# Looking for A/C answers...



## munkittrick (Aug 27, 2006)

Hey guys!
I just got in my car after a two week paint job (Had to special order paint). When I left her there, the AC worked well, but since I picked it up, the AC isn't playing nice anymore. The low side pressure reading is nearing 100psi and God only knows what the high-pressure side is measuring at. The AC clutch kicks on and off as expected, and all of the fuses in the car are intact.

Even with everything in seemingly good order, the vents blow hot (Actually, more like really warm). All of the climate controls work and there doesn't seem to be any reason for them not to. 

So, my questions are;
A.) Being that the clutch is engaging properly, is it possible that something has gone awry with the compressor that would allow the compressor to cycle correctly, but not blow cold?

B.) Is it possible that a fan-switch is telling the compressor not to function due to an electrical short or failed relay at that (or any other relay for that matter)?

C.) Could the extra week of sitting in the local body shop have been stagnant enough to kill something?

One other note, I changed my accessory belt and the tensioner a few weeks ago, but it worked really well after doing so.

Anything you guys can offer would be appreciated.

http://attachments.techguy.org/attachments/158339d1256727741/funny-dog-pictures-now-panic.jpg


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## munkittrick (Aug 27, 2006)

Quick update. I verified that the compressor clutch is also operating properly when I press the AC button as well as off when I depress it.


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## cuppie (May 4, 2005)

Um, maybe it's just a low charge?  
(Low charge may be due to 'real' leak; may be due to normal minute seepage from A/C system. Car in your sig is 10 years old....)

What's that low-side pressure value you stated represent: system off (engine off, or compressor not engaged), or system on (enigne running, compressor is engaged)?


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## munkittrick (Aug 27, 2006)

cuppie said:


> Um, maybe it's just a low charge?
> (Low charge may be due to 'real' leak; may be due to normal minute seepage from A/C system. Car in your sig is 10 years old....)
> 
> What's that low-side pressure value you stated represent: system off (engine off, or compressor not engaged), or system on (engine running, compressor is engaged)?



Understood. My apologies. I should probably further clarify. The entire A/C system, right down to the vent tubes was replaced due to the previous owner having a LOT of mold. I'm highly allergic....so it was all swapped out for a recently wrecked system. It worked flawlessly for an entire season and there are no leaks visible with a UV check.

In lieu of doing a full evac and replace, I'm hoping to find a logical problem as it might simply return if it's not taken care of. Beyond that, an evac and refill is $209 at the dealership and there's only one other place I'd trust with it...and he's a week behind schedule already.

The low-side pressure measurement was taken at A/C ON FULL. It reads a very clear 100psi and goes up slightly when you cycle the system off, but lowers slightly if cycled back on again. I got a PM saying that maybe my gauge was wrong, but I borrowed a known good set and it was the same. Another user suggested a problem with the system dryer, but the pressure test also showed NO internal humidity or condensation....so the dryer is functioning as expected.

Could there be something that caused a blockage in one of the refrigerant lines? Is there a known area of failure for these systems...other than the compressor?

Thanks again.


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## cuppie (May 4, 2005)

Since you have (or, borrowed) a gauge _set_, what are both the *high *and low side pressure readings when the compressor is engaged?
(A low-side pressure, by itself, is relatively meaningless. Ned both to say what the system is actually doing.)
(also need ambient temperature, and approximate humidity, as enviroment does affect what to expect here.)


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## munkittrick (Aug 27, 2006)

cuppie said:


> Since you have (or, borrowed) a gauge _set_, what are both the *high *and low side pressure readings when the compressor is engaged?
> (A low-side pressure, by itself, is relatively meaningless. Ned both to say what the system is actually doing.)
> (also need ambient temperature, and approximate humidity, as environment does affect what to expect here.)



The high-side reaches roughly 155PSI when on full or off. The low-pressure side reads at almost 100PSI when on full and a bit less if the AC is off. The ambient temperature is fluctuating pretty rapidly, but I'd say the its about 82 degrees at roughly 30-35% environmental humidity.

A local parts store (I realize that they are not a really helpful consultant, but it's what I've got immediately) said that it might be that the car got a bad charge, but he couldn't tell me if that would raise or lower the line pressure with any degree of certainty.

Thanks guys!


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## cuppie (May 4, 2005)

That doesn't make any sense.

With the engine off (or, engine running, compressor not engaged (and, has been off for at least 30 seconds), the high- and low-side pressures should be equal.

Let's try this. Fill out the following blanks:
*Engine off*
Low side pressure: _____ psi
High side pressure: _____ psi

*Engine running, compressor engaged:*
Low side pressure: _____ psi
High side pressure: _____ psi

Ambient temperature: ____ *F

Being a FWD car especially, make sure that the radiator fan(s) is working properly (should be on with compressor on); a large fan in front of the vehicle may be needed to maintain airflow over the condender (high temps will skew pressure readings.)

_(note: ambient temp is outside temp, not vent outlet temp. It's not going to fluctuate at all!)
_


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## munkittrick (Aug 27, 2006)

cuppie said:


> That doesn't make any sense.
> 
> With the engine off (or, engine running, compressor not engaged (and, has been off for at least 30 seconds), the high- and low-side pressures should be equal.
> 
> ...



