A/C warms noticeably at stop, or when accelerating

[dbrown]

Anyone else experience this? Air blows ice cold until slowing to a stop light, then warms significantly for a few seconds, then cools back down. Same thing happens when accelerating from stop. Had it in at the dealer for a windshield "whistle", and asked them to check this, and they said everything is fine, but this service dept. is hot garbage, so.... who knows. Just curious if anyone else has dealt with this. FWIW, it's been 105-110° here.

 

[Toad86405]

Anyone else experience this? Air blows ice cold until slowing to a stop light, then warms significantly for a few seconds, then cools back down. Same thing happens when accelerating from stop. Had it in at the dealer for a windshield "whistle", and asked them to check this, and they said everything is fine, but this service dept. is hot garbage, so.... who knows. Just curious if anyone else has dealt with this. FWIW, it's been 105-110° here.

Mine does the same set on Auto, external air. It's 114 today

 

[Got Pipe]

Mine also does this. I've decided(in no scientific way shape or form whatsoever) that there is some AC pump trickery going on that the stop start systems use. Or also a vacuum cutoff like the exhaust uses. Maybe to gain that extra little power upon acceleration. I dunno.

 

[Dubai_Bulldog392]

Anyone else experience this? Air blows ice cold until slowing to a stop light, then warms significantly for a few seconds, then cools back down. Same thing happens when accelerating from stop. Had it in at the dealer for a windshield "whistle", and asked them to check this, and they said everything is fine, but this service dept. is hot garbage, so.... who knows. Just curious if anyone else has dealt with this. FWIW, it's been 105-110° here.

Mine does the same and we have really hot temperatures in dubai...lasts for about a minute but then quicks back in...

 

[MDrums]

Mine does this too but I only notice it a little....more on really hard acceleration. I've had other vehicles other brands do the same thing.

 

[WHLN392]

Totally normal under hard acceleration, the a/c pump belt clutch disconnects to minimize drag on the engine under hard acceleration. Most vehicles are programed this way.


As for heating up when at a stop, not surprising, the condensers are not getting airflow, the engine rpm is at idle contributing the the a/c not being at its optimal efficiency range.


Plus these engines are shoehorned like nobodies business in these Jeeps and tend to run hot.

 

[@JC]

Mine also does this. I've decided(in no scientific way shape or form whatsoever) that there is some AC pump trickery going on that the stop start systems use. Or also a vacuum cutoff like the exhaust uses. Maybe to gain that extra little power upon acceleration. I dunno.

We don't have start/stop.

 

[Got Pipe]

We don't have start/stop.

Yeah, I'm aware of that. I was just stating I think it might be related to some of the tech those systems use.

 

[dbrown]

Just glad that I'm not alone

 

[Rounder]

Mine has always done that. I quit using auto and do manual and adjust the blower more when needed. Still happens. If i crack my window for a smoke it cannot be turned up high enough. My 16 JKU never had any problems. Agree big engine shoehorned in a small space. Gets hot in there.

 

[jeepguru]

Both my 2024s do that and here in AZ when it's 120 degrees out...it sucks!

 

[RAMSTEEL]

Same and I switched to MAX AC, internal air circulation, seems to help.

 

[392 Jeep]

You’re not alone. I think that’s just how it is.

 

[Rampage]

Mine does it too

 

[Excaliburitto]

Same here.

 

[carbonalpine]

Mine does it also, but I tried to dive deep into it and found:

-When in recirculating mode, the a/c compressor has cycles lasting about 5 seconds. It cools nice and in 5 seconds you can feel the temp rise, then 5 seconds lasted you feel it cool again. I took it in and they say it’s normal


I have another JL non 392 and it doesn’t do this, but they’re saying it’s cause of the v8.

 

[dbrown]

Mine does it also, but I tried to dive deep into it and found:

-When in recirculating mode, the a/c compressor has cycles lasting about 5 seconds. It cools nice and in 5 seconds you can feel the temp rise, then 5 seconds lasted you feel it cool again. I took it in and they say it’s normal


I have another JL non 392 and it doesn’t do this, but they’re saying it’s cause of the v8.

Yeah, I basically got the same info last week from the dealer. My '18 JLRU doesn't have the issue, and I don't think Jeep will admit it, but the V8 is so crammed in there, I'm sure they had to make significant modifications to basically everything.

 

[guruman]

TLDR; It's a wall of text, sorry, and good luck.

All A/C systems work like this. You may not notice as much, but they are all basically the same.

The compressor is not directly tied to the HVAC controls. It's controlled by a simple electric clutch. When you turn on the A/C, power is sent to the wire to activate the clutch, but before it gets there it passes through 2 or so pressure switches. One on the low pressure side of the system. If it's too low, the compressor will not come on, as too low of a pressure o the low side indicated not enough refrigerant in the system. It will not start the compressor, because if there was no refrigerant, the compressor would never turn off and it would burn up.

Provided there is enough pressure o the low side switch, the wire then passes through a high pressure side switch. The high side switch tells the compressor when to stop. The compressor turns on, and pumps refrigerant from the low side to the high side until the high side switch reaches its limit. Then the high pressure refrigerant leaks through the oriface (or similar) into the low side, becoming cold I the process. Once the high side pressure leaks down below the high side pressure sensor, the compressor comes back on. This is the cycling on and off, which is not controlled by the computer or the HVAC controls.

