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Defroster Vent Problems

UNCC_99XJ

Twisted Up
NAXJA Member
Location
Harrisburg, NC
Went outside this morning to clear the snow and ice off the windows, and it was to thick to do w/o warming it up. So I started it, turned the defroster on, moved the temp over to heat, and turned the speed up to 4. Right as I got to 4 I heard this aweful POP!, like some sort of door being slammed shut in the vent system. Didn't think much about it, considering it had been sitting out in the snow and ice all night, figured things were cold. So 10 min later I go back out and clear the stuff off my windshield, and immediately the right side of my windshield fogs up on the inside. Cut the defroster on and off several times, leaving it on 4, and still getting that popping sound. I then drove it around the block a couple of times, thinking maybe I would wake something up with a little more heat. No luck. Brought it home, and started feeling around the vents, to obviously find the right side of the defroster vent was not working. So I cut everything but the engine off for a few seconds, and tried it again. This time leaving it on speed 1. Hey it's working again! Turned it up to 2, still working, next came 3. Still working! The minute I hit 4, POP!, and it's back to half a vent again.

I don't have my Hayne's manual any more. Returned that yesterday cause I've got my eye on a FSM on ebay for a decent price. Anybody have any suggestions as to what might be wrong? I rarely use my defroster, let alone on 4, but every now and then when I don't have time to let it warm up, i'll move it to 4 to defrost the window quicker. TIA.

forgot to add it's a 99, 4.0, automatic.....not that it would really matter, but it seems to be a common response when somebody posts a question.
 
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that sounds like a weak vacuum signal to the defrost motor...usually on the older jeeps, its one plenum to two hoses, the two hoses go to the two vents-
maybe on the newer jeeps its two flaps..but I sure dont see why.

I usually get one loud slap noise with mine when im low on vacuum, and it switches to defrost only mode- the flapper loses vacuum, and the motor retracts hitting against the stop. Sometimes it even sounds like a creaky door closeing. spooky.

with so much volume of air rushing past a flap with little or no vac to hold it open, it probably would slam it closed. Find that motor.
 
So basically your saying the defrost motor is robably going bad? or the vacuum lines to it are going bad?

Sounds like I gotta pull off the entire dash board to do it......
 
BRINGING BACK A DEAD THREAD FROM AGES AGO
but my jeep is doing the EXACT same thing as the original post in the thread
and I was wondering if anyone has some tips on what this could possibly be or what the best way to fix it is
thanks in advance
and yes I have been searching I am on page 14 of a current search hence how I found an almost 2 year old thread and revived it this was the first relevant thread I found with a similar problem
all the rest of my vents seem to work fin its only when I use defront I will only get it out of the drivers side
right side is almost always dead
 
I agree with Ed. The newer xj's have two doors, one for drivers side, and one for passenger side. Using this method the newer jeeps have far better blowing power from the fan that older ones do. I would start by tracing from that doors vacuum motor and go from there testing for leaks. If you have good vacuum at the line going into the vacuum motor, chances are the motor itself is bad.
 
Sounds like a vacuum problem, but I'm totally not sure why it would happen when you turn the fan to speed 4 - that's electronic, no vacuum lines with the blower motor.......

Unless his blower switch blug is melting/really heating up and then it does something with a vacuum line sitting beside it? Although if it melted the vacuum line it would always have a problem, regardless of fan speed........I can't think of anything else. I'd pull the center dash and look behind the HVAC controls for signs of weakness in the lines extending off of the HVAC controls.

Not that hard, beats taking the entire dash apart for a small fix.
 
Check the vacuum lines that run to and from the little vacuum ball behind the front bumper on the passanger side. One day my heater would only work in defrost mode and the problem was because the vacuum line has developed a hole after getting stuck under the battery tray. After fixing the leak, all was well.
 
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