1993 Jeep Cherokee A/C Performance Issues

[mtwask01]

I’m having A/C issues on a 1993 Jeep Cherokee with a 4.0L engine. The background on this Jeep is that it had an original R-12 system that worked well up until it was wrecked two years ago. The wreck damaged the condenser and I believe this ultimately led to it leaking out all of the refrigerant. Below is my attempt to detail what I know in a coherent and detailed manner and thanks in advance for reading through this lengthy post.

A few months ago, I bought a complete kit that included a new compressor, condenser, receiver drier, and expansion valve. I chose not to replace the evaporator due to the labor involved and I did not suspect any problem with it. I drained all of the shipping oil out of the new compressor which I believe was PAG 46. I did several flushes with mineral oil and then finally measured out 3.5 ounces of oil to fill the compressor. The factory service manual shows that the evaporator retains 1 ounce of oil so I deducted that from the 4.5 ounces system capacity to give me 3.5 ounces to add. I installed the compressor and other components and performed an evacuation. After the evacuation, I charged with nitrogen to check for leaks. After I was satisfied that there were no leaks, I performed another evacuation and then charged the system with 2.38 lbs of R-12 using a 30# tank and scale. After charging the A/C seemed marginal at best. I can’t remember the exact readings that day so I took some pressure readings today. Below are the details from today’s readings.

Ambient temp was 95 with about 50% humidity. With the windows down, blower on max, and a/c mode on normal (non-recirculate) the vent temps were about 70 degrees. With the engine at about 2000 rpm, low side was 40 and high side was around 290-315. When I let it back down to idle, low side goes to 52 and high side about 265. I had a temperature clamp on the suction line just past the expansion valve reading about 63 degrees. The manifold gauge set I was using has superheat calculations. I was getting between 4-12 degrees of superheat throughout this testing session. After letting the system settle for a while, I sprayed down the condenser and the high side came down to around 230-250 psi.

Here are my thoughts from these observations. From my understanding, these pressures seem kind of high for R-12 and it would seem like this is a condensing issue. The new condenser is a serpentine style as was the original. The original condenser did seem to have more cooling fins compared to this new one. Another variable is that the new expansion valve is tuned for R-134a. The expansion valve is a UAC unit (P/N: EX 9448C) and I looked up their evaporator core for this vehicle and it is a plate and fin style (P/N: EV 4178PFXC). The original OEM evaporator that is still in this vehicle is a tube and fin design. Perhaps the expansion valve has a different capacity/properties with the difference in evaporator core design and refrigerant. Some other things to note is that the fan clutch is a new OEM unit that was installed a few months ago. The auxiliary electric fan is functioning and seems to move plenty of air. The sight glass on the receiver drier is clear with no bubbles.

Sorry for the lengthy post but I wanted to give you guys as much detail as possible. Any advice or insight would be greatly appreciated.

Thanks,
Matt

[tbirdtbird]

Of course you purged all your manifold hoses while you were setting up after vacuum.?
Does your vacuum pump use an electric motor?
How deep a vacuum did you achieve?
How long did you vacuum?
You are certain the oil capacity is 4.5 oz?
Where did you add the 3.5 oz
I agree that you do not seem to have enough condenser. Are there air dams in place so that the air doesn't just take the easy way and go around the condenser then into the radiator?
Is your condenser plumbed correctly? (large diam hose into TOP)
The FD is directional, are you sure it is correct?
Was the system open to the atmosphere during the two yr interval?
Did you flush the hoses?
I suspect your true superheat is higher than you posted.

Answering all these questions will help us help you

Here is a list of reasons for a high side to be too high, from one of the masters (Ice-n-tropics):

Non condensables (air) inside refrigerant circuit
Blocked circuit inside evaporator coil
Evaporator fins loose from tube contact or corroded away
Evaporator coil with internal by-pass due to faulty manufacture or over pressurization during service
Expansion valve blocked open mechanically or not closing due to bad contact with capillary tube bulb against suction tube. Causes frost/ice on suction line.
Too much oil in system will give warmer temps than indicated by the pressure (doubt this is your problem)

[mtwask01]

Thanks for the reply! To answer your questions:

Yes, all manifold hoses were purged before charging. I vacuumed for 3 hours and the system got down to 29 in Hg. I did not have a micron gauge to get a precise reading but after evacuating for a while I isolated the vacuum pump from the system and there was no decay. Also when I reopened the valve to finish the evacuation there was no tone change or any additional vapor coming out from the vacuum pump so I'm pretty confident the evacuation was successful. The vacuum pump was an electric Harbor Freight 2.5 cfm unit brand new out of the box with fresh vacuum pump oil. Also while evacuating, I started the engine and allowed the entire engine bay to warm up. I also turned on the heat and recirculated it so that the evaporator core also got warm to help with the evacuation. I am confident that the system takes 4.5 ounces of oil as this was the specification listed in the factory service manual. I added all the oil to the compressor. All hoses were also new so no flushing was required. Everything that I see indicates that the condenser is plumbed correctly as it has a #8 fitting at the top where the high pressure discharge vapor will flow at the top and then a smaller #6 hose at the bottom where the liquid discharge goes to a hose leading to the filter drier. The filter drier has "IN" stamped on the inlet side which is where the hose exiting the condenser is attached. As far as the superheat is concerned, when I first started the a/c it was much higher (like 20-30) and then worked its way down when the pressures settled. In the two year period since it last worked well, the system had not been opened to atmosphere. I recovered the remaining R-12 (about 1/2 lb) and then put a very small amount of nitrogen in the system so that I could take components apart without sucking air in. I think I've covered my bases as far as not introducing non-condensables into the system and following procedure. I've purchased a parallel flow condenser from a later model Cherokee that I think I'm going to try next and I may also try the original equipment expansion valve that was tuned for R-12.

