Air door blend issues

[Squamish_Steffen]

Are you guys aren't going to believe this one. I think I know what it is. The air conditioning evaporator is frosting over. when I was checking the vacuum line in the engine bay, I noticed that the lines coming off the compressor and off the dryer were completely Frozen over. In fact the dryer was a block of ice. I'm wondering if the coil in the dash isn't completely Frozen over. Test that, I just unplugged the air conditioning compressor. about 5 minutes later I've got full heat again.

Interesting, why would the evap coil be freezing over? - unintended operation of the AC compressor?? To my mind that would produce cold air prior to no-air. I'll check mine on Wednesday and report back.

 

[JeepJeepster]

Bad pressure switch a-top the accumulator.

I never use the A/C in the winter, but mine froze up in the summer when using the A/C. Had to turn the A/C off to let it thaw, then kick it back on till I got around to figuring out what the issue was and replacing it.

 

[sleazy rider]

Well, that’s a different issue to deal with for heat. lol Good on ya to figure that one out.

 

[lfhoward]

On the KK the AC runs on defrost mode to help dehumidify the defrost air. So my AC compressor running in winter is not so out of the ordinary. I don’t know if this is true on KJ’s.

 

[JeepJeepster]

On the KK the AC runs on defrost mode to help dehumidify the defrost air. So my AC compressor running in winter is not so out of the ordinary. I don’t know if this is true on KJ’s.

It is, but I took my hvac controls apart and put a piece of tape over the contact for the A/C. The 05+ controls actually turn the A/C on in the floor positions also. Im not using the A/C in the winter unless I need it. The only time I need it is when I get in the vehicle and I'm wet from snow or rain, then I'll manually turn it on till the windows are clear and turn it off.

Issue is, literally no one knows to turn the A/C on to quickly defog the windows.... So you end up with stuff like this.

 

[Raybird2112]

It is, but I took my hvac controls apart and put a piece of tape over the contact for the A/C. The 05+ controls actually turn the A/C on in the floor positions also. Im not using the A/C in the winter unless I need it. The only time I need it is when I get in the vehicle and I'm wet from snow or rain, then I'll manually turn it on till the windows are clear and turn it off.

Issue is, literally no one knows to turn the A/C on to quickly defog the windows.... So you end up with stuff like this.

We'll, I ended up putting I whole new ac system in my Jeep because my compressor blew. I'm hoping that fixes my sir blend issues. I think my air blend issues were related to my ac compressor.

 

[sota]

the compressor in these things turns on CONSTANTLY. It was explained to me, by someone on here, that it's to keep the seals lubed or some other excuse. All I know is, whenever the HVAC fan is on, the compressor will cycle every minute, regardless of it being needed. In defrost mode it's commanded on anyways.
I suspect the problem wouldn't happen if I were in recirc mode instead of dash or floor+dash all the time, but you can't split that function in these things.

 

[sota]

It is, but I took my hvac controls apart and put a piece of tape over the contact for the A/C. The 05+ controls actually turn the A/C on in the floor positions also. Im not using the A/C in the winter unless I need it. The only time I need it is when I get in the vehicle and I'm wet from snow or rain, then I'll manually turn it on till the windows are clear and turn it off.

Issue is, literally no one knows to turn the A/C on to quickly defog the windows.... So you end up with stuff like this.

It doesn't help that the evap coil is before the heater coil, so you can wind up with cold, moist air hitting the evap, causing it to frost over.
It's done this now both times i've gone plowing this season.

 

[JeepJeepster]

Try replacing the pressure switch on the accumulator and see if it quits freezing up. Or just unplug the compressor.

So far I've only used the A/C on those hot humid summer days and my compressor seems just fine. I did have to replace the A/C hose from the compressor to the condenser but thats just old hoses failing.

 

[Squamish_Steffen]

I've spent the last day learning the HVAC vacuum system and preparing for a complete diagnostic run, following the service manual instructions, thinking that vacuum component was at fault.
But I'm quickly converting to the new theory about the evap coil getting frosted and blocked ! Good find @Sota.

To my understanding, all the air (recirc or fresh) is blown across the evap coil before "deciding" to pass over the heater core or to bypass it. If the evap coil is blocked, this would explain why I can still hear the mode-door when adjusting the position switch (floor/face/window), even with no airflow.

In particular, following on Jeepsters train of thought, I'm curious if the AC low-pressure switch has failed..? (See page 24-19 of the service manual). This switch is meant to disable the AC clutch/comp when accumulator pressure falls below 21 PSI (and re-enable above 38 PSI) OR if outside/ambient temp fall below "approx" 30F "due to the pressure/temp relationship of the refrigerant.". A low refrigerant charge (leak) causes frosting of the evap coil. But the low-pressure switch is meant to avoid AC operation if this happens of course.

Alternatively, maybe an excessive refrigerant charge would keep the suction/accumulator pressure high (>21 PSI) despite cold ambient temps. The evap coil might still frost if the ambient temp is too cold to transfer enough heat to the coil, and especially if humidity is high. Just need to test refrigerant pressure either way.

Another suspect could be the ambient air temp sensor located in the grille. Though, it seems that is an input to the BCM and CMTC, not the PCM which controls the AC system.

I'll be doing two 1h drives tomorrow in -5C to +5C temps. Guaranteed the airflow will stop after 15min of driving each way, as the issue has been perfectly consistent for two months now. I'll pull the AC clutch fuse and verify that the AC doesn't enable. Lets see if that solves it until the refrigerant pressure can be tested and corrected. (I really hope this is it, as it sounds way easier to fix than a blend door actuator.)

