What are AC pressures supposed to be

[MrShorty]

A few years ago, I bought simple 134A AC pressure gauge (my '92's system was converted to 134A). The package had a little chart on it that showed what high and low side pressures should be based on underhood ambient temperature. Of course, the package hasn't survived a couple of years.

This year, my AC system doesn't cool very well. I hooked up my pressure gauge and (engine at idle), I get ~40 psi low side and 300-350 psi high side. Without the chart, I don't know if this indicates a low refrigerant charge or some other condition.

Basically I need some basic diagnostic help. Do my pressures indicate that I need to add refigerant and see what that does, or does it indicate other problems.

 

[Blee1099]

Whats the ambient temperature right now? If its about 90 degrees, It should be 200-220 PSI on the high side & 25-30 PSI on the low side at idle.. With the tac at about 1500-1750 your high 220-240 and low should be 20-25 PSI. Just a guess but I wonder if your by chance overcharged or have a bad orifice tube since your running so high at idle.

 

[Glacier991]

I'm with Blee. It is a little bit of black magic to have "absolute" pressures.... because there are so many variables. The cooling you feel in the cabin is based on the low side pressure. The lower it is, the lower the vent temps should be. Remember that the pressure in PSI of 134 closely mimics the air temps it will produce when the system is running. So 40'ish will produce 40'ish at the evaporator... plus some rise in temps due to cooling etc.

High pressures on the high side can be due to several factors... but often is due to inadequate cooling at the condensor. Overcharge can also cause this condition.

A partially clogged orifice tube usually leads to high high side pressures but too low low side and a faster than normal cycling.

If you have not added any refrigerant to the system recently.... I am suspecting your condensor is not operating efficiently.... the obvious first check is for obstructions... bugs, leaves etc....
Next make sure you are getting full airflow over the evaporator. If the low side does not have the heat removed it will also produce the type of sympoms you have. Blend door problems are well known in our beasts.

 

[MrShorty]

Thanks for the reminder about cleaning the condenser. I had thought of it once, but then forgot. I went out and cleaned out the radiator and condenser and found a chunk of cardboard stuck in front of the condenser, so I got that removed and cleaned out the fins. Haven't had enough chance, yet, to see if that makes much difference. But we'll keep working on it.

 

[Maniak]

Just an extra FYI for anyone else looking for pressures.. If you have a Variable Oriface tube, your pressures will be different.

With the VOR, my high side pressures are higher at idle than they were with the regular oriface tube (I changed to a VOR when I changed a leaky connector). At idle, at 95-100F ambient temp and using a VOR my high pressure side is about 300+.

~Mark

 

[ma96782]

msmith65 had posted this link in another thread.....it has some hints on how he got his conversion cold.

http://www.autoacforum.com/messageview.cfm?catid=2&threadid=15368&FTVAR_MSGDBTABLE=

Aloha, Mark

PS....as for pressures on your conversion:Operating pressures and R-134a capacity and oil are your guess for your conversion vehicle (AS A COMPARISION my '94 Explorer stock w/ R-134a, calls for: a low side at 22-50 psi and high side at 160-250 psi, 36 oz. R-134a, and 7 oz oil)