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Discussion Starter · #1 · (Edited)
2009 XC90 - Can AC compressor be bad if it still cools some?

An indie shop we have no experience with (just moved here) just diagnosed a bad compressor and $1800 fix, I'm looking to make sure that's accurate and it's fairly unlikely to be something cheap like the control solenoid (such as in this thread which seems rather similar.) Can a compressor be bad if it's not leaking and still able to cool at least a little? Here's what we've got going on, I'll try to add whatever info the shop collected to come to their conclusion if I can pry that information from them:
  • It has the proper 2.2lbs of 134a
  • It's not leaking
  • It only cools on the freeway, but not always. Sometimes I might have to turn it on/off a few times and it will eventually cool at freeway speed
  • When it does cool, it only cools a little. I've measured between 3-27 degrees below ambient. Never below about 60* measured at the vents.
  • When it cools, it cools regardless whether it's 70* or 100* ambient temperature outside.
  • Once we pull off the freeway, it starts blowing ambient air (or slightly warmer) from the vents.
  • Condenser appears to be clean, fan appears to work.

One of my main concerns is that I asked about the possibility of just the control solenoid being bad, and they didn't seem to know what I was talking about. When I asked how they came to the conclusion of compressor vs solenoid they insisted their measurements show the compressor isn't putting out enough pressure, but he didn't know any more than that. I believe he mentioned the hi and low pressures being pretty much the same (which is not at all what I've measured - I was seeing extremely low low-side pressure, like 0-20, and high side around 150. I've not observed it cycling at all though, but I've also not observed it cooling while sitting in my driveway.)

Anyway, is it worth trying to replace the solenoid myself, or is the whole compressor being bad likely accurate?
 

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The real question is how did they determine it had the correct amount of refrigerant? 20 PSI low side is way too low, indicating it is undercharged. Getting the high side any higher would further lower the low side, so, I'd say the compressor is not the problem. First thing I'd do is add some refrigerant and get the low side up to the correct pressure. I'm a little rusty, but, IIRC, it should be at least 30.
 

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Interesting. It's the suction side of the compressor that makes the low side pressure. Since yours is too low, the compressor is "sucking" just fine, so it's also compressing fine.

Another cause of low pressure on the low side is a blocked expansion valve, or orifice, depending on the design of the system. This prevents the high pressure from passing through the evaporator to replenish the low side. It would be accompanied by unexpectedly high pressure on the high side, but that can easily go unnoticed. Of course, if little to no refrigerant is passing through the evaporator, it won't cool so pretty good, either... the compressor will cycle on and off rapidly. (Maybe this is why they think it is bad...)

Are you taking it to an AC specialist?
 

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Low pressure on the low side also leads to icing of the evaporator. 20 PSI R134a is going to be about 20 degrees F. While this might sound good, it means that any humidity in the air will freeze on the core and block the airflow. Do you feel like the ventilation fan is working up to snuff, or is the airflow reduced?

Any idea what the pressure is when the system is off? How fast do the low and high sides equalize?
 

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Aside from all this... a new compressor can be had from Tasca parts for 600 bucks....
 

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Discussion Starter · #7 · (Edited)
OK, so the only information I got from the shop when I picked it up was that they measured 70low, 80high, so concluded bad compressor.

I'm not sure why my measurements differ so much, but I just hooked it up again and I'm measuring 5-10low, and 125high. It doesn't cycle (do variable displacement compressors 'cycle' per se or do the pressures just float back and forth some?), but that's what I'd expect at the moment since it's only working on the freeway and I'm not on the freeway taking these readings. HOWEVER, by holding RPMs around 1500-2000 I was able to get just over 15 degrees of cooling under ambient, down to 55 degrees which is actually the coldest I've measured from it (even colder than my best readings on the freeway.) I still didn't see the pressures doing much of anything though - the high pressure eeked up a little higher and maybe fluxtuated slowly between 120-130ish.

Static pressure is reading ~7low ~75high after 20 minutes - but my understanding is that these don't equalize necessarily due to the type of expansion valve?
 

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Either this system is so far advanced from what I'm familiar with that I'm clueless, or it's under charged. 7 PSI is to low under any circumstances. 130 is not high enough, either. I suggest you take it to a specialist or (cough) the dealer.

OK, so the only information I got from the shop when I picked it up was that they measured 70low, 80high, so concluded bad compressor.
If these are running pressures, then I'd agree, but, they don't jive with your readings.
 

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The A/C compressors on our cars are variable controlled compressors. They do not cycle on and off like a lot of compressors typically. The solenoid on the compressor is used to regulate the output of the compressor from what I understand. When they fail they can definitely cause the car to not cool the A/C properly or at all. That's not to say that your compressor hasn't failed, but you should probably look for a second opinion. Most shops will have no idea about the solenoid. Even if the car needs a whole compressor, $1800 sounds like a lot to me even with a shop markup on parts. You don't have to buy the compressor from Volvo, it is readily available from Valeo (the likely OEM for your compressor) for about $400. There's also brands available such as Denso and Hella who are OEMs for other companies for less than that. Maybe $1800 is a normal price, but get some other quotes. The control solenoid itself is located on the compressor. If you open the hood and face the compressor, look at the bottom right of it. The should be a circular cutout with a connector underneath where the A/C lines attach. That is the solenoid. Part number appears to be 31305844 from Volvo. Maybe take the car to a Volvo specialist who has probably seen this issue before.
 

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Still, even with a magical compressor, the pressure readings don't make sense. If you suck the low side down to nothing then the high side should be higher than normal (the freon has to go somewhere...)
 
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