New A/C compressor pulls a vacuum on low side (1 Viewer)

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Hi everyone!
I am having some troubles with my air conditioning system on my ‘95. My knowledge about A/C is not very extensive (and it shows :hillbilly:) but I wanted to tackle this job myself since I’m 19 and have no money


Here’s what happened:

I replaced my A/C compressor because the old one was acting up. It wasn’t totally locked up, but often times the belt would slip when it kicked on. Compressor would turn on, belt would slip for a few seconds, then the compressor would finally free up. It was like this for a while, maybe 6 months, before I decided to do something about it. It cooled fine, but the squeal when the compressor started was annoying. BTW the A/C belt was new and tight.

I drained the refrigerant, pulled the old compressor and installed a Denso remanufactured unit that I ordered from Rock Auto. I installed the new o-rings on the lines going to the compressor, but I didn’t touch any of the other lines. Once I had it all sealed I pulled a vacuum and it held 30 in Hg overnight (I followed the A/C video by ChrisFix). I let the vacuum pump run for another half hour then I added refrigerant. I added about 2.5 of those 12oz bottles of refrigerant. With the compressor running and the correct amount of refrigerant in the system, the low side was at a vacuum around 20 in HG. I read that this means there is a blockage somewhere in the system.

I have a new expansion valve and drier too. Would it be stupid to blow compressed air through the lines when everything is disconnected?

As it sits right now, the lines at the evaporator and the drier are disconnected.

I read that replacing a bad A/C compressor without flushing the system can cause lots of problems (after I was all done of course) and that there might be debris in the lines. The other thing I was unsure about was the oil in the compressor. It came with oil in it already, but I’m unsure if it was the correct amount.

Any help would be greatly appreciated, I really wanna get this thing working again.
 
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It sounds like a faulty expansion block as it's common for the low side to go into vacuum when it fails. It's unusual for the 10PA series Denso compressors to shed debris into the system so once you pull the drier and expansion, check the lines for any sign for debris. If it was a Denso brand compressor, they come with a full charge of refrigerant, so once you replace the two parts you should be fine without adding any more oil. While you've got it apart, I'd replace all the O-rings.
 
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I just went through this.
My drier's filter bag ruptured and blew the little bb's of desiccant into the expansion valve( TXV). They also plugged the liquid line from the fire wall to the TXV.
I blew mine out with compressed air. The vacuum will boil off any moisture introduced.
You could have a plugged condenser, can you blow air through the condenser?
Did you blow air through the evaporator while you had it apart?
 
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No, but the evaporator is out now and I can do that. I’ll try blowing air through the condenser too.
 
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I just shot some air through the evaporator, so it’s clear. Nothing weird there. When I did the same thing to the condenser, a bunch of oil came out. I also blew out the other two lines, the one running to the drier and the one going to the condenser. My understanding was that the oil is supposed to stay in the compressor, could it have made its way through everything?
 
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These may help: the second one is better, but both are usefull.



put "sanden a/c system diagnosis (r134a)" into google search bar
You'll get this auto downloading chart.
SANDEN_SYSTEM_DIAGNOSIS_CHART.pdf
 

smritte

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I agree with what everyone has said and want to know what your high side reading was, what the ambient temp of the day was and how much refrigerant had you installed?
If you have a stuck or plugged TXV, high side component or line, the high side goes through the roof (400psi and up).
 
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Got everything back together. New o-rings all around. Added exactly 2 bottles of refrigerant and here’s the pressures I’m getting
9155B1E8-B4E1-41F5-8ACD-14AF0C4EE4A6.jpeg
 
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What RPM are you at?
What's your ambient temp?
What is your vent temp?
How long was it running? Needs 10 minutes at idle to stabilize, remember you got engine heat that plays a part.
 
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Engine at idle, ambient temp ~90F, didn’t get a vent temp. Felt cold, not crazy though
 
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I added refrigerant through low side. Let her run for about 10 mins with blower on high
 
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Doors open and vent set to recurculate?

According to the ambient temp, your pressures are just outside the normal range, Low is a touch low and High is a touch high.
You need to get a vent temp, it should be around 25 to 30 degrees less than ambient, got a cooking thermometer?
1 In. Pocket Thermometer
Make sure to calibrate it in a glass of ice water.
Your readings look exactly like mine does. After the initial repair, I took it all apart twice looking for a blockage I could never find. Put it back together with a new TXV and drier/receiver and its running about like yours.

If the vent temp checks out, I'd be tempted to leave it well enough alone.
 
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Cool. I have one of those infrared thermometers too, but I can probably borrow a cooking thermometer just to test. I’ll get a vent temp tomorrow!
 

smritte

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His pressures look like he's at idle and about 95 ambient.
Like the others said, meat thermometer in center vent (I like those because the numbers are bigger), High fan, 1400(ish) rpms. Depending on how hot your dash is will depend on how long it takes for the temp to stabilize (your cooling off the dash vents).

Temp guns are not accurate here because they don't read air temp. They read the temp of the plastic or metal. As long as what they hit is same as air temp then your good. Stem thermometers are way faster.

Put your thermometer in a glass of ice water. Melting ice is 32 degrees. I've had new thermometers off as much as 10 degrees.
Make sure you let the water sit long enough to stabilize.

Low gauge will sit in the 20's, high gauge @95 degrees will be around 300 psi, Vent temp should be around high 30's-low 40's degrees. Your high gauge will go up and down as your fan clutch turns on and off.

Your Low gauge pressure is what your evap temp is (not vent temp). If its too low it freezes and a safety switch will "cycle" (turn on and off) your compressor. This pressure is controlled by your TXV
Your High gauge is your condenser and radiator fan. How much heat is being removed. If the high gauge goes over 450-500, a safety switch will cycle the compressor to keep it from being damaged. This pressure is controlled by fan/condenser.

Normal pressure readings @ 1200 - 1500 rpms, fan on high.
Low Gauge 20psi - 30psi. Over 35psi (at speed) can mean a stuck open TXV or compressor valve issue.

High Gauge is based on ambient temp. Basic formula is Ambient X 2.45 (min) Ambiant X 2.5 + 50 (max). 100 degree ambient = 245 - 300 psi. Humidity will effect it. high humidity will raise the range and low will drop a little.

Refrigerant oil will effect these numbers. Know EXACTLY how much refrigerant you installed and how much oil. With the correct amount of refrigerant, if your low on oil the high gauge will read low. Too much and it reads high.

You said you put 2 cans in?
Two 12 oz cans?
If so that's 24 oz or 1lb 8 oz or 1.5 lbs. Your spec is 1.8 lb.

This should keep you busy. Remember AC just needs to be close. Kinda like hand grenades. Vent temp is everything as long as the gauges are somewhere near spec.
 
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