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AC problem, variable displacement compressor diagnosis

6K views 8 replies 3 participants last post by  IanH 
#1 ·
I went and checked my AC pressures as it has not been cooling well for several summers, and now is to the point it isn't working enough to be usable.

So normally when you rev it up from idle to 2000 rpm the low side pressure decreases and high side increases as you increase the freon volume across the condenser.

Mine the low side increased a lot and high side increased very little, air coming out of vents increased to 85 F. (And it got worse with more Freon.)
I have never seen this before.

My diagnosis is the compressor but help would be appreciated.

Thanks Ian.
 
#2 ·
BTW, I had a blocked expansion valve once and it caused high high-side pressure and Low Low side pressure so I don't think its the problem here.
 
#3 ·
ok so more reading and it seems the compressor could be working normally.
so why is low side pressure high enough to stop cooling ( before I added more Freon.)
 
#4 ·
No comments?

Let Freon out this morning watching pressures and temps.
Got marginally cooler then hotter,
so not overcharged,
low side is still above 40 - 50 Lbs/sq inch.
high side is still low,
so not enough freon moving round system
either compressor is not pumping
or expansion valve is not working correctly
 
#5 ·
You need to take it to a shop and let them diagnose. And not a Brake-O or similar, a shop that really knows auto AC. They can pull out the refrigerant and weight it to see if it was the right amount. Most AC issues are causes by wrong refrigerant level and leaks.
 
#6 ·
It sounds like a bad compressor. The factory service manual gives the causes of incorrect pressures. It'll show pics of the low and high side pressure gauges in various settings (i.e. a high side gauge with a low reading and and low side gauge with a high reading) along with a list of potential causes. I would download the service manual, or at least the heating and A/C chapter, for your vehicle. NissanHelp.com has service manual downloads in their "knowledge base" in chapter form and Nico Club's site has entire manual downloads. I also agree that it's tough to get a good read on the system without the equipment to recover and charge the system accurately, but gauges are the "eyes" of the system.
 
#7 ·
Thanks for the reply.
Yes have been reading the service manual.
I pumped out the system with my industrial Vacuum pump for a couple of hours and refilled with about 14 oz of freon.
Pressure is still high on low side, low on high side.
when driving around today temp got as low as 45 F, then drifted high and never got below 70 again. stopped effectively cooling for a bit then dropped to 80 and then 70 F, 88 - 92 F outside.
Its strange it works better at low engine rpm and is so variable. compressor is engaged whole time. This A/C does not behave like my 134a systems on the Sentra or SER.
I think the mechanical control mechanism for the variable capacity feature on the compressor is faulty or worn out.
So I am ordering a compressor kit today.
 
#8 ·
Well replaced the compressor and dryer. still putting back together but is looking better.
high side up to 250 lbs per sq and low at 40.
lost some freon because packing leaking on valve in low side hose, so don't know how much freon in system.
I think its low so off to get another can.

BTW this took a lot more time than it should have. lower compressor bolts were difficult to get to and very tight.
dryer bolts were nothing short of impossible.
did not replace expansion valve (yet) when I saw what was involved.
 
#9 ·
final pressure was 36 low side 310 high side in garage 86 F

First trip out in stop and go traffic, temp at outlet is 40 F on recirc, fan max, 46 F on outside Air, high 80's outside.

So it was the compressor
 
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