Help with R134a A/C pressure readings

cartercd · 07-16-05, 11:45 AM

After hitting 118 degrees in Phoenix the other day, I decided to check my a/c pressure levels to ensure it is operating at its best. I'm having some problems interpreting the results.

Engine warmed up, recirc on, high fan speed, 1500 rpm:

90 ambient temperature the low side was 24 psi and the high side was 295 psi
110 ambient temperature the low side was 33 psi and the high side was 335 psi

The FSM says at 86-95 degrees the low side should be between 22 and 36psi. So my pressure is probably on the low side, but in range. The FSM says the high side should be between 199 and 228psi, so my pressure is high (even after adjusting for above averge tremperature).

In both cases the site glass showed a whitish foam.

The FSM has a troubleshooting guide, but it does not indicate a condition where low side is okay, but high side is high. Could I have air trapped in the system?

Incidentally, my A/C is working fine. It blows cold air and can keep up with 100+ degree temperatures, but I am trying to maximize performance for those really sweltering days.

Thanks for any advice.

Rich · 07-16-05, 12:33 PM

Two guesses. Firstly, I believe the orginal factory installed viscous fan couplers to be marginal at best. So, the too high, high pressure reading could be due to inadequate airflow through the condensor. Second guess is that you could have a partial blockage in the system. Expansion valve in the evaporator would be one likely culprit.

One of these days I will replace my fan coupler and see how that affects the high pressure reading. I may also install an auxillary condensor fan.

For what it is worth, I am informed, but have no extraordinary expertise with AC maintenance. So, appreciate why I say guess.

cartercd · 07-16-05, 01:17 PM

I did take a floor fan and hold it in front of the radiator. It brought down the pressure a little bit, but not much. I am interested in an auxiliary fan that cycles with the compressor, but I want to make sure the sytem is operating correctly before I start making mods. I agree there could be a partial blockage - I was hoping for an easier and cheaper problem! We'll see what other IHMUDers think.

cary · 07-16-05, 02:02 PM

The pressures aren't that far out of line. The usual rule of thumb for 134 a/c is 2.2-2.4x ambient temp. Also 134 works slightly better charged on the low side compared to r-12 that works better with a full charge. Given the lack of airflow at idle on the LC, I don't think you are that far off, if at all.

Riley · 07-16-05, 03:01 PM

Quote:

Originally Posted by Rich

Two guesses. Firstly, I believe the orginal factory installed viscous fan couplers to be marginal at best. So, the too high, high pressure reading could be due to inadequate airflow through the condensor. Second guess is that you could have a partial blockage in the system. Expansion valve in the evaporator would be one likely culprit.

One of these days I will replace my fan coupler and see how that affects the high pressure reading. I may also install an auxillary condensor fan.

For what it is worth, I am informed, but have no extraordinary expertise with AC maintenance. So, appreciate why I say guess.

I have less experience with AC than anybody probably but going through some trouble shooting today at an AC shop and they were commenting that ths high side look too high and the low side normal. They "guessed" the fan clutch as well. They commented that they loose about 10% of their pull every year of age.

Probably not a bad idea to replace it anyway. It's a good thing to do in high temps

inacoma · 07-16-05, 03:25 PM

Quote:

Originally Posted by cartercd

I did take a floor fan and hold it in front of the radiator. It brought down the pressure a little bit, but not much. I am interested in an auxiliary fan that cycles with the compressor, but I want to make sure the sytem is operating correctly before I start making mods. I agree there could be a partial blockage - I was hoping for an easier and cheaper problem! We'll see what other IHMUDers think.

I put electric fan in, with manual cutoff if needed via switch in the cab. works great. oh, and I went throught the whole AC mess last year, replaced a shit ton of parts, had system checked, etc. all to get the damn pressure down and ac cold. there were NO line blockages or anything...vaccum held great. In the end, the charge value was still higher than fsm, but lower than a friends AC (which was super high, and didn't put out as cold ac as me after my dump/charge). So i put the fan in as an Insurance measure...With the fan, AC charge, higher pressure readings than FSM....AC been working strong for a year. Buddies truck was at super high pressures and he ran that way for a long long time with less cold air. I figure there is a pressure bypass if things get too high, so I'm going to stick with the way I did it, and stay cold!!!

later.

kumar

Rich · 07-16-05, 05:19 PM

Excessive pressure results in accelerated wear on compressor. I would think that you don't want to be too far out of line. With respect to compressor life, I don't know what too far out of line is.

inacoma · 07-16-05, 07:39 PM

Quote:

Originally Posted by Rich

Excessive pressure results in accelerated wear on compressor. I would think that you don't want to be too far out of line. With respect to compressor life, I don't know what too far out of line is.

That's something I considered during my AC issues of last year. I also know that the compressor has a pretty conservative pressure valve...so I am pretty sure i'm within spec or I would have blown that a long time ago. I worked hand in hand with an AC shop. We checked almost every inch of the system for temp differential, etc. We just couldn't get the pressure down and still keep cool.. Keep in mind I replaced the evaporator and some switch under the glove box (I forget the name, it's late). After all that, we vaccuumed it out. Then we put in the EXACT amount of freon the Specs called for. Tested the system, nice and cold. tested the pressure .... again, higher than what the FSM stated, however the amount of coolant was dead on. We couldn't figure out why the pressure was so high. I should note that the pressure on my system before I started all this was higher than where I finished...and my truck, at that time, had about 180k miles on it. I also put in the died freon so that if there becomes a leak over time I will be able to find it, but I think I'll be ok

later.

kumar

Rich · 07-16-05, 07:58 PM

With my low pressure in spec my high pressure cycles up and down, at times exceeding spec. I get good cooling.

inacoma · 07-16-05, 08:14 PM

Quote:

Originally Posted by Rich

With my low pressure in spec my high pressure cycles up and down, at times exceeding spec. I get good cooling.

right...I should have clarified my pressures

My low pressure is normaly very good...but my high pressure at 1500 rpm is a good (not gross) amount over spec. But it does VARY quite a bit as I hit the good ol accellerator.

kumar