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another AC cycling question- '96 OB wagon
Posted 04 August 2010 - 10:15 AM
So for the few years I've had my 96 OB wagon, whenever I've used the AC, the compressor always seems to be kicking on and off every 10-20 seconds or so. I always assumed that was just how the system was designed, and there wasn't enough damping of the controls, resulting in the noticeable cycling. Whenever the compressor was on, the air always blew sufficiently cool.
Here's where my problems start: a friend noticed this the other day and said it's not supposed to do that, you must be low on refrigerant, get a can and the cheap gauge/hose from the local store and you'll be golden. I thought, wow, all this time I thought this was normal, but I guess not... so I got the can and cheap gauge and started adding some r134a to the system. And yes, I realize now that I shouldn't have proceeded in this manner, AC systems are complicated and best left to a pro, etc.
It was around 70-75F yesterday, and the low side was initially at around 25 psi, with the compressor running, AC on high, etc. I didn't measure the air temp at the vents before I started. I bumped the low pressure reading up to about 30 psi, and I expected the air coming out to be a little colder, but it wasn't... but the compressor did stop cycling on and off, so I thought I was in good shape. I checked it again an hour later though, and this time it was still cycling on and off again. This time I took the temp at the vents, it was around 48-50F. Also, I checked that there were no bubbles in the sight glass when compressor's on, when it turns off, the bubbles show up but disappear right away.
At this point I realized that I had somehow hit the re-circ button on the dash, so it was pulling inside air. Once I changed this to outside air, the cycling stopped, and the compressor runs continuously. If I switch it to inside air, it starts cycling. Either way, the air coming out of the vents is roughly 48-50F.
Does it sound to everyone like my system is functioning properly? I'm guessing that the refrigerant *might* have been slightly low to begin with, and adding some might have slightly helped, since the AC doesn't cycle on and off if it's switched to outside air... but to be honest, it may have done that anyway, as I almost never use the AC without having the re-circulate button depressed.
Thanks for reading my novel. Any input is appreciated.
Posted 04 August 2010 - 12:03 PM
The reason your compressor is cycling when on recirc is because the incoming air is already "sufficiently" cool and tells the thermo switch to cut off the compressor. When the temp increases, compressors kicks back on, and the cycle repeats. If the incoming air is always warm, then the thermo switch doesn't kick off.
What I have found is that some cars don't have as good of a control system and as a result the temperature swings between when the compressor kicks off and on are greater. Additionally, compressor design will affect this as well.
The older legacies 90-94 had a variable geometry compressor. So instead of cycling, they would alter the compression ratio internal and allow for less refrigerent to flow to the evaporator which would help moderate temperatures. IMO, this system is superior in terms of keeping a more constant temperature. The downside is the compressors tend to be more complex and may have a more likely chance of failure.
Not sure if I answered your question....if not let me know.
Posted 04 August 2010 - 12:23 PM
Posted 05 August 2010 - 07:30 AM
Thanks to all of you knowledgeable Soobie people willing to help us n00bs!
Posted 05 August 2010 - 12:27 PM
I've got a 97 Impreza Outback Sport and yes the AC compressor clutch is noticable. It's normal.
The AC systems do have expansion valves and function the same way as others. However, like I mentioned, I think some manufacturers use less expensive components and control systems than others, which causes more noticable cycling and temperature swings.
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