A/C refrigerant pressures

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Oct 22, 2010
Parker, CO
My rig is a '94 and has a vortec 5.3l swapped in. I am using the compressor that came on the vortec (2006 MY). It's a R134A system.

I had custom hoses made up and installed a new receiver/drier and replaced all of the orings that I could get too.

I vacuumed the system down first to check for leaks (non found), and then to dehumidify for 45mins.

I charged the system using a digital kitchen scale to measure out the freon going in. The engine was at a fast idle of 1200 rpm while charging. I think I goofed it though.

I am seeing vent temps ranging from 45 to 55 degrees and my pressures on the manifold show about 25psi on the suction and 275 on the discharge side. These pressure willl drop if I lower the vent fan speed.

Did I overcharge the system? I feel like I need to have it discharged, suck it down again, and the recharge again. Maybe my compressor is not up to par?
One indication of overcharging is a rise in temp while charging. Did you measure the air temp coming out of the vent while you were doing it?
One indication of overcharging is a rise in temp while charging. Did you measure the air temp coming out of the vent while you were doing it?

I was measuring the temp as I charged. I think the problem is that the day I was charging it, the ambient temps were in the 50s.. I didn't see a rise in temps - it just came down to about 40 degrees so I stopped charging.
Ref press will vary with ambient, evap load, condenser airflow, and comp speed. If it was only in the 50s, and your discharge press was 275, you could have overcharged it slightly. Also, the low suction pressure is likely an indication of low evap load due to the low ambient. At low evap loads, the expansion valve will close, resulting in higher discharge pressure (i.e. the compressor is pumping fluid into a valve that is almost closed). I would recommend waiting for a warmer day (80s) to check the system perf before altering the ref charge. When I used to design, and build a/c systems, we would always determine the "critical" charge at max ambient, max evap load, and comp speed. Anything above the critical charge will result in abnormally high discharge pressures (i.e. 350 psi).
well... i went back in to check my pressures and when I disconnected the high side port, the schrader wouldn't reseal - I therefore lost my charge.

With that, I am done. It's going to a professional tomorrow to get charged correctly. It's supposed to be near 90 tomorrow so it should be a good day to test it.
Hey rockrod,

How did you wire up the compressor? It's getting hot in Texas and I need to get my AC installed.

Thank you.
I wired it up using the trigger wire from the toyota ac amp. I just cut the stock toy connector off and installed a weatherpak in place of it and ran the wire. You do need to ground the other wire so that the compressor will work. the toyota ac amp will command the compressor but i have found with the DBW set up that there's no way to kick the engine idle speed up. the engine does compensate but it takes a moment for the idle to stabilize.

i took it to a shop and they charged it - it seems to be working well now. i am getting 40 degrees at the vent.
So cut and splice the black wire, connect it to one of the wires of the compressor, and ground the other wire comming out of the compressor?

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Cool man. Thank you for your help.

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The black wire from this connector? because I am not getting any voltage from that black wire.

thank you

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nevermind got it running. how many cans of freon did you put yours?

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BTT :)

Today it's about 100F in denver. I jumped in the cruiser to head home (it was sititng in the sun so needless to say it was hot inside), and fired up the a/c (step 3 on blower fan). It started cooling, but about 10 minutes into my ride, the compressor started cycling rapidly and the a/c started blowing warm air. I shut it off for a moment and then turned it back on, but this time set the blower to speed 1. The compressor no longer short cycled and the vent temp was about 40 degrees...

if I tried to increase the blower speed, the compressor would start short cycling again about 3-4 minutes later. It seemed to become less of an issue as I got closer to home.

Still overcharged? Logic tells me the system is still over charged and the heat load is pushing the pressure switch to trip...it's either that, or I don't have enough air flow through the condenser.

I am running a dual electric fan on it's own controller. The controller has an input for A/C and will ramp the fans up in response to the compressor being commanded on. I think the fans are sufficient - they are able to keep the 5.3l in it's operating temp range (197f - 202f).
Just got my ac working. The guy that charged it said if the compressor hit 400psi it will turn off. He had mine running 300psi at idle.
not sure what my pressures are. not really wanting to hook up the gauge because the last time i did that i lost all of my charge.

while toting the kids to camp, here is what i noticed. the outside air temp was about 70f.

a/c blower fan on step one and step two, I saw 40 degrees at the vent. it would climb to about 44 sitting at a light. I am really thinking this is an air flow issue through the condenser.
figured I should circle back and update this thread.

The problem was solved. I ended up taking it to a shop and having them evacuate and recharge the system. Since it was still spring we were not seeing the usual summer time temps so as it got hotter I started noticing the compressor would kick on/off rapidly and stop cooling. This would happen only when the ambient temp exceeded 90f.

This summer hasn't been as hot so I haven't had this issue come up too ofter. Well this last weekend it was really hot and the a/c couldn't keep up.

I waited until it was about 95f outside, rolled down all the windows, set the idle at 1200 rpm with HP tuners, and cranked up the a/c with the pressure gauges in place. I immediately saw the low side was exceeding 60 psi. I bled off refrigerant until the low side was about 40 psi and the high side about 240psi and saw that the vent temps were right at 45f, and the compressor was staying engaged.

a few trips on the road rewarded me with vent temps at 41 degrees and holding regardless of outside temp and either driving or sitting at a light. I do have the aux fan and it makes no difference in vent temps at all. I will leave it there as a back up but it's not needed.

Cool stuff.
Good info, thanks for following through.
You mean you RECOVERED the excess refrigerant. Big brother is always watching. There are hefty fines for the deliberate release of HFC's, HCFC's, and CFC's and equally hefty rewards to those who snitch. I'm glad you got it cooling.

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