So, recently, the A/C system on my 1996 Ford Escort LX 1.9L wagon has been acting funny. When I turn the AC on, it works well for about 5 seconds, but then I hear the clutch disengage and the RPMs go down. Then a few seconds later, the RPMs go up (presumably as the clutch re-engages), and then the cycle repeats itself.
My first suspicion is that the system is low on refrigerant. With the outdoor temperature around 80 F, I measured the pressure from the low side service port while the A/C was on full blast with the doors open (to keep the compressor continually running), and it was continually fluctuating (every 5 seconds or so) between 24 and 45 PSI. According to the instruction manual that came with the R-134 canister I bought, it states that refrigerant should not be added if the PSI doesn’t exceed 45 PSI.
So, I am confused over whether or not I should add refrigerant to my system. On one hand, the manual says no, but I also know that low refrigerant can cause the compressor to cycle more often. Isn’t that the whole point of adding extra refrigerant – so that the pressure is stabilized on the low side and the frequency of the compressor cycling is decreased? Thanks!
Most likely you are low on refrigerant. The system is set up to engage the compressor when the pressure reaches 45 PSI. As the compressor operates the pressure will drop (and the evaporator gets colder) When the pressure gets to 24 PSI the compressor cuts out to prevent the evaporator from freezing.
When the pressure drops that quickly causing the compressor to cuts in and out quickly, it is a sign of not enough refrigerant.
The system needs to be leak checked and the leaks repaired (the refrigerant went somewhere). Then evacuated, and charged with the proper amount of refrigerant. IMHO this probably is not a DIY project if you intend to do it right.
Neither does either of these statements.
Back to the OP, what you describe is the textbook description of short cycling, the typical result of low refrigerant. There may be a detectable leak, or there may not - it could be the result of gradual (and usually undetectable and unrepairable) seepage, or it could be that a significant leak has developed recently. Knowing when (if ever) it was last charged could give a clue as to which.
In your situation (i.e., planning to recharge it yourself), the logical thing to do is add refrigerant and see if the cooling improves. This would go pretty much hand in hand with the short cycling giving way to a normal cycling cadence. The trick is figuring out how much to put in.
It could take anywhere from half to nearly all of the system’s specified full charge. It’s probably safe to start with a 12 oz. can. Monitor the system pressures and the (dash vent) outlet air temperature with the blower on high and the A/C on Max or Recirculate. Then add in 2 oz. increments, and after each increment let the system stabilize for 5 minutes. When the outlet air temperature ceases to drop, or the high side pressure starts to rise, it time to stop adding. Outlet air temp will normally range from 35-45’ F, depending on the particular car, ambient temp, and humidity.
If the short cycling recurs before this year is over, there’s a significant leak, which should be found and repaired.
I last charged the system about four years ago, when, IIRC, I experienced similar issues with the compressor cycling a little too quickly (and the A/C was not delivering particularly cold air). I had this done at a shop, however, and this would be my first time charging it myself with one of those DIY kits.
BTW, here is the instruction manual which indicates not to add refrigerant if the pressure readings exceed 45 PSI. As I wrote, when I measured with the gauge, it would fluctuate between 25 and 44 PSI. Because of this manual, I wanted to find out if it would be okay to add refrigerant even if the pressure readings didn’t exceed 44 PSI.
Thanks for the helpful advice. I bought a 16 oz. can of refrigerant, so I’ll recharge the system in small increments as you suggested, and will report back my findings.
If this doesn’t work, then I suppose the next step would be to replace the clutch…
Then a small undetectable leak is a reasonable possibility.
Which is good advice (see clarification below) - but in the OP you wrote “refrigerant should not be added if the PSI doesn’t exceed 45 PSI.” This sounds like it needs a minimum of 45 psi before adding, and is essentially what you’re now saying by “I wanted to find out if it would be okay to add refrigerant even if the pressure readings didn’t exceed 44 PSI.” It appears you’re getting confused here.
Now here’s the clarification: the instructions are referring to low side pressure when the compressor is engaged. The 44 psi you’re seeing is with the compressor off, as the system starts to equalize towards its rest pressure. It’s only significant here as the pressure at which the clutch is switched back on after it gets switched off. Ignore it. Futhermore, you’ll likely see a pressure spike when the can valve is opened, and it doesn’t count either. System operating, compressor clutch engaged, system closed (i.e. not currently open to something external like a refrigerant can) - those are the conditions to observe low side pressure.
Those instructions are pretty sloppy about how much to add. Low side pressure will usually run between 25 and 44 psi on cars as a group, but that doesn’t mean that just anywhere in that range is normal for your car. And that “0-24 psi: System may require additional refrigerant” is only useful on a system that doesn’t use the cycling clutch strategy, which as you have observed won’t even let the pressure get below 24.
Get a thermometer (not a medical type, something quick responding and easy to read) that you can stick into an outlet vent and charge by temerature. Watch the low side pressure as a double-check, but don’t try to charge by pressure. It’s too imprecise, and especially difficult without a high side gauge.
Only if you have evidence that the clutch is failing, such as its slipping or disengaging when it has 12 V power and ground applied. When the clutch is turning off when the pressure gets to its low shut-off value, as you have described, it’s working exactly as it’s supposed to.
less than 45 pounds generally means you have a significant leak and will only be bleeding halocarbon into the ecosystem.
Considering you seem to be doing this every five years like clockwork points to a small difficult to detect leak. You can add refrigerant anyway as a temporary fix, kind of like adding a quart now and then to a car that burns oil… except for that whole halocarbon thing.
Less than 45 psi rest pressure would indicate that (normal rest pressure is 80-110), but those instructions are referring to low side operating pressure, which is typically in the 25-44 range. And habs got it backwards, they say don’t add if the (low side operating) pressure does exceed 45.
Also the instructions don’t mention that all of the these pressures (both high and low) are temperature dependent. The high side pressures will vary with the ambient temp at the condenser, humidity, and airflow over the condenser. The low side pressures will vary with the in car temp, sunshine load on car, and the fan speed.
On a hot day, if you don’t have a fan forcing air though the condenser, you will never get the high side pressures down to spec. The system will appear to be overcharged. If you are charging a black car outside on a sunny day, you will never get the system down to 24 PSI.
On the other hand, if you were to measure the pressure in the system in the dead of winter, it could easily be < 24 PSI
FTR while I don’t think it is a problem here, a dead electric radiator fan, or an overcharged system can also short cycle. In these cases, the compressor is cutting out due to the high side safety interrupt switch turning off the compressor before a hose bursts.
D’oh, you’re right! :smack: Sorry for the original confusing statement.
Thanks again to everyone for all the useful advice. Reading through this thread has definitely cleared up some things for me. I probably won’t have time to charge the system until next week, but I’ll keep you all posted with the results.
Isn’t that the whole point of adding extra refrigerant – so that the pressure is stabilized on the low side and the frequency of the compressor cycling is decreased? Thanks!
