Which part of my A/C system is bad?

Yoda

Yoda

TJ Enthusiast
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May 5, 2021
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Location
MA
Sorry....not TJ, it is KJ related.....but it's all the same-ish stuff so it shouldn't really matter. My 02 Liberty is my daily and the AC has been giving me fits. I'm hoping someone knows more about this stuff than me. Here goes...

Last summer my AC compressor went on me, a bearing or the clutch was making a racket. I replaced it and everything was fine last summer. I also replaced the low and high pressure lines along with the drier/accumulator at the same time. Installed new o-rings and gaskets too.

This spring the AC was cool....now it is just plain warm air. I figured I was low on freon so I took it to a shop to have them evacuate it and recharge it which they did. It passed the leak test and they filled it up to whatever it says on the sticker under the hood. No change.

I started poking under the hood last night and noticed a couple of things:
  • The low pressure line going from the Condenser to the drier/accumulator is ICE COLD. This makes me think the condenser is working.
  • The AC compressor is short cycling. 5 seconds on, 5 seconds off or something roughly like that. Not sure if that is normal or not. Perhaps a restriction somewhere?
  • I'll check tonight but I don't remember if the line from the drier/accumulator to the evaporator is cold or not. I think the drier/accumulator should be ice cold too right?
  • Line coming out of the evaporator is warm/hot.
I will be checking the blend door position also. My blend door actuator is toast so I just manually turned the screw to cold on the heater box...but its been a few weeks, maybe it opened itself back up from vibration?

I really really don't want to put an evaporator in this. I've already changed the heater core before and it is an all day job. Thanks in advance!!!
 
You really need gauges on it to diagnose it properly but short cycling makes me think either they didn't actually get a full charge (or it's already leaked out despite their testing - did they check the service ports themselves after disconnecting their machine?) or you have a restriction in the system.
 
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It'd be good to know the high side pressure. If it's normal or high then restriction, if it's low then low charge.

The fact that the tube coming out of the evaporator is warm tells me that the refrigerant mass flow is so low that it's all evaporating and then warming up (gaining "superheat") without absorbing enough heat from the air to make it cold.
 
Yoda said:
Sorry....not TJ, it is KJ related.....but it's all the same-ish stuff so it shouldn't really matter. My 02 Liberty is my daily and the AC has been giving me fits. I'm hoping someone knows more about this stuff than me. Here goes...

Last summer my AC compressor went on me, a bearing or the clutch was making a racket. I replaced it and everything was fine last summer. I also replaced the low and high pressure lines along with the drier/accumulator at the same time. Installed new o-rings and gaskets too.

This spring the AC was cool....now it is just plain warm air. I figured I was low on freon so I took it to a shop to have them evacuate it and recharge it which they did. It passed the leak test and they filled it up to whatever it says on the sticker under the hood. No change.

I started poking under the hood last night and noticed a couple of things:
  • The low pressure line going from the Condenser to the drier/accumulator is ICE COLD. This makes me think the condenser is working.
  • The AC compressor is short cycling. 5 seconds on, 5 seconds off or something roughly like that. Not sure if that is normal or not. Perhaps a restriction somewhere?
  • I'll check tonight but I don't remember if the line from the drier/accumulator to the evaporator is cold or not. I think the drier/accumulator should be ice cold too right?
  • Line coming out of the evaporator is warm/hot.
I will be checking the blend door position also. My blend door actuator is toast so I just manually turned the screw to cold on the heater box...but its been a few weeks, maybe it opened itself back up from vibration?

I really really don't want to put an evaporator in this. I've already changed the heater core before and it is an all day job. Thanks in advance!!!
Click to expand...

You appear to have low pressure issues (or by chance airflow issues over the evap) as the liquid line will be heavily frosted. Coming out of the cab to the accumulator then compressor is the low side hot gas line and should not be cold.
 
Have you tested the high and low side pressure switches? If one or both is failing it may be flickering on/off/on/off and causing the symptom you mention - AC clutch engaging/disengaging intermittently. Of course first check the system is filled and high/low side pressures are within the correct range as low fill can also cause this problem.
 
***EDIT***
I had said previously that the low pressure line went to the accumulator, then to the evaporator. It is the other way around. Low pressure line goes to the evaporator.

Good News!!!
Well sort of. First....thank you everyone for the advice so far!!! So last night I tinkered a bit more trying to diagnose the issue. I jumped the terminals on the low pressure switch connector. The compressor stayed on and after a few seconds the low pressure line iced up. This made me think low freon.

I went to the auto parts store and bought a can of R134a and figured I'd add a little at a time and see what happens. I added some and the compressor stayed on longer but still short cycled. Added a bit more and it stayed on even longer. After the third try the compressor stayed on permanently. The air was starting to get cool.

The can came with a gauge, although probably not very accurate, with instructions to fill the system between 30-50psi. The gauge was all over the place when the compressor was cycling but once it stayed on the pressure was steady at roughly 20psi. I added the whole can and the pressure was stable at 44psi and the air was ice cold!!!

On the drive to work this morning it was still pretty cold! The bottle did have some "conditioners" to help seal leaks in rubber o-rings and gaskets and it also had a dye to help diagnose leaks. At the moment it appears that I either did not get a full charge or I had a leak, but otherwise my system seems functional.

I bought a $30 cheapy freon detector and a UV light so if the charge doesn't last I'll hopefully be able to find the leak. Fingers crossed its not the evaporator!!!

I may also purchase a set of decent gauges for the AC system. My understanding is that the ratio of high pressure to low pressure is pretty important for best performance. The FSM says at 90F ambient High side should be 250-350psi (call it 300psi average + 14.7psi) and the low side should be 30-40psi (35 psi average +14.7psi). That is a ratio of 6.3:1. Or an overall range of 5.9-6.7:1 at 90F.
 
