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How much refrigerant to add 1999 Ford Ranger

primo382 on Wed August 31, 2011 12:22 PM User is offline

Year: 1999
Make: Ford
Model: Ranger
Engine Size: 2.5L
Refrigerant Type: R134a
Ambient Temp: 80 Deg F
Pressure Low: 23
Pressure High: 240
Country of Origin: United States

Thanks for having me on the forum. I've been a spectator for quite a while but decided to throw my hat in the ring.

I purchased a 1999 Ford Ranger 2.5L that I am fixing up to sell and the last step is charging the A/C.

I added one 12 oz. can of refrigerant and my compressor was still short cycling. Before I added another can a day later I took static pressure readings from both sides and they were pretty far off. My low side static reading was just over 50 lbs and high side was 105 lbs. As soon as I started the vehicle and turned on the A/C the low side would drop down to 20 and the compressor would shut off.

I added a second can which brought my low side pressure to 24 lbs while running and high side to 240 lbs. but as soon as the engine was revved to 2000 RPM the low side would drop and the compressor would kick off again. The system is blowing very cold (i did not put my thermometer in the vents) but the compressor is still short cycling.

I left my gauges hooked up overnight so I could take the static reading again in the morning. They are still pretty far off. Low side static reading of 63 lbs. and high side of 110 lbs. At 1000 RPM the low side drops to 24 lbs and high side pressure is around 240 lbs still.

Approx. how many more ounces of refrigerant do I need to bring the low side pressure to 35 lbs at 1000 RPM?

Thank you in advance for your help.

HECAT on Wed August 31, 2011 1:42 PM User is offline

30 oz is the factory fill.

Systems are charged by weight, and performance is evaluated by pressures.


HECAT: You support the Forum when you consider for your a/c parts.


primo382 on Wed August 31, 2011 3:51 PM User is offline

Thank you for your response.

So if my low side pressure is at 24 lbs. and drops from compressor suction is it safe to assume I need at least one more 12 oz. can of R134a, perhaps more?

Edited: Wed August 31, 2011 at 3:52 PM by primo382

HECAT on Wed August 31, 2011 4:32 PM User is offline

You cannot make any assumptions regarding pressures until the system is at full charge (by weight). Static pressures mean nothing more than refrigerant is in there; they do not have any bearing on how much is in there. The system can cool with low charge but will not move the lubricant properly; so the vent temps currently mean nothing. Running low on charge is the number one way to kill a compressor from lack of lubrication. I think some of your readings may indicate either a gauge problem or an orifice problem, but who knows until the full charge is installed.


HECAT: You support the Forum when you consider for your a/c parts.


primo382 on Fri September 02, 2011 8:57 AM User is offline

I appreciate the insight. I guess my confusion stemmed from measuring capacity (how much liquid is in the system). But I did realize that low side pressure was not much different with 20 oz. versus 30 oz. however as RPM's increased 20 oz. was not enough to sustain adequate low side pressure, tripping the pressure switch and disengaging the compressor clutch.

Thanks again for all your help. I used to just hook up my junk blue parts store hose and put a few cans in until it got cold and the compressor stayed running. I learned quite a bit this time around. Again, Kudos to this great resource you guys have here.


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