Low pressues after changing compressor

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Low pressues after changing compressor

Postby Soundside » Mon Jul 04, 2016 2:18 pm

Hi, I'm a newb to this list, but I have HVAC experience. I have what seems odd behavior of a 2011 Honda CRV upon changeout of the compressor.
- As far as I can tell the problem with the old compressor was the clutch (I didn't disassemble to look inside, but the clutch wouldn't engage and the comp seemed to pump when turned by hand)
- I changed the compressor, expansion valve and drier insert, added 4 oz oil (provided with kit)
- Pumped down system
- I have gauge set, and used autoparts store cans of r134
- data plate says 17.4 Oz capacity
- figured two cans would be more than enough (12 oz by weight), and I could bleed some off or stop short on second can if the pressures got a little high
- filling system saw normal behavior, but noticed the low pressure switch never operated, which is normal when you first start to fill. Low side pressure at/below 20 psi didn't open it.
- after putting both cans in, during idle, system pressure looks fine, maybe even a bit low, but at 1500-2000 low side drops below 20 psi
- got a little flash of frost on suction line
- all indications were that it needed more refrigerant ???
- with great trepidation, I got another can and started adding, finally saw a little more normal pressures, but nothing indicating overcharging (high side hardly went over 200 psi (temp outside was 75-80 F)
- after dumping all three cans in, at high idle (2000 rpm) the thing started rapidly cycling even though the low side pressure was around 30 psi (figured high pressure switch was cycling - I didn't have the high side gauge on at the time)
- I got scared and started bleeding gas off
- now I'm back to what looks like low pressures but the system seems to operate normally 20 psi at 1500 rpm, about 200 high side

What is going on? Is the low pressure switch bad? Is it still overcharged? Is it undercharged?
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Re: Low pressues after changing compressor

Postby Tom Greenleaf » Mon Jul 04, 2016 2:30 pm

Welcome:

Wow - all over the place w possible troubles with this. You really don't know what went wrong or even needed a compressor TMK and blasting in refrigerant with a system with a very low capacity a couple ounces will throw it all off.

What did the oil look like OUT of the old compressor?

Did you prep the replacement or what? If just started up and didn't have it blowing cool replacement compressor probably isn't any good nor condenser now or wasn't much good the first round?

System is designed to shut down with pressures too high or low, many stay disabled on purpose.

IDK - Think you'd be best to start over again. Hope you can warranty the compressor if that's all you did most would be void.

Info on what the original problem was is chiefly lost so think you have to start again and made a credible history lost yet again,

T

(edit added) " added 4 oz oil (provided with kit) " That quoted from top post. That is close to many system capacities in total not just a compressor. You need to know how much you get out of any parts to even guess or flush all out - impossible/impractical with HE condensers where some or a lot would accumulate......
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Re: Low pressues after changing compressor

Postby Soundside » Mon Jul 04, 2016 3:27 pm

Some history. Car belongs to my daughter. The history on the first failure is fuzzy. The only info I got was that it stopped cooling. In thinking further through it, I think it might have gotten low on charge, then the clutch failed from cycling on and off.
In diagnosing it, it was clear the clutch was inop. and the compressor turned freely by hand. It's a long story, but in discussions with daughter and boyfriend, I said something like, "it's about as easy to swap the compressor as to rebuild the clutch" - assuming only the clutch was bad.
The next thing I know, a mail-order compressor shows up on my porch, and daughter/boyfriend show up with car.
Retracing all my steps, I now see at least part of the problem. I put too much oil in it. Oil in old compressor looked good. But there was not much in it.

I didn't flush the system, based on what I thought to be the failure mode, visual inspection of the filter dryer internals, old oil, etc. The new compressor didn't come with any instructions on oil charge, so I asked boyfriend to look it up. I think he got confused, and I didn't double check the info he found. Long story short, it probably only needed about an ounce and half. Now trying to backtrack and find oil capacities, I'm finding 5.4 oz (don't have the vehicle here, so relying on online sources). So there's probably about 3 ounces too much oil in there.

