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91 4.0 AC does not work well in heat

xjerod

NAXJA Forum User
Location
maryland
I have a 91 with the R134 AC that does not work well in the heat. It works ok when the weather is cool outside.

It only blows cold when the blower is on lower settings
Works better when its cool outside

I have washed out the radiator and condenser and there seems to be plenty of air flow... enough that it can hold a rag to the grill while parked.

I have added Freon with dye but I have not found any leaks.
I have confirmed that the electric fan comes one when the AC is on and the compressor clutch engages.

I dont know much about AC systems and need some help on this one... I tend to think the system isnt getting enough coolant and that could be why the hot outside temps kill my cooling.

Thoughts??

XJerod
91 Cherokee Sport 4.0
105k
 
Some basics on AC systems. If you have moisture-laden air mixed in the refrigerant, bad things can happen;
1. Maximum heat is extracted from hot air flowing through the evaporator coil when the capillary tube/throttling valve produces a phase change (liquid refrigerant flashes to gas) before entering the evaporator coil. Moisture, in refrigerants, can freeze into ice crystals and block throttling valves, ie.orifices or capillary tubes , etc. This prevents/degrades the liquid refrigerant from changing phase to a gas so that less heat is absorbed from the hot air flowing through the evaporator coil.
2. Typically, a vacuum is pulled on the AC systems to remove all moisture and the AC is then recharged with refrigerant.
3. Small pin-hole leaks in a AC system can also cause a loss of refrigerant. Most times the product "Red Angel" can be used to seal small leaks without disassembling the whole system. Large leaking cracks in the AC system typically require the defective parts to be replaced.
4. Once the moisture is removed, small leaks stopped, then the AC compressor can be checked to see if it can achieve the spec'd intake/discharges pressures for the actual ambient hot air temperature. If the intake/discharge pressures can't be achieved by the compressor( i.e. leaking compressor valves), for a given ambient hot air temperature, it may indicate it's time for a new compressor or a rebuild.

Best regards,

CJR
 
I recently replaced my compressor, expansion valve, drier, pulled a vacuum on the system and back-filled with refrigerant, and the cooling was still terrible on mine, so you're not alone. For round two I'm installing a new parallel flow condenser, so we'll see how that does.
 
So if you're going to work on your own AC, you're going to need the proper tools. At a minimum, you'll need a vacuum pump and a manifold gauge set. On Amazon there's a set that works well enough for about $100-$150 ish.

First question: Did you do any work to this outside of what you already have stated?

2nd: I need pressures. That's where the manifold gauge set comes in handy. At idle, high and low side pressures AND the ambient temperature w/ relative humidity.

Off the bat, I'd say it's either charged too much or too little. Trapped air in the system can also make it perform poorly, especially when the vehicle is stopped.
 
Thanks for all of the input so far.

I have added the Red Angel but I have been using the gauge that comes with freon refils.. I probably should get an actual manifold gauge.
I actually had a shop look at the AC but they could not figure out that there was a service valve that had to opened before you could get any readings off of the high or low pressure side connects.
Is there anything that regulates how much freon is used at one time? for example something that gives it more freon on hot days and less when its not needed or is it just a set amount no matter what?
 
The service valves (I also just learned this) are for replacing the compressor without discharging the refrigerant. All the way back out and you can't charge. All the way in and the system won't work. Mid way is service position.

3cFsLpB.jpg


I recommend doing a litle reading up about Thermal Expansion Valves. It's not necessarily more refrigerant when hot/less when not, but it's what controls the refrigerant flow based on evaporator temperature. On the older XJs it looks like an aluminum block with a disc on top. It should be at the very back of the engine bay.

Since you have the gauge that comes with the 134a charge kit, what's that one read with the AC on full blast at idle?
 
Some additional thoughts to consider when working on AC systems;

1. Pulling a vacuum on an AC system, recharging,and then poor AC cooling.

If the AC system HAS PIN HOLE LEAKS and we pull a vacuum on the AC system, ambient moist air can be drawn into the AC system. The additional moist air added in the refrigerant can now degrade the cooling as ice-crystals form in the refrigerant.

2. Adding Red Angel FIRST is the better approach as it FIRST seals any minor porosity of AC components. Sometimes, adding another treatment of Red Angel improves the sealing of leaks in the AC system. If poor AC cooling is still occurring, after two(2) treatments of Red Angel, it's probably time to pull the AC system apart and replace some leaking components.

A well sealed AC system prevents the pulling of ambient moist air into the AC system when you pull a vacuum on the AC system. Now the vacuum only extracts the moisture from within the AC refrigerant and does not pull in any additional moisture through unsealed pin-holes in the AC system.

3. Some people use the refrigerant gauges to pull a vacuum on the AC system. Not a good practice. The best technique is to use a small portable "micron gauge" with the vacuum pump. Evacuate the AC system till you get to about 5000 microns or less, then recharge with refrigerant. How low a micron reading you can obtain is typically a good measure of how well-sealed the AC system is.

4. Rely on accurate AC gauge kits to measure the operating suction and discharge AC pressures at a particular ambient temperature. Small gauges, on refrigerant charging cans, are not very accurate. Remember to "jumper-out" the low pressure switch as mentioned in the FM. This eliminates the cycling of the low-pressure switch when reading the pressures. When done, pull the "jumper-wire" off the low-pressure switch.

Best regards,

CJR
 
Thank you for the picture that describes the service valve.. It explains a ton.

When I hook up the refill gauge and have the ac on max at idle the pressure reads 0 and I charge it the pressure shows it is building but then slowly goes back to 0... I assumed there was a leak but I could not locate any dye and they AC still blows cold on cool days.

I think CJR is right... I need a proper gauge and vacuum pull to evacuate the system an see if there is a leak.. I will have to research what a jumper-out / jumper-wire means.

Thanks for all of the input!
 
Please don't do that jumper thing. I don't even care of the FSM says to, don't. It's there to protect the compressor. The pressure at which the switch turns it off is around 20 psi or so. The refrigerant can will have way more pressure than that. Start charging with the AC off. Add as much of the can as you can until pressure in the system equalizes, then start it on max. The compressor might cycle a couple of times, but it's not a big deal if it does. If it runs constantly without refrigerant to entrain oil and lubricate/cool the compressor, you'll burn it up.

Now, with that out of the way, you can most certainly vacuum the system through the manifold gauges. When I'm on a side job, it's exactly how I do it. After vacuuming the system, isolate the yellow hose and you can watch for leaks. If there's a leak, even a pinhole, you'll see it on the manifold gauges. It should hold vacuum. If it doesn't, you obviously have a leak.

You say the pressure is zero and it goes back down to zero. That sounds under charged. It will read the pressure from the can at first as it equalizes. Keep charging until the pressure reads a stable 40 psi. You should refer to pressure/temperature charts for the exact pressure, but about 40 will get you functional AC. Charge for a few seconds, wait, check, repeat.
 
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