One more thing I can think of that could be an issue. Is it possible that more than 1 ounce of oil accumulated in the evaporator? The service manual said 1 ounce but if the refrigerant from the original system was slowly leaking out, is it likely that more could end up in the evaporator? I'm thinking that the manufacturer probably developed that chart from fully charged systems that were then recovered and then analysed for the amount of oil in each component. This may be a different result than a slowly leaking system. If more than 1 ounce of oil was in the evaporator, than I could see this causing too much oil in the system.

[tbirdtbird]

Your testing should be done on recirculate.
When you sprayed down the condenser, what was the lo side reading and the center vent temp?

[Cusser]

Here is a list of reasons for a high side to be too high, from one of the masters (Ice-n-tropics):

Non condensables (air) inside refrigerant circuit
Blocked circuit inside evaporator coil
Evaporator fins loose from tube contact or corroded away
Evaporator coil with internal by-pass due to faulty manufacture or over pressurization during service
Expansion valve blocked open mechanically or not closing due to bad contact with capillary tube bulb against suction tube. Causes frost/ice on suction line.
Too much oil in system will give warmer temps than indicated by the pressure (doubt this is your problem)

How about poor/malfunctioning condenser cooling fans (fan clutch or electric fans)

[tbirdtbird]

For sure, Cusser

[JohnHere]

A few months ago, I bought a complete kit that included a new compressor, condenser, receiver drier, and expansion valve. I chose not to replace the evaporator due to the labor involved and I did not suspect any problem with it. I drained all of the shipping oil out of the new compressor which I believe was PAG 46. I did several flushes with mineral oil and then finally measured out 3.5 ounces of oil to fill the compressor. The factory service manual shows that the evaporator retains 1 ounce of oil so I deducted that from the 4.5 ounces system capacity to give me 3.5 ounces to add. I installed the compressor and other components and performed an evacuation. After the evacuation, I charged with nitrogen to check for leaks. After I was satisfied that there were no leaks, I performed another evacuation and then charged the system with 2.38 lbs of R-12 using a 30# tank and scale. After charging the A/C seemed marginal at best. I can’t remember the exact readings that day so I took some pressure readings today. Below are the details from today’s readings.

I think your oil balancing logic is sound. You installed a new condenser, which will also retain 1/2 ounce to 1 ounce of oil after running, and a R/D which will retain about 1/2 ounce. So after running and stabilizing the system, the compressor will theoretically retain about 2 ounces of oil.

In addition, the refrigerant and oil capacities match what I have for your vehicle.

With what PSI did you nitrogen-pressurize the system? The evaporator, a low-pressure component, can tolerate about 100 PSI maximum.

One more thing I can think of that could be an issue. Is it possible that more than 1 ounce of oil accumulated in the evaporator? The service manual said 1 ounce but if the refrigerant from the original system was slowly leaking out, is it likely that more could end up in the evaporator? I'm thinking that the manufacturer probably developed that chart from fully charged systems that were then recovered and then analysed for the amount of oil in each component. This may be a different result than a slowly leaking system. If more than 1 ounce of oil was in the evaporator, than I could see this causing too much oil in the system.

I don't think so. Typically, only 1/2 ounce to 1 ounce of oil will accumulate in the evaporator as you intimated.

[mtwask01]

Thanks for all the responses guys. To follow up on some questions. When I sprayed down the condenser the low side dropped to 40 psi and high side was around 225-250 psi. I didn’t take a reading at the vent to see what it dropped down to. I would take a guess that it dropped about 10 degrees from 70 down to 60. When I pressurized the system with nitrogen, I went to about 130 psi. I figured that was a safe pressure for the evap since car interiors on a hot day can reach 120 degrees or more which would correspond to about 150 psi with R-12.

Another observation I had is that the system voltage drops quite a bit with a/c on. It shows just above the red area on the gauge (maybe 12-12.5 volts). How bad of an impact do you guys think this would have on the performance of the electric fan?

[tbirdtbird]

OK, we don't guess in AC repair. Repeat the water on the condenser test and measure the vent temp and post, please. At the conditions specified (max fan, max cool, recirc, doors open, RPM 1500-1800.)

Your voltage observation is a good one. If you are only getting 12.5 volts at speed, then either you have a bad/corroded connector someplace or your alternator is shot. At speed, a modern alternator should put out a regulated 14.2 volts or thereabouts. Voltage lower than that will create a very high amp draw by the fans and the comp and put quite a drag on the motor.

Please post a pic of your voltmeter, or multimeter, have never seen one with colors on it ("just above the red" ?? what is that)

[JohnHere]

When I pressurized the system with nitrogen, I went to about 130 psi. I figured that was a safe pressure for the evap since car interiors on a hot day can reach 120 degrees or more which would correspond to about 150 psi with R-12.

Yes, but...
The TXV between the high and low sides regulates the evaporator pressure to about 30 PSI with R-12 so that the evaporator can do its job. The TXV doesn't "care" about the ambient temperature. The evaporator by its very nature isn't designed to withstand high pressures without leaking or disintegrating.