Humidity is always very high where I am, and I only noticed the whole issue when it got cold outside (<40F).

 

[sota]

@Squamish_Steffen you're pretty much on the track I am, for sorting this out.
I can only offer my observations at this point:

• "Failed" heat in terms of there's barely any air moving
• that air that is felt is hot
• all the vacuum operated doors work as expected
• the electrically controlled blend door works as expected (drilled a hole in the box, stuck a screwdriver in the shaft, saw it turning when commanded)
• refrigerant lines off the accumulator/dryer were frosted over, as was the accumulator/dryer.
• ambient temperatures we at freezing, +/- 1 or 2 degrees Fahrenheit either way.
• air was moist, as it was snowing/sleeting/raining.
• problem went away when vehicle was in a garage with the door closed for at least an hour, or when the a/c compressor electrical controls were disconnected for 5-10 minutes.
• air flow gradually was restored after compressor was disconnected.

I'm open to ideas/suggestions as to why it's doing it. failed sensors or switches are always possible/plausible.

 

[sota]

@JeepJeepster any suggestions/ideas/shop-notes on how to test that pressure switch?

I'm pretty drunk right now, after having plowed for 25 hours over the past 48, so forgive me if I've asked an obvious question.

 

[sota]

I'll add... after disconnecting the compressor electrical on the first day, I had no problems with heat for the second day.
I might try at some point, to fish my inspection camera up through the recirc intake port and see if I can see the evap. If I can, and conditions are ripe for it to occur, i'll attempt to recreate what I suspect is a freezing over.

 

[JeepJeepster]

One test would be to hookup a set of gauges and watch the pressures while watching the clutch engage and disengage. Does the clutch stay engaged?

You wont be able to get past the fan when going through the intake like that. You can remove the fan and see the evap though, its very simple to remove. If I recall correctly, it doesnt even use screws.

 

[Squamish_Steffen]

I popped the hood after driving for 30min on Tuesday night and observed the accumulator as one big ice block. AC clutch cycling 3-6 times/min. I'm not certain, but don't think either of those are normal.

I removed the AC clutch fuse (pos 21, 20amp, engine bay) for both my winter highway drives yesterday, and viola - problem solved. There is no vacuum controls issue. Perfect airflow and heat. Given that my airfow issues have been totally consistent for 7 weeks now, I consider this result quite conclusive. I did not notice any other side effects, excess humidity or otherwise, though I'll say yesterday was not particularly wet/humid.

Going to leave the fuse out for a week or two while I buy a gauge/refill kit and probably a new low-pressure sensor. I'll report back if I find humidity issues during that time. This forum has done it again - thanks for the help folks.

 

[Squamish_Steffen]

any suggestions/ideas/shop-notes on how to test that pressure switch?

The jeep service manual has a detailed procedure to test that switch and others, page 24-19 in my manual. Sounds easy, just need a ref pressure gauge.

 

[Raybird2112]

New ac compressor in for a 4 days now and no air blend issues.

 

[sota]

I popped the hood after driving for 30min on Tuesday night and observed the accumulator as one big ice block. AC clutch cycling 3-6 times/min. I'm not certain, but don't think either of those are normal.

I removed the AC clutch fuse (pos 21, 20amp, engine bay) for both my winter highway drives yesterday, and viola - problem solved. There is no vacuum controls issue. Perfect airflow and heat. Given that my airfow issues have been totally consistent for 7 weeks now, I consider this result quite conclusive. I did not notice any other side effects, excess humidity or otherwise, though I'll say yesterday was not particularly wet/humid.

Going to leave the fuse out for a week or two while I buy a gauge/refill kit and probably a new low-pressure sensor. I'll report back if I find humidity issues during that time. This forum has done it again - thanks for the help folks.

definitely keep us posted.
if we get another storm in the next couple of days, I might try and test the theory again as well: hook the compressor up and see how long it takes for the vents to stop working.

think my procedure will be:
1) run with everything normally connected, attempting to time until freeze up, defrost or floor/defrost only.
2) disconnect compressor, time how long until air flow is restored.
3) get cabin interior face melting warm (easy enough to do.)
4) reconnect compressor, time again to freeze up.
5) disconnect compressor, time how long until air flow is restored.
6) reconnect compressor, change mode to recirc, time again to freeze up.

I have a suspicion that, as long as it stays in recirc mode at point #6, it won't freeze up; no major amounts of cold/humid air will be brought into the system, and the ambient temp will be high enough to keep the evap from freezing over. I might be totally wrong on that though.

 

[sota]

New ac compressor in for a 4 days now and no air blend issues.

That might lend credence to the low A/C charge theory some had floated. A new compressor would mean a vac and fill of the system, thereby restoring a proper charge.

 

[Squamish_Steffen]

New ac compressor in for a 4 days now and no air blend issues.

Did you/they replace the accumulator or any sensors at all? And what temps have you been driving in during the issues? (Glad to hear your issue is solved btw)

What still bugs me is that the low-pressure switch is supposed to prevent operation during a low-charge scenario. And supposedly prevent operation at all below 30F. The service manual (p24-2) reads "To maintain min evap temp and prevent evap freezing, the low-pressure switch on the accumulator cycles the compressor clutch."

The previous owner of my car said they had to replace the AC compressor twice in the two years prior. Sounded like they didn't really understand the cause of failure. Perhaps related. But the AC worked great for me all last summer.