I'm still a bit confused as how this might cause the behavior I saw on the gauges. But I suppose when one thing is out of whack it can create havoc elsewhere.
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Re: Low pressues after changing compressor

Postby cornbinder89 » Mon Jul 04, 2016 3:44 pm

High side SHOULDN"T go over 200 on a 75-80 deg day! 200 PSI is 130 deg condensing temp or 50 deg above ambient temp. High side pressure should be no higher than the temp required to condense the refrigerant to liquid. You don't want high pressure, it is a necessary evil based on the outlet temp of the condenser. The lower the temp, the lower the pressure.
Low side not cycling is a problem if this is an orifice tube system, not if it is a Tx system.
Never try and charge to pressure! PERIOD, END OF STORY. With the correct charge in the system compare pressures to temps observed and they should be close.
Sounds like you had one problem and now you made it into 2-3.
get the correct charge in the system and than troubleshoot from there.
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Re: Low pressues after changing compressor

Postby cornbinder89 » Mon Jul 04, 2016 3:51 pm

Frost on suction line DOES NOT indicate "low on refrigerant". It indicates a flooded evaporator, where there is still liquid in the line after the evaporator.
If you have a Tx system you can see low suction pressures as the Tx throttles the flow. The 28-32 psi switch only applys to CCOT type systems. A Tx system may use a low cut-out (safety) switch as low as 7-10PSI if on the suction line, or 40-50 psi on the high pressure line.
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Re: Low pressues after changing compressor

Postby Tom Greenleaf » Mon Jul 04, 2016 4:10 pm

Just FYI - site issues AGAIN - read back as things may be out of order or didn't show in order now should be fine. Dang site!

Yes - was thinking too much oil - the results give real stranger performance. I just looked for system capacity which will vary without specs for this exact model. Some only hold that much total some double that - yadda, yadda.

How to know condensers are the issue alone? Ha - use water all over them and if everything behaves, pressures actually lower than you expect is somewhat normal but if then vent temps on target it's got some debris. Dang things are like a filter for systems and no fixing them that really works.

Sorry about site issues. I check in a lot and probably the lone moderator for a while to block or fix spammer going wild on this joint which is a long standing problem.

Again - read back for what might be now in place,

T
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Re: Low pressues after changing compressor

Postby Soundside » Mon Jul 04, 2016 4:47 pm

Thank you for all the feedback. It's a txv system. Admittedly, I went after this a bit like a bull in a china shop, but it's a long story. In a big hurry, had no reference materials, took some short cuts.

I realize that frost on the suction line usually means an overcharge. But with low pressures, it was a bit confusing and just a data point I wanted to provide.

High side pressure never went above 200 (while I was monitoring it). Most of the time was around 175. I was not trying to add refrigerant to increase this pressure. I was concerned more about what seemed low suction pressure to me. Again, not having access to specific info on this model, and not having a lot of experience with what to expect on a txv model with 134, I think i let the pressure fool me into thinking I needed more charge. That, in combination with having added too much oil seems to be the problem. The system is cooling fine, so far, but I think it's still a bit overcharged, and it still has too much oil in it. Problem is the kids live out of town, so I no longer have access to it. Probably should have them take it to a shop.
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Re: Low pressues after changing compressor

Postby cornbinder89 » Mon Jul 04, 2016 5:45 pm

The thing to remember is pressure equates to temp of refrigerant. Check the pressure/temp chart and see if that is a reasonable temp for a system in those conditions.
With a Tx valve throttling the feed to the evaporator, when temps are not that hot, you may see lower low pressure reading than equate to the boiling point, This just means little refrigerant is needed in the evaporator to cool to the set point, with the compressor turning at speed, the low pressure is below the boiling point but all if it has flashed to vapor. This is esp true if the gauge port is on or near the compressor.
At a full heat load the TX would open up and you would expect to see higher low side pressures, as more is allowed into the evaporator and it takes more to cool it down to the set point.
It is why it can be hard to work on system in the cooler weather.
My Trucks have a low side cut-out, that doesn't trip until the low side pressure is between 4-7 psi, then it "locks" In the off position until re-set to prevent cycling on and off if the system has a slow leak. Its not un common for me to see low pressures (measured at the compressor) at or below 20 PSI with the engine running at 1500, in cooler weather.
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Re: Low pressues after changing compressor

Postby Soundside » Mon Jul 04, 2016 6:21 pm

Thanks again for the information.
Assuming the system has say 28 to 30 oz of refrigerant in it (supposed to have ~18), and 7 oz of oil (supposed to have ~5), what is the immediate risk of damage? I assume the biggest issue is probably slugging the compressor with liquid. Probably more likely if ambient temps are low?
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Re: Low pressues after changing compressor

Postby cornbinder89 » Mon Jul 04, 2016 6:34 pm

High head pressures, with slipping clutch or belt. The excess is stored in a liquid state in the receiver, and when that is full will back up into the condenser, making its effective size smaller as liquid starts to fill it, so poorer cooling as temps rise. You're making the system work harder with less effective condenser. The Tx should minimize the chance of liquid making all the way back, but that is a possibility.
I'd try and get the correct amount in there ASAP.
Not so concerned about the excess oil, that will reduce heat transfer a bit, but you are not that high on the level (2 oz?)
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