I replace the switch or is the switch and inner parts serviceable? - in Europe parts are a pain to get not like in JEEP parts mecca USA... :(
any help on this is greatly appreciated.

thanks and cheers
Martin from Hungary in Europe - because there is three "Hungary" in the USA... LOL
 
#221 · (Edited)
@phatzo

Could be the resister
 
#222 ·
blower motor resistor. its located right above the passenger vent door. 2 screws hold it in place, and there is a single plug that goes to it, 3 wires in one plug. super easy fix.
 
#224 ·
From what I understand, jeepair is/was the OE supplier to old Wranglers. I don't think you can go wrong with one of their kits as long as you buy from them and communicate directly with them when purchasing. Make sure they know the year and engine of your Jeep and that the kit you want to buy includes all the brackets, hardware and wiring if your Jeep isn't already wired for A/C.
Being in Europe, any miscommunication is ten times the headache to resolve than it is here in the US due to monster shipping charges.
If you buy the A/C system, have them put the resistor in the box that they ship the kit in. That'll at least save the shipping charge on the resistor.

ALWAYS ask for a discount.

Good Luck, L.M.
 
#225 · (Edited)
I think I can contribute some details about the Right Cowl Gate Vacuum plumbing and controls that were left out of the original write-up that the OP might want to add that I discovered while trying to troubleshoot an air flow restriction issue.

The Vacuum System opens the R Cowl Gate when the engine is running, and when the heating system is in any other position other than off.

The Vacuum line powering the heating system tees off the 4wd Vacuum line in engine compartment near the driver side firewall. It passes through the firewall where it terminates into a small manifold within the heating system control lever console. This manifold has three ports only two of which are connected to vacuum lines. This is visible when looking up behind the heater control lever console from the driver side. The center port has a tube that runs to the R Cowl Gate actuator. In my Jeep, this tube is a soft ductile rubber line compared to the semi-rigid plastic lines that make up the rest of the system. (Could possibly be a replacement part) The port nearest the driver side is where the vacuum supply pipe attaches. The port nearest the passenger side has no tube attached to it on my jeep and must be a vent port.

The cause of my Air Flow restriction was the vacuum supply line was hooked up to the vent port and was holding the gate shut even with the heater system on. It was not enough to fully block all airflow, but was enough that it was not running at full potential.
 
#228 · (Edited)
I have a 93 YJ 2.5. Not sure if this was asked in the thread....

The heat stops producing (then starts feeling warm randomly) sometimes even at idle and low speeds after the car is warmed up. Weather is in the 60's . I'm also seeing the temp gauge jump up a few degrees then back down. The thermostat isnt very old. Trying to figure out if the thermostat is an issue or the heater core is blocked. This happen on my Discovery and it was the heater core blocking water flow and causing it. Just curious if you think it's the same issue here.

Incredible write up btw!!! Best explanation of the system and wonderful feedback. Thank you!
 
#229 · (Edited)
I have a 93 YJ 2.5. Not sure if this was asked in the thread....

The heat stops producing (then starts feeling warm randomly) sometimes even at idle and low speeds after the car is warmed up. Weather is in the 60's . I'm also seeing the temp gauge jump up a few degrees then back down. The thermostat isnt very old. Trying to figure out if the thermostat is an issue or the heater core is blocked. This happen on my Discovery and it was the heater core blocking water flow and causing it. Just curious if you think it's the same issue here.

Incredible write up btw!!! Best explanation of the system and wonderful feedback. Thank you!
Are both your hoses to the heater core hot under the hood?
Random changes is sometimes low coolant level.

Sent from my SM-G960U using Tapatalk
 
#230 ·
Welcome to the Forum Joel YJ,

How long have you had the Jeep? Is the problem new or has it been going on for some time?
As Shark_13 states, it may be air in the system. With a cool engine, make sure the radiator and the coolant reservoir are topped off. Then run the Jeep at highway speeds for a few miles (or more).That should force any air in the system out through the top of the radiator into the coolant reservoir. The way a coolant reservoir is set up, once the air is pushed out, coolant will be drawn back into the cooling system as the engine cools. After doing this a couple or few times and the issue still exists, I'd suspect the thermostat or some intermittent blockage in the cooling system.

A good time to flush the cooling system is with replacement of the thermostat. If you suspect the thermostat to be the cause, be sure to use only a MOPAR or Stant thermostat. If you're unsure about flushing the cooling system, post again in the YJ Tech section.

Good Luck, L.M.
 
#232 ·
First thing, great write up. I have a 1992 4.0, heat has always worked great. It summer here in Colorado and I am noticing a lot of heat in the cabin while driving. On really hot days (like we have been experiencing this week) the cabin is really hot. From what I can tell, I am getting a lot of radiate heat from the heater core, radiating into the cabin thru the wall of the heater box.. Makes for an uncomfortable ride home from the trails.

Are any of you experiencing this?

I know I could put a manual block valve in one of the heater hoses going into the cabin under the hood, and I will if necessary but I would rather resolve this correctly than to just create a band aid.

Any input would be appreciated.
 
#233 ·
Are you running the fan at all? in vent mode? I get the same thing to a small degree with our AZ summers, the box will radiate some heat since the coolant is flowing through the core. I usually run a bypass to keep the flow through the manifold.
Checking your cable that directs the opens the air flow to make sure that it is still connected and the heat diverter is actually closed, is about all you can do. I'm guessing you have already checked for any coolant leaks from the box and that there are no open holes from the firwall to allow hot air from the engine bay into the cab.
 
