Seeking Help in Troubleshooting AC

[Rtd16]

I just finished rebuilding a 1993 Cherokee that I converted from R-12 to R-134A, I replaced the entire system - all hoses, compressor, condenser, evaporator, expansion valve, drier and pressure switch.
Drew down vacuum, found and fixed a leak at the hot side connection to the expansion valve. Drew down again and held vacuum. I drew in the first can of refrigerant (I'm using a high and low pressure gage manifold set up).
Seemed to be ok until I went to get the next can in, but can’t draw in any more refrigerant. Low side pressure just climbs and sits at 80-90 psi. High side sits at 150 ish. I have cool but not cold air from the vents, the line from the outlet of the expansion valve to the evaporator valve frosts up, and the evaporator outlet lines to the compressor are hot. I unplug the pressure switch at the drier and the aux fan turns off and I can hear the compressor click. Same if I plug it back in, fan turns on and I can hear the compressor sound like it's kicking on, but the low side pressure just stays at 80-90. I don't think the compressor is bad as it’s new. Any help you provide would be appreciated

[JohnHere]

The 80-90 PSI reading on the low side is considerably more pressure than that in the can of refrigerant and is why the system won't accept any more.

Is the compressor running at an engine speed of ~1,800 RPM when you're attempting to charge it?

Are you sure you have an unobstructed path from the can tap valve, through the yellow hose, through the Manifold Gauge Set (MGS) low side, through the blue hose, through the service-port adapter, and finally, into the system?

Sometimes, MGS hoses have check-valves inside them and won't flow when inadvertently reversed end-for-end.

Maybe you've already tried these additional suggestions, but sometimes we overlook the obvious:
The can tap-valve must be in the fully clockwise position for self-sealing cans, fully clockwise then fully counter-clockwise for the older piercing-style cans; The MGS low-side handwheel must be fully opened in the counter-clockwise position; The MGS high-side handwheel must be fully closed in the clockwise position; And, the thumb-wheel on the low-side service port adapter must be fully opened in the clockwise position.

Note well "clockwise" and "counter-clockwise" in the above suggestions.

Also worth checking is whether the Schrader valve or GM ball-style valve in the low-side service port haven't disintegrated, possibly blocking refrigerant flow into the system.

Lastly, does the can of refrigerant have anything in it?

[Rtd16]

Thank you for the reply
No, I was not revving the engine up when charging. That's not something I've had do with other vehicles so I didn't think about that.

This weekend I tried again. Low side pressure dropped down to about 15-20 psi a few times, and I was able to get about 1/3 of a can in. After I got some additional refrigerant in, it exhibited the same behavior of rising to about 80-90 psi and just sitting there. So I think I have 1.5 12 oz cans in, which is still short of the total amount that should be in the system.

Should I give it another go with revving the engine? I'm worried this compressor is constantly running and not properly cycling which may cause damage.

Thank you again for your help

[JohnHere]

No, I was not revving the engine up when charging. That's not something I've had do with other vehicles so I didn't think about that.

Run the engine at about 1,800 RPM with the A/C system turned on while charging it. If everything is working as it should, the compressor will engage and draw-in the rest of the refrigerant through the low side as soon as you reach the minimum low-pressure threshold.

This weekend I tried again. Low side pressure dropped down to about 15-20 psi a few times, and I was able to get about 1/3 of a can in. After I got some additional refrigerant in, it exhibited the same behavior of rising to about 80-90 psi and just sitting there. So I think I have 1.5 12 oz cans in, which is still short of the total amount that should be in the system.

If you have only 18 ounces in the system now, that's a lot short. According to my reference material, the original specifications for your vehicle call for 38 ounces net weight of R-12, and 4.5 fluid ounces of mineral oil. So you should have at least 30 ounces of R-134a in the converted system for it to cool properly.

Should I give it another go with revving the engine? I'm worried this compressor is constantly running and not properly cycling which may cause damage.

As mentioned above, the compressor won't run with the system turned on until you reach the minimum low-pressure threshold—in other words, the low-pressure cut-off switch must "see" sufficient pressure to energize the clutch and engage the compressor. If the switch is working properly, there's little risk of damaging anything "assuming" the proper amount of oil is in the system.

