Low air flow from heater fan
 

It seems like there's not much air coming out of the vents on my '01 Cherokee. The fan sounds like it's working well, it definitely speeds up as you go up through the positions. But with the fan on high it doesn't put out much more air than some of my other vehicles do on low. It doesn't seem to matter where I have the air coming out, floor, vents or defrost all seem to have pretty low flow.

I'm thinking that most likely there's some blockage in the air ducts before the diverter valve. This weekend I hope to dig into it a little, but I was wondering if anyone had any suggestions of where to start looking. Do Cherokees have a cabin air filter that might be plugged? Is there a screen somewhere that tends to block up? Or is the heater core the most likely place to get blocked?

Thanks

the best bet is that one of the blend doors is either broken or the motor to move it has broken. youll have to tear into the HVAC

Do you see this problem with the Mode Selector in the Recirculation setting (first setting up from OFF)? In Recirc you are bringing air in from the cabin instead of the outside.

You can see the Recirc door and a portion of the fan by rolling the glove box down (two rubber stoppers removed) and, using a flashlight, looking inward towards the HVAC unit. With vacuum available (engine running) move the Mode Selector knob to Recirc. You should see the door open. The door also defaults to open when the Mode Selector is positioned to OFF.

If there's any rat's nest materials or foam seal materials in there you should be able to see it. Operate the blower and see if the fan is rotating. It could be loose on the motor shaft. You can poke the fan through the recirc opening grill using a screw driver (fan turned OFF). See if it flops around. If everything seems okay, there may be crap downstream of this point.

Edit: If the airflow is normal when Recirc is selected there could be blockage upstream of the fan. The fan brings air in from the grill under the windshield wipers.

I'm thinking that the blend doors are working correctly because the air comes out where it's supposed to: floor, vents or defrost, whatever is selected. It's just not very much air.

Thanks, that gives me some more specific things to look for. I don't know right now if "recirc" makes a difference, but that will be easy enough to check.

By the way, just from the sound I believe the fan is working correctly. It certainly changes its sound as well as the amount of air in each of the 4 speed positions. The motor sounds like it's under some load, like it's trying to move air, not just freewheeling. And there are no unusual noises to indicate bad bearings or anything. So I'm thinking a blockage somewhere is likely.

Again, thanks for the specifics on where to look. Now I know what I'm doing on Saturday!

Here's a pic of the recirc air door, as will be viewed through the glove box opening. This pic does not represent what a glove box roll down looks like - it's a view of the glove box completely removed.

I wonder why this thing is still called a "Glove Box". I don't think anyone has put gloves in that box for decades. LOL

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Found a mouse nest inside the fan cage when that happened to me. No air volume out of any of the vents, in spite of the fan obviously running well. Had to pull the fan out and clean out all the stuff the mouse had put in there. Haven't the slightest idea how a mouse could get in there, but they're tricky little devils.

You're not the first to find baby rat nesting materials in there.

I too am wondering how they get in there. They can slip through the eye of needle so I suppose they could sneak in through the the grill under the wipers, even though there's a screen to prevent that. Through the recirc opening? The recirc door defaults open when the system is shut down, but how do those buggers get in the Jeep to begin with?

I know...most floor boards are rusted so bad under the front wheel well liners that they climb through the rust holes in the floor.

I catch Chipmonks and Red Squirrels jumping up in my wheel wells all the time.

I think I've located the problem, but I don't think I like what I've found. The air flow is pretty much the same whether it's on recirc or not, so that must mean it's either in or downstream of the blower.

I can see the blower through the recirc door. It's clean and spins freely, so that must mean it's not the blower.

From what I can see in my service manual, it looks like the air flows from the blower, through the AC evaporator, to the blend door. Then, depending on the position of the blend door, none, some or all of the air goes through the heater core. From there it looks like the air, whether it went through the heater core or not, goes to the floor, panel, or defrost, depending on what is selected.

So I tried it with the heat turned all the way up and all the way down. There's good flow with the heat down, but it slows way down when I turn the heat up. So what it looks like is that the air flow through the heater core is restricted.

It sounds like a lot of work to get the heater core out of a Cherokee (evacuate the refrigerant, disconnect the refrigerant lines, drain the radiator, disconnect the heater hoses, remove the dash, remove the HVAC assembly).

So my questions: Am I right in what it takes to get at the heater core, or is there any short-cut? Is it normal for the heater core to significantly restrict air flow, or does it sound like there is a blockage?

If that's really what it takes I'm not going to go any farther with it myself. And I don't want to spend the money on someone else doing it if this is just normal.

I think your analysis is correct.

HVAC Unit R&R is a hard job. If you can get by with airflow with a blend of heat and cold maybe you should leave it be for now.

OK, thanks. I wish the answer was something better, but it's nice to know what reality is. It usually works out better that way. I think I'll check with my shop and see what they think it will cost. Not sure I'm going to pay it, but Minnesota winters are a lot less fun without effective heaters!

Here's something you may want to experiment with:

Locate the AC Accumulator in the engine bay. Remove the electrical connector from the Low Pressure cycling switch on top of the Accumulator and tape the harness connector and switch over with electrical tape to protect them. This will prevent the AC Compressor Clutch from engaging.

Now, you can use any selection on the Mode selector and the AC cold air won't be involved in the Blend-Air door position, only outside air. Except that if you select Recirc the cabin air will be recirculated without having your heat diluted with cold outside air.

Any selection on the Mode selector can be used for heat. Adjust your temp selector to get optimum air flow and heat.

The only draw back with this is you won't have AC air to help defogging the windows.

Edit: You can always plug it back in if you are having defog issues (above 30*F).

If I'm understanding you correctly, the only real advantage this would have is allowing me to use the recirc position without running the AC. This would let me get warmer air because recirc doesn't bring in cold outside air, but it normally uses the AC to cool the inside air, and with the low pressure cycling switch disconnected it wouldn't run the AC.

But it wouldn't have any effect on the output with the mode selector in the "floor" or "panel" positions that don't run the AC.

Am I getting that right?

You are correct.

I'm fixing to do this to my Heep shortly because winter is coming. Last year it got down to 13 below zero so I do enjoy having Recirc and the panel/floor selection with heat only.

OK, got it. Thanks for the suggestion.

By the way, I sort of did something like that on a CJ5 a long time ago. I was replacing the rusted out tub with a fiberglass one and I never bothered to cut the air opening in the cowl, so the heater just pulled air from under the dash. I figured a heater in a rag top needed all the help it could get.

I did something similar on my '90 to have the floor/dash panel combo or recirc with heat only, but to accomplish it, I cut the wire from the selector switch that signals the A/C system and installed an inline switch on the dash board to disable the A/C whenever I want.