My Expy sat for 2+ years and due to a new child, I put it back on the road late last fall. Last week, we had our 1st "must use AC" day and it didn't work.

I listened to see if the compressor was cycling and it never engaged. After some suggestions from the Expy/Navi forum, I picked up a gauge and r134a kit.

Outside temp was about 60F, I think my therm was showing 57F to be exact. When I checked the low side pressure with the engine off, the gauge reading was below 15. It made sense that the compressor wouldn't start at the pressure. I put in about a 1/2 can with the motor running A/C on max and it was showing 40, but still no compressor cycling. I then shut the motor down, let it sit for about 3 mins and restarted. This time, the compressor started to cycle, it was cycling quickly though, I'd say about every 3 seconds.

With the compressor cycling, the pressure would jump from 17 to 40. With the motor off, it would read 50. Still no cold air. I put in about 14oz of R134a.

I guess what I need to know is if I should I keep adding R134a until it stabilizes around 40, even when the compressor is cycling? Also, where should the static pressure be? It's sitting at 50 now. I've been told it should be around 80, but I'm thinking that might be dependent on outside temp.

BTW, I'm now thinking that the reason that I'm not getting cold air and the compressor keeps cycling is because I need to add a lot more R-134a. I think when the compressor starts and draws coolant, the low side pressure drops to low, causing the compressor to turn off. When the compressor cycles off, it sees the low side pressure at 40 and starts it up again causing the coolant draw and low side pressure to drop to low again. Am I right on this thought? It looks like I'll need to add another can to get the pressure to stay above 22 when the compressor is running.

One other thing... Does anyone know how much R-134a the system takes? I need to know because I have access to a vacuum, but I need some adapters.

Thanks!!

 

The ONLY way to know how much refrigerant is in the system is to put it there. As in, extract what's installed, and add the amount you need. There should be a label for refrigerant capacity on the radiator support, the underside of the hood, or on the evaporator. Most Ford's I have seen have the label on the radiator support.
You describe a system that has the symptoms of low refrigerant cycling. BUT, the pressures, 17,40 and 50 all depend on ambient temperature. R134a will have a specific pressure at 57F. At 32F it will have a different pressure. When the compressor pulls the R134a out of the evaporator, it boils, and absorbs heat. When the compressor has pulled it all out, or the boiling pressure would be below 32F[well, maybe 27F or so] the compressor will stop pulling, or evacuating the evaporator so you don't get an evaporator covered with a coating of ice. The idea being to keep the temperature/pressure right near the freezing temperature of water. That dehumidifies the air, and produces the most cooling without freezing the evap.
Given it sat, I would look at all the snap-lock connectors for dirt mixed with oil. If the oil got out and trapped the dirt, then refrigerant can take the same path. You may need new O-rings on your snap-locks. You could also put Nylog[I think] on the fittings before you put them back together. You would then pull the vacuum, and then put in the correct amount of refrigerant. Other than that, it is a guess on how much to put in.
tom

 

Thanks for the explanation on how the system works! Knowing what the system is doing makes diagnosing the issue much easier. Also, thanks for the tips on what to look for.

As of yesterday evening ambient temp of 60f, I added more refrigerant for a total of about 20 oz. When I hit a total of about 18oz, the compressor stayed running and the low side pressure stayed at a consistent 25. I added the extra 2 oz's because I was trying to get the pressure up to 35, but it didn't budge. Since it didn't move and I figure it might be because of the outside temp, I figured I'd better leave it alone for now. I'll check it again when the outside temp is closer to 80F.

Thanks again on what I might need to do to make sure the system no longer leaks. When I get more time, I'm going to do what you said. It makes sense to replace the o-rings and I'll look up what Nylogs are to get them added too.

 

Nylog is a chemical goo used on the connectors to help them stay sealed. It is similar in consistency to STP. A long-chained molecule of some sort. Once you get it on something, it is hard to get it off. Always seems to leave a coating. It functions to cushion the O-rings from the constant vibration and motion that they encounter in a moving vehicle bouncing and from the flow of the refrigerant through the system. I *think* it make a longer lasting lubricant for the O-rings, and thus extends their life significantly. If there is motion of any sort, the Nylog tends to dampen it down.
There are other words used to describe Nylog[there are two varieties, IIRC], such as snot. It is weird stuff.
tom