#234 ·
Thanks for your quick response. Yes have checked for leaks, no leaks. I have almost always felt some heat in the summer in the cabin, but most days in Denver when we are in the high 90s I don't drive the YJ. Yesterday I was headed to the Jeep club meeting so wanted to take the jeep. It was hot here so felt the heat more. Most years I remove my hard top, but I can't this year due to various reasons.

The cable that controls the blend door is fubarred to I manually just flip the blend door, of course I always make sure it is in the closed position when it's warm out.

Thanks again, I appreciate your suggestions and help.
 
#236 ·
My jeep has had whatever was on the floor removed leaving open mounting holes and what I assume are purpose drilled drain holes. Passengers have told me the floor nearest the seat is hot to the touch and they are glad they weren't wearing flip flops. The cat is under that part of the body and I am contemplating removing the cat. Before anyone says something about inspection I could ride a bicycle to the station for my jeep and Silverado at the same time here in NC.

Back to the heat think of your jeep like a steel barrel with a propane burner at the bottom and you will be fine. Between the road and engine heat with hot pipe going to the muffler in a short space is a recipe for baking a cake. The cake being you of course.lol

Sent from my moto g(7) supra using Tapatalk
 
#237 ·
Ok, I went looking to figure out why my blower would never turn off. It turns out my switch plate was broken and it was on the part that was broken off.

I also realized that mine doesn't have the vacuum switch on the left side to control the cowl vent doors. What are the effects of not having the vacuum switch hooked up?
Sleeve Grey Motor vehicle Automotive exterior Bumper
 
#238 ·
Ok, I went looking to figure out why my blower would never turn off. It turns out my switch plate was broken and it was on the part that was broken off.

I also realized that mine doesn't have the vacuum switch on the left side to control the cowl vent doors. What are the effects of not having the vacuum switch hooked up?
Cowl gate closed. No fresh air into cab.
 
#243 ·
That should plug onto the vacuum switch on the heater control housing.
This is the best pic I have. The switch is that yellowed piece of plastic on the left side of the control housing.

Let us know how it goes.

Good Luck, L.M.
 

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#244 ·
The green line in the center goes to the door gate behind glove box. the line from the firewall plugs into that rubber plug. no one seems to know what the third port is for, vacuum purge maybe....
Here's mine installed
Hood Motor vehicle Gas Electrical wiring Cable
 
#245 · (Edited)
I glued up my heater switch plate, and just bought the vacuum switch from someone who also has a spare broken heater switch plate.

What sensor is this above the vent on the passenger side?
Tire Automotive tire Motor vehicle Chair Wood
 
#246 ·
That's a heater switch resistor to give you hi-med-lo fan speeds.

Good Luck, L.M.
 
#247 ·
Great write-up! Thank you!

Has anyone had cold air (in winter) blow in at the floor level along with the hot air? It seems on both driver and passenger side that there is another vent bringing outside air in. On the driver's side it seems to come in from under the dash and blow down to the left side of the actual floor heat vent. Thanks for the help in advance!
 
#248 ·
Great write-up! Thank you!

Has anyone had cold air (in winter) blow in at the floor level along with the hot air? It seems on both driver and passenger side that there is another vent bringing outside air in. On the driver's side it seems to come in from under the dash and blow down to the left side of the actual floor heat vent. Thanks for the help in advance!
On closer inspection, I'm guessing it's coming from the floor heat vent, but it blows all the time from there regardless of the placement of the blend door lever.
 
#249 ·
Blend gate may not be closing all the way. It can be adjusted at the cable connection, but having seen the inside of the heater box I don't think it can be completely sealed.
 
#250 ·
Thank you. I found a video of a heater core replacement that gave me a look into the box and I'm prone to agree. Have you seen any recommendations on maintenance for the cables? Mine are brutal to try and move. I usually just reach over to the box to move the blend door lever because it's too stiff for the cable to move on it's own.
 
#251 ·
There are I believe two of the three cables that are still available. But I'd shoot a little pb blaster down them let soak and work the cable. Rinse and repeat a couple times. It just might loosen them up.
Also check to make sure there are no kinks.
The plastic tabs that hold the cable will be brittle after 30 years to take care when removing.
The push on retainers can be found at your local ace hardware store in the misc nuts and bolts section.
 
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#252 ·
The searching continues...

After reading the OP again and looking at the drawings then looking more closely under the dash, the cold air is coming from the cowl gate on the driver's side. It seems to be closing tightly at the top but open by maybe 1/2 to 3/4 of an inch at the bottom. When I cycle the top cable on the climate control, that driver's side cowl gate will open more when I go far right to "vent," but cycling back to heat or defrost, that gate never closes completely. Anyone had this issue? This is definitely where my cold air blowing on my feet is coming from.

Thinking about this further I was wondering if anyone has tried to recirculate the warmed air in the cab back through the core. I'm wondering if the air intake vent on the hood was blocked off, and the cowl vents or the vent near the glove box were kept open, if the blower would then be able to circulate the warm air repeatedly, hopefully making it warmer in the cab? It's been in the sub-single digits around here in the morning and I don't think the little 2.5 can generate enough heat to fight that cold air coming from outside. Or, I need to try a new heater core. Probably that. 🤷🏻‍♂️

One more note, the green vacuum line/plug is dangling in the dash....just found that today. The three-port plug only has the green line in it. With this being completely disconnected, could it be part of the problem? I feel like it's a dumb question but don't really understand exactly what those vacuum lines are doing.

Thanks for the help and insight thus far!
 
#253 ·
S/B only one green line to cowl gate, passenger side. Black line comes in from engine bay vac.

Those cables have sliders at point of connection for adjustment. Maybe you can slide enough to fully close drivers cowl gate.
 
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