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Adding refrigerant not stopping compressor short cycle

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Jul 5, 2007, 4:09 PM

Post #1 of 8 (10326 views)
Adding refrigerant not stopping compressor short cycle Sign In

Just replaced my condenser, accumulator, and hoses on my 95 Cherokee Sport (4.0L, inline 6) with a great deal of help from Tom on my previous "high side port" thread. While charging the system with 12oz R134a cans I used an off the shelf Interdynamics $15 charging hose with gauge. I dispensed about 2 cans in the system with the AC on max and noted that although the system was blowing cold the compressor was short cycling. When off the pressure on the low side rises to 45psi ("normal" or "filled") and the compressor engages. In about 20 seconds the pressure falls to around 20 psi ("low") and the compressor disengages. Normal enough I thought and I started adding can number 3. Problem is after the third can it seems to be acting the same way. The 'compressor on' pressure doesn't seem to want to go above 20 psi even though I added another can. I can't hear anything to indicate a rapid leak behind the dash. There are also no evident leaks in the engine compartment (used UV dye). I've read in several places that it is unwise to go over 3 cans. I would suspect the gauge but the consistency in the behavior seems to suggest the compressor switches are doing what it is supposed to. Could it be the ambient temperature since I was charging when the air temp was a cool 70 degrees F? A coworker (who is also an AC tech) has also suggested that if water is entrained in the system the evaporator could be freezing very quickly. If this were so wouldn't my pressure be rising with the addition of more gas? Should I attempt to add another can? I'm considering just waiting for a hot day (85 deg +) and taking more readings.

Tom Greenleaf
Ultimate Carjunky / Moderator

Jul 5, 2007, 10:34 PM

Post #2 of 8 (10318 views)
Re: Adding refrigerant not stopping compressor short cycle [In reply to] Sign In

Don't add any more refrigerant now! My chart shows 32oz for a 95 Jeep Cherokee and you are already 4oz over that which is already enough to cause problems.

The short cycling could be the high side switch shutting system down and you need that # now. It would shut down with the low reading hitting 20 -- that part is expected.

Let system sit for a while and check for static pressure. The 45 mentioned when compressor is off was too quick for it to stabilize. It could have been leaking out fast while you were charging it. Static pressure should be about the same # as the ambient temp in F. It would read that on both sides when at rest. If lower than that system is very low as it doesn't take much to get the static pressure to appear proper.

That gauge/can tap is not telling you the system is filled -- it can't!! You can't tell without both pressures while compressor is on and even that only tells you that you are in range or not. To know what's in there you would have to recover and weigh it.

Evap could be icing up which will drop low side pressure and cycle compressor. From just what you have right now it does behave like it's low on charge but don't be hasty.

If the system was charged without a vacuum there is moisture and air inside and oil has absorbed more than it would like and new drier has absorbed about all it will. A VERY long well held vacuum is in order and start over with just the 32oz charge.

You might be ok and ambient temp of just 70F is low enough to have normal cycling.

How much oil did you add for the new parts?

I may be away without computer for the day but will be right back or someone else here can suggest away. I would just leave it alone for now and gather some more info if you can and if you have to go by low side pressure only a warmer day will help a lot,

Long retired now


Jul 6, 2007, 10:45 AM

Post #3 of 8 (10315 views)
Re: Adding refrigerant not stopping compressor short cycle [In reply to] Sign In

Thanks for the info. I am going to try it today now that it is warmer out. It has been sitting for a few days too so that should help determine the leak status. If it still short cycles I am going to bring it to a coworker who has proper gauges and a vaccuum pump. I guess you were right about having that high pressure port! When I initially charged it last week it kept the car very cool since I ran it for about a half hour while driving. I turned it off when I realized it was short cycling and also it was only about 75 out that day.

I added about 4 oz of oil by pouring it down the hose. I was unable to tell how much oil was in the old accumulator, condenser, and hoses because everything was rusted together. Some did pour out and some was lost over the years from the leaks and cracks. I did some research and could only find a few sources who said as a rule of thumb to add 2 oz for the accumulator, 2 for the condenser, and 1 for hoses. I made sure I used R134a which did not have oil or stop leak since I didn't want to overdo it. I used one can that had UV dye in it which is carried by some oil as well I'd imagine.

Tom Greenleaf
Ultimate Carjunky / Moderator

Jul 7, 2007, 5:21 AM

Post #4 of 8 (10313 views)
Re: Adding refrigerant not stopping compressor short cycle [In reply to] Sign In

That sounds good with the oil. I don't think dye can really be counted as oil but the 4oz would have been my suggestion for this vehicle. Remember that with oil it starts to be a guessing game that can only be ended with flushing it all out and replenish. Kind of like an engine without a dipstick.

Let us know how you are progressing with this,

Long retired now


Jul 7, 2007, 11:28 AM

Post #5 of 8 (10311 views)
Re: Adding refrigerant not stopping compressor short cycle [In reply to] Sign In

Yesterday I gave the system a test again since I hadn't been running in a few days. Again it was only about 70 degrees out (good news since I'm having AC trouble!). When I first started the vehicle and the AC it ran fine (no short cycle) for a couple of minutes. The low side pressure was just around 20 psi. After those couple of minutes the short cycling began again. I'm probably going to have this vacuumed down anyway since I know people with the tools but perhaps this sheds some more light on the system.

Tom Greenleaf
Ultimate Carjunky / Moderator

Jul 7, 2007, 11:34 AM

Post #6 of 8 (10310 views)
Re: Adding refrigerant not stopping compressor short cycle [In reply to] Sign In

70F is still cool for this. Try putting heater on full blast for a while in car so the A/C would have to work harder for a few minutes. What do you get for output temps? Try fan on full blast, norm or outside air with windows open and cycling may cease especially at idle speeds,

Long retired now


Jul 7, 2007, 12:29 PM

Post #7 of 8 (10308 views)
Re: Adding refrigerant not stopping compressor short cycle [In reply to] Sign In

So this may be OK. I don't have a thermometer or thermocouple here right now so not sure what the output temps are. Weather Channel says the ambient temp here is 75 degrees. I just measured the static pressure without the system running and it was 95 psi (my quotes before were when the system was running and were not stable as you pointed out). I ran up the heater for a while then turned the AC on. It ran for a solid 5 minutes without cycling before I shut it down. The low side pressure port read about 21 psi or so throughout the run. I still would like to get it checked out with both high and low side pressure gauges but for now it seems to be running fairly normal, albeit a bit low, given the tools I have at my disposal. Would you agree?

Tom Greenleaf
Ultimate Carjunky / Moderator

Jul 7, 2007, 12:54 PM

Post #8 of 8 (10306 views)
Re: Adding refrigerant not stopping compressor short cycle [In reply to] Sign In

Thermometer: Just go to WallyWorld or similar and pick up a silly $8 wired digital remote "in-out" thermometer. They are fast and you can measure the real ambient temp taken about 2" in front of condenser and put the wired end in a vent and see both just switching from in and out. The weather's day's temp doesn't count for this. It's what your vehicle thinks it is -- in the sun or not or over a hot driveway, whatever,

Long retired now

Adding refrigerant not stopping compressor short cycle

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