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Invest in the gauge manifold before adding the refrigerant so you know what the system is doing not that the compressor is cycling due to the refrigerant stacking up on the high side which can be caused by an restriction in the metering oriface.
Before you connect the gauge manifold to the A/C service ports; I would recommend having someone that is knowledgeable assist you.
 
Yoda said:
I may also purchase a set of decent gauges for the AC system. My understanding is that the ratio of high pressure to low pressure is pretty important for best performance. The FSM says at 90F ambient High side should be 250-350psi (call it 300psi average + 14.7psi) and the low side should be 30-40psi (35 psi average +14.7psi). That is a ratio of 6.3:1. Or an overall range of 5.9-6.7:1 at 90F.
Click to expand...

I leave open the possibility that the pressures follow a ratio in orifice tube systems because that's not typically my area of experience, but generally, the compressor capacity (mass flow) is a function of it's inlet and outlet conditions, and the smaller the lift (discharge pressure - suction pressure), the more it will flow and the less it will have to work to do it. It all feeds back into itself and ends up being a system of equations:

(neglecting heat gain and pressure drop in refrigerant lines for the sake of simplicity)

1. Evaporator outlet conditions = compressor inlet conditions

2. Compressor flow = f(inlet and outlet conditions)

3. compressor outlet conditions = condenser inlet conditions

4. condenser pressure = f(compressor flow, condenser inlet air and refrigerant conditions)

5. condenser outlet conditions = expansion device inlet conditions

6. expansion device outlet conditions = f(compressor flow, expansion device inlet conditions)

7. evaporator pressure = f(compressor flow, evaporator inlet air and refrigerant conditions)

back to #1

If you had the compressor, evaporator, and condenser performance curves you could actually solve the equations to model the system using tools like GoalSeek or Solver in Excel.
 
Yoda said:
***EDIT***
I had said previously that the low pressure line went to the accumulator, then to the evaporator. It is the other way around. Low pressure line goes to the evaporator.

Good News!!!
Well sort of. First....thank you everyone for the advice so far!!! So last night I tinkered a bit more trying to diagnose the issue. I jumped the terminals on the low pressure switch connector. The compressor stayed on and after a few seconds the low pressure line iced up. This made me think low freon.

I went to the auto parts store and bought a can of R134a and figured I'd add a little at a time and see what happens. I added some and the compressor stayed on longer but still short cycled. Added a bit more and it stayed on even longer. After the third try the compressor stayed on permanently. The air was starting to get cool.

The can came with a gauge, although probably not very accurate, with instructions to fill the system between 30-50psi. The gauge was all over the place when the compressor was cycling but once it stayed on the pressure was steady at roughly 20psi. I added the whole can and the pressure was stable at 44psi and the air was ice cold!!!

On the drive to work this morning it was still pretty cold! The bottle did have some "conditioners" to help seal leaks in rubber o-rings and gaskets and it also had a dye to help diagnose leaks. At the moment it appears that I either did not get a full charge or I had a leak, but otherwise my system seems functional.

I bought a $30 cheapy freon detector and a UV light so if the charge doesn't last I'll hopefully be able to find the leak. Fingers crossed its not the evaporator!!!

I may also purchase a set of decent gauges for the AC system. My understanding is that the ratio of high pressure to low pressure is pretty important for best performance. The FSM says at 90F ambient High side should be 250-350psi (call it 300psi average + 14.7psi) and the low side should be 30-40psi (35 psi average +14.7psi). That is a ratio of 6.3:1. Or an overall range of 5.9-6.7:1 at 90F.
Click to expand...

Well try this again for normal operation, the LP line is the outlet of the evap that connects the the accumulator then to the compressor where it then becomes the the HP hot gas then to the condenser where it condenses back to liquid that goes to the evap and so on.
 
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Did you replace, or at least clean the condenser, when you replaced the compressor? If there was debris in the system from whatever failed in your compressor, it usually gets trapped in the condenser. With newer vehicles, the condensers seem nearly impossible to flush clean (very small transfer tubes), so along with the drier, I usually just replace the condenser when replacing a compressor that grenaded. Also, it seems that oil quantities and refrigerant quantities are often screwed up, so it might just need a proper charging, as you've deduced.
 
***UPDATE***

I got a decent set of gauges and hooked them up. It was 85F and the jeep was fully warmed up and the AC on full blast. Low pressure read 43psi and the high pressure read 290psi. FSM says that at 80F ambient the low side should be 25-35psi and at 90F it should be 30-40psi. FSM also says high side should be 200-300psi at 80F and 250-350psi at 90F.

If I split the difference at 85F the low side should be 27.5-37.5psi and high side should be 225-325psi.

So it looks like I put a little extra freon, but not that much. If I idle at around 1200rpm the low side goes down to about 38psi and the high side goes up to about 310psi. Overall the AC is better and so far holding the charge. At idle it is about 60 degrees and gets down to about 50 degrees while moving. Not ideal, but better than a sauna.

I have a new accumulator in the box and I'll be replacing the condenser and high pressure AC line and orifice tube as well. As far as I can tell the condenser is 20 years old and original. I'll evacuate it and put a fresh charge in it and hope for the best!
 
Before drawing the system under and vacuum, wasting the refrigerant that has already been charged into the system and recharging to lower pressures and temperatures.
Remember that when the Jeep is actually moving and air is being forced thru the condenser instead of being drawn thru by the radiator cooling fan; the heat of compression will be removed quicker, the system suction and discharge pressures will be lower which will also lower the refrigerant temperature and the discharge air temperatures coming from the ventilation ducting.
You need to check the ventilation temperatures while the Jeep is moving.
 
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