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Low side pressure quickly jumps, possible causes?

jacob_coulter on Mon August 12, 2013 6:18 PM User is offline

Year: 2000
Make: Infiniti
Model: QX4

I have a relative that has an older Infiniti QX4 that's worth next to nothing, it's basically a high milage beater that's falling apart. She had warm temperatures coming from the vents, and asked me my opinion on it. I put a thermometer on the vent and it was around 60 degrees. When I hooked up a low pressure gauge, it looked like it was low on refrigerant, gauge read about 20psi. I told her there was a "redneck" way to just charge it by adding a can or 2 and get it close, might get it colder.

Anyway, got some pure R-134 (no sealers) and looked at the charts to where it said to charge for the outside temperature. Everything seemed to be going great when I was adding refrigerant, and I got it to around 45 psi on the low side and the vent temperatures were coming down dramatically with the vent thermometer. It looked like it was a success, but just as I was about to pull the line off, the low side started "spiking" to around 100psi when the compressor would engage. It kept doing this every few minutes in a cycle.

Any ideas what happened? Could the expansion valve have become clogged during the charging?

Thanks for the help.

webbch on Mon August 12, 2013 7:29 PM User is offlineView users profile

If I read it correctly, you had a low side pressure of 20 psi when you started. You attempted to charge by pressures using only the low side gauge on the can of refrigerant. While I'm not condoning the attempt to charge by pressures, why didn't you stop around 30 psi, which should correspond to a refrigerant temperature closer to freezing than 45 psi? I'd be VERY concerned about excessive high side pressure right now due to an overcharge.

A low, low side pressure COULD be due to a blockage in the high side line, which would mean already-elevated high side pressures when you first started. Adding refrigerant would have exacerbated this issue drastically. Granted, it's only a theory because we don't know what the high side pressures were when you started, nor when you finished.



Edited: Mon August 12, 2013 at 7:32 PM by webbch

mk378 on Mon August 12, 2013 7:39 PM User is offline

Rapid cycling without the air getting very cold means you probably overcharged it and it's tripping the high pressure cutoff. Never attempt to charge by pressure without seeing the high side. Late-model systems don't use much refrigerant. Many are less than 24 oz total capacity (starting from completely empty). The days of throwing in a can or two are over. You have to be pretty exact with the charge amount to get good performance, which ideally means recovering the remaining charge and then charging by weight to the manufacturer's specification.

Edit: The capacity chart says 25 oz for your friend's QX4, so two full cans on top of whatever was already there is definitely an overcharge.

Edited: Tue August 13, 2013 at 10:24 AM by mk378

jacob_coulter on Tue August 13, 2013 11:30 AM User is offline

Regarding charging it to 45 psi, I was going off the charts included with the gauge since it was about 90 degrees outside.
http://www.mastercool.com/media/89660-INST_web.pdf

And it was not two full cans, but about 1 and 1/3 cans.

I know it's not exact and the right way is to do it by weight, but unless you have a $5,000 machine, I'm going to have to go with some combination of gauges. Just trying to get a AC that gets a little colder to help somebody out.

webbch on Tue August 13, 2013 12:51 PM User is offlineView users profile

That table lists high side pressures. VERY dangerous to charge using that method without knowing the high side pressures. I believe that table is intended for ballpark diagnostic purposes, not a charging-by-pressures table, although I can see how one might interpret it as such.

Also, look at step d in the "Charging Refrigerant" section of that same document:
"Open manifold low side (blue) valve slowly until pressure reaches 40 psi. Do not exceed 40 psi during the recharging process. Exceeding 40 psi could damage the compressor."

Nobody's saying you need $5000 worth of equipment. I know you're trying to help a friend, and that's admirable. However, the last thing you want to do is cause further damage to their system and/or get hurt in the process. Thus, the MINIMUM requirements are an A/C gauge set to monitor both the low AND high side pressure, which will help you to diagnose many problems AND keep you safe.

Granted, unless you have a recovery unit and vacuum pump, you'll still be tied to an a/c shop for a few things.


Edited: Tue August 13, 2013 at 12:53 PM by webbch

jacob_coulter on Tue August 13, 2013 1:37 PM User is offline

But does it make sense for it to jump like that purely because of a slight, possible overcharge? I could obviously understand the low side reading 100 psi if I kept charging it until it reached near that level, but to have a stable reading of 40-45 psi after several minutes, and then all of a sudden it jumps to 100 when the compressor engages seems to indicate something else is going on, like a restriction somewhere.

webbch on Tue August 13, 2013 3:48 PM User is offlineView users profile

A low, low side pressure in combination with a high, high side pressure is indicative of a blockage. You saw only the low side pressure, assumed a low charge, and added refrigerant. What if your high side pressure was already high (because of a blockage) when you started? By adding refrigerant, it brought your low side pressure up to "normal" levels...but now the high side pressures are sky high.

Of course it's just a theory, but one that could easily be verified or dispelled by throwing a gauge on the high side and saving the cranial exercises for a more deserving problem.

I don't know specifically about the low side spiking, other than it's likely related to an extremely high, high side pressure. You REALLY need to measure the high side pressures ASAP - you could be destroying the compressor right now. I mean no disrespect, but there's a good chance you have more than just a "slight" overcharge. Worst case scenario was the charge level was correct when you started (operating on the theory that low side pressure was down due to blockage), and you now have ~41 oz of refrigerant in a system with a capacity of 25 oz, or a 64% overcharge. That's probably over-estimating the problem, but I don't think that a 25% overcharge is too far fetched, which would likely cause some major problems.

HECAT on Tue August 13, 2013 4:28 PM User is offline

Quote
Originally posted by: jacob_coulter
But does it make sense for it to jump like that purely because of a slight, possible overcharge? I could obviously understand the low side reading 100 psi if I kept charging it until it reached near that level, but to have a stable reading of 40-45 psi after several minutes, and then all of a sudden it jumps to 100 when the compressor engages seems to indicate something else is going on, like a restriction somewhere.

If it is running at 40-45 on the low side, it may be kicking off (not on) and you are then reading 100 psi static pressure. Just a thought.

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mk378 on Tue August 13, 2013 6:02 PM User is offline

It's normal for the low side to come up should the compressor stop. What little instructions come with the useless single gauge seldom explain that.

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