The mechanism that controls the compressor is pretty dumb and simple. When you change the settings to ask for A/C it just goes through its on-off cycle, keeping the high side pressurized every time it gets low. Turning the fan down does affect who quickly heat is transferred inside the HVAC unit, and thus can influence the timing of the on-off cycles, but it's not directly commanding the compressor. Likewise, changing the temperature setting moves a blend door inside the HVAC unit to mix in hot and cooled air to get to your desired temp. that may again affect the rate heat gets transferred I the system and change the timing of the on-off cycle, but it;s not directly turning the compressor on and off.

It's easier to notice the on-off cycle when you are passing already cooled air over the coils, as you are not transferring as much heat as you would be if you were drawing outside air in, so you will get short on-off cycles, because the compressor does not need to work as hard to cool the already cooled air.

As for the output temperature.... The refrigerant pressure are set carefully to get maximum cooling WITHOUT freezing. If the temp inside the HVAC unit gets to the freezing point, any moisture in the air will freeze in the unit until its blocks airflow and effectively stops cooling. Altitude changes the freezing point, but since the refrigerant is in a sealed system it is not affected by altitude, so they have to account for that. So the refrigerant is probably getting down to a safe 36-38 degrees F, but the air passing through is only getting down to 44. Any colder, and the system will freeze.

The performance of the refrigerant cannot really be improved. You could move more air, thus transferring more heat. But there is not much else you can do to get colder A/C.

If you live in a hot climate, focus on keeping extra heat from getting in:

Paint the hardtop white
Install a hard liner
Lyzardskin thermal coatings for the roof/floor
install heat rejecting window tint
header/exhaust wrap?
???

If you want to go really extreme, you can cut a hole in the top and install a second, DC powered A/C unit on the roof.

 

[carbonalpine]

TLDR; It's a wall of text, sorry, and good luck.

All A/C systems work like this. You may not notice as much, but they are all basically the same.

The compressor is not directly tied to the HVAC controls. It's controlled by a simple electric clutch. When you turn on the A/C, power is sent to the wire to activate the clutch, but before it gets there it passes through 2 or so pressure switches. One on the low pressure side of the system. If it's too low, the compressor will not come on, as too low of a pressure o the low side indicated not enough refrigerant in the system. It will not start the compressor, because if there was no refrigerant, the compressor would never turn off and it would burn up.

Provided there is enough pressure o the low side switch, the wire then passes through a high pressure side switch. The high side switch tells the compressor when to stop. The compressor turns on, and pumps refrigerant from the low side to the high side until the high side switch reaches its limit. Then the high pressure refrigerant leaks through the oriface (or similar) into the low side, becoming cold I the process. Once the high side pressure leaks down below the high side pressure sensor, the compressor comes back on. This is the cycling on and off, which is not controlled by the computer or the HVAC controls.

The mechanism that controls the compressor is pretty dumb and simple. When you change the settings to ask for A/C it just goes through its on-off cycle, keeping the high side pressurized every time it gets low. Turning the fan down does affect who quickly heat is transferred inside the HVAC unit, and thus can influence the timing of the on-off cycles, but it's not directly commanding the compressor. Likewise, changing the temperature setting moves a blend door inside the HVAC unit to mix in hot and cooled air to get to your desired temp. that may again affect the rate heat gets transferred I the system and change the timing of the on-off cycle, but it;s not directly turning the compressor on and off.

It's easier to notice the on-off cycle when you are passing already cooled air over the coils, as you are not transferring as much heat as you would be if you were drawing outside air in, so you will get short on-off cycles, because the compressor does not need to work as hard to cool the already cooled air.

As for the output temperature.... The refrigerant pressure are set carefully to get maximum cooling WITHOUT freezing. If the temp inside the HVAC unit gets to the freezing point, any moisture in the air will freeze in the unit until its blocks airflow and effectively stops cooling. Altitude changes the freezing point, but since the refrigerant is in a sealed system it is not affected by altitude, so they have to account for that. So the refrigerant is probably getting down to a safe 36-38 degrees F, but the air passing through is only getting down to 44. Any colder, and the system will freeze.

The performance of the refrigerant cannot really be improved. You could move more air, thus transferring more heat. But there is not much else you can do to get colder A/C.

If you live in a hot climate, focus on keeping extra heat from getting in:

Paint the hardtop white
Install a hard liner
Lyzardskin thermal coatings for the roof/floor
install heat rejecting window tint
header/exhaust wrap?
???

If you want to go really extreme, you can cut a hole in the top and install a second, DC powered A/C unit on the roof.

Thank you so much for this, I was just wondering why my non 392 JL doesn’t have this short cycle problem but I’ll just chalk it up to it being a common issue with the v8. I’m sure my answer is inside what you wrote, I appreciate you taking the time

 

[guruman]

Thank you so much for this, I was just wondering why my non 392 JL doesn’t have this short cycle problem but I’ll just chalk it up to it being a common issue with the v8. I’m sure my answer is inside what you wrote, I appreciate you taking the time

I'm betting it does the same thing. If you are running already cooled air through, the compressor just does not need to run as much, we just notice it more in the 392. The temp fluctuation and compressor cycling may just be more noticeable due to the engine packaging and the heat it produces.