While we're at it, let's review some of the reasons why the low side can register such high pressures:
1. Overcharge.
2. Air and moisture in the system.
3. Lack of condenser airflow.
4. Stuck-open TXV.
5. Too much oil.
6. Faulty compressor reed valve.
7. Blockage or restriction.

I recall your mentioning that the system had a vacuum leak early on. That leak definitely sucked-in air and moisture. If you didn't replace the R/D a second time and then thoroughly evacuate the system again before recharging it, then air (a non-condensable gas) and moisture are still present in the system.

I don't recall your mentioning the condenser fan. But if it doesn't come on with the A/C system, then the fan is suspect.

How much PAG oil did you add, where did you add it, and did it come from a sealed bottle?

[Rtd16]

I replaced everything when I did the work as it came as a kit, so I followed the instructions that came with the compressor - drained the existing oil it came with and added half of the 8oz bottle of PAG oil the kit came with to the compressor then the rest to dryer, condenser and evaporator.

After I fixed the leak I did not replace the dryer, but I did pull vacuum on the system again to get the moisture out before adding the refrigerant.
I am also using the gage manifold set as you described.

It's been raining the last few days but when it clears off I'm going to try again revving the engine up to see if that makes a difference.

I did want to clarify what I'm seeing on the several attempts I've tried to charge the system:
-As soon as I turn on the AC (max) with the engine running, I can hear the click of the compressor and the auxiliary fan turns on. I never hear the compressor kick off during this entire time.
-Initially, I will see low side pressure drop down to 10-15 psi, and high side rises (probably around 75-100psi if I remember correctly).
-I can do 2-3 cycles of admitting refrigerant (I can see it flow through the manifold sight glass), where low side pressure will drop when I close the low side manifold valve and rise up to 50ish when I open it. High side pressure rises as high as 125-150 when I do this.
-After about the third try, it seems low side pressure will just rise to 80-90 and sit there, regardless of cycling the gage manifold valve. High side sits at about 150psi
-The high side outlet of the expansion valve becomes covered in frost (not sure if that's normal)
-Again, I'm not hearing the compressor cycling at all during this entire process, it appears to be running the whole time. In other vehicles I've charged I can distinctly hear and see the compressor cycling on an off even at idle.

Thank you again for helping me get through this, I really do appreciate it.

[Rtd16]

I also got this from JeepAir which is the company I got the parts from:

"That system uses a binary switch which is not a clutch cycling switch. A binary switch is a high and low safety cut off switch. It closes the circuit to the compressor (shuts off) if the high or low goes outside it's normal operating ranges. Typically if the binary switch fails you would have no power to the a/c clutch. Your Jeep should have a thermostat that has a probe that inserts into the evaporator, the probe will sense the temperature and send a signal to the compressor to cycle it off to prevent evaporator freeze up.
CCOT (Clutch cycling orifice tube systems) found on TJs for example cycle the clutch on and off regularly. I would check your high and low side gauge reading to verify you are in normal ranges as well."

So this explains my misunderstanding of why the clutch isn't cycling.

[JohnHere]

I also got this from JeepAir which is the company I got the parts from:
"That system uses a binary switch which is not a clutch cycling switch. A binary switch is a high and low safety cut off switch. It closes the circuit to the compressor (shuts off) if the high or low goes outside it's normal operating ranges. Typically if the binary switch fails you would have no power to the a/c clutch.

That is correct. A binary switch (two functions) is a safety switch, just like a trinary switch (three functions), which also controls a fan (or fans).

But it should say, "It opens the circuit to the compressor (shuts off)... .

". ... . Your Jeep should have a thermostat that has a probe that inserts into the evaporator, the probe will sense the temperature and send a signal to the compressor to cycle it off to prevent evaporator freeze up. CCOT (Clutch cycling orifice tube systems) found on TJs for example cycle the clutch on and off regularly. I would check your high and low side gauge reading to verify you are in normal ranges as well."

So this explains my misunderstanding of why the clutch isn't cycling.

No misunderstanding at all. The compressor will cycle depending on the temperature of the evaporator, as Tim explained earlier. If the evaporator gets too cold, the thermostat will cycle the compressor off before evaporator freeze-up. When the evaporator temperature rises, the thermostat will cycle the compressor on again.