I know, right? It makes no sense. Before I say anything further, please understand that while I'm in over my head in this instance, I'm very well versed in getting any other project handled, so I'm not exactly a rube, but not exactly a certified tech either.

*Engine off*
Low side pressure: 100psi
High side pressure: 100psi

*Engine running, compressor engaged:*
Low side pressure: 98 psi
High side pressure: 155psi with erratic spikes up to roughly 175 psi

Ambient temperature: roughly 82*F, but the temperature shifted here today from a quite normal 79 degree to a dumbfounding 88 degrees in less than an hour.



The fan control module was ruled out today with a brand new "loner" from a local shop (Thanks Mick!).

My temperature is as expected and both fans cycle on and off appropriately when expected and have both low, and high speeds. There is no smell of antifreeze, she's burning NO oil, and I'm using only genuine parts in the car, so I'm unable to blame a substandard component.

The last thing to check is my radiator temperature sending unit, but if I'm correct, it can be simply jumped as a means to diagnose a failed component without damage. Although, the fact that my fans are operating well seems to point to an good-working-order temperature sending unit as well.

What should I try next?


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## cuppie (May 4, 2005)

Thanks for the pressure readings. Your earlier posts (that had only part of the info, plus how you had worded things) made it make no sense; but, with all of the data, it makes perfect sense now.

Sorry, dude - compressor's dead. The gauges don't lie: your compressor is unable to, well, compress (or pull) refrigerant anymore.
It occasionally _tries_ to do something (your erratic high-side spikes to 175psi (roughly normal running high-side pressure)); but, it then slacks off again.

If it were overcharged, both the low- and high-side pressures would be higher than normal; undercharged, both sides would be lower than normal.
Air in the system looks just like an overcharge; moisture in the system results in normal pressures, but 'warm' outlet temps.
And, restrictions in the system (say, from a clogged or stuck expansion valve) will throw the pressures way out of whack.


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## munkittrick (Aug 27, 2006)

cuppie said:


> Thanks for the pressure readings. Your earlier posts (that had only part of the info, plus how you had worded things) made it make no sense; but, with all of the data, it makes perfect sense now.
> 
> Sorry, dude - compressor's dead. The gauges don't lie: your compressor is unable to, well, compress (or pull) refrigerant anymore.
> It occasionally _tries_ to do something (your erratic high-side spikes to 175psi (roughly normal running high-side pressure)); but, it then slacks off again.
> ...




Actually, according to the manual, both sides are way over the normal pressure. The low side should be at 55-65 peak when off and the high side should read in the low 140s when off. When on, they are supposed to regulate to an even pressure to keep the cycle going, but I think my neighbor may have figured it out anyway.

Since the compressor is engaging and there is evidence of an offset pressure during operation, he suggested that it is most likely the "pressure switch". Now, I'm aware of what it does, and it sounds logical that the switch may be failing...which explains the random compressor cycling....but I was also told that it may be easier to just have an entirely new compressor installed.

If you have any other thoughts, let me know. I'll update from this end tomorrow after I get a better look at the underside.

Thanks again!


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## cuppie (May 4, 2005)

> Actually, according to the manual, both sides are way over the normal pressure. The low side should be at 55-65 peak when off and the high side should read in the low 140s when off. When on, they are supposed to regulate to an even pressure to keep the cycle going, but I think my neighbor may have figured it out anyway.


 :what: What manual did that come from? That's entirely backwards... 

With the system OFF, high- and low-side pressures WILL be equal. Why? With the compressor not doing anything, the system will equalize itself.
The job of the compressor is, well, to compress the refrigerant. An A/C system works on thermodynamic principals (droppping pressure in same volume reduces temperature; increasing pressure in same volume increases temperature.) The lower pressure in the evaporator (which is created by the expansion valve) removes heat there; the higher pressure in the condenser (created by the compressor) helps it dump the heat.

If the pressures on both sides remained the same _during operation_, it wouldn't do anything. Mebbe you read it backwards? 

I will say this:
were I working on this car, I would do a recovery (and, note how much I pulled out), evac, and charge on the vehicle (it's good to know just how much really was in there) before condemning any part (especially for what a compressor can cost); but, it's still looking like yours is dead.
And, you didn't say what year/model you're working on, either - if it's new enough to have a swash-plate (variable-displacement) compressor... well, they can hide some things, and mislead you in other directions, when the charge quantity is incorrect. Hence, the habit to do a pull-n-fill (R2D2 makes it sooo easy....), as most of what I work on these days has a swash-plate compressor.
If it's a fixed-displacement compressor, though.... they tell no lies.


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## munkittrick (Aug 27, 2006)

Okay, now I'm frigging baffled. VW did a compressor test today and said that the compressor was working, even said "working really well", but since I couldn't afford to leave the car for the day, that's the only bit they could get done before I had to go to work.

So, the compressor is good, the evaporator is solid, the condenser is spotless and sealed, the FCM is working as expected, the compressor clutch is engaging as it should, the coolant temperature sending unit is working as well and I'M UP TO MY EYEBALLS IN *FAIL*.

For the love of Snickers, someone give me another point to go on.

Can I safely have the system vacuumed/evacuated out to drop the pressure to a far manageable pressure, or does this require me to rebuild the compressor from inside out?


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