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A/C system question

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I recently purchased a 91 Nissan hard body pickup with 138k miles...it's a great little truck for an even better price. The A/C isn't working and the previous owner says that it worked great until he let the truck sit for a pretty long while as he no longer used the vehicle. My question is if the issue is just that the Freon leaked out of the system shouldn’t the A/C compressor still kick on when you turn it on? I'm trying to figure it if I'm just out of Freon or if the compressor has gone out. Any help is much appreciated!

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I'm by no means a pro at HVAC, but I know that some if not all automotive compressors are fused. I chased my tail on one of my cars for a long time before making that discovery. Problem solved with a 50 cent fuse.

Last spring, the a/c in my Silverado began getting warmer and moving less air in fairly rapid fashion. The bottle of recharge with the pressure meter on it indicated that the charge was full. I couldn't hear the compressor kicking on, so going through the manual and a service book, I realized that I had completely forgotten about the dual filters in the system, below and behind the dashboard. Upon opening the filter access door, I got a nice surprise of about a quarter-bushel of old leaves, pine straw and dirt dumped in my face. Two new filters and a half hour later, viola! High level CFM and COLD air once again.:applause:

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I recently purchased a 91 Nissan hard body pickup with 138k miles...it's a great little truck for an even better price. The A/C isn't working and the previous owner says that it worked great until he let the truck sit for a pretty long while as he no longer used the vehicle. My question is if the issue is just that the Freon leaked out of the system shouldn’t the A/C compressor still kick on when you turn it on? I'm trying to figure it if I'm just out of Freon or if the compressor has gone out. Any help is much appreciated!

No, the compressor wont run if theres no freon, the freon is what lubricates/cools the compressor, without it, it would burn up quickly. The system has a pressure switch on it, if that switch doesnt detect a near full load of freon, the compressor wont kick on.

for VERY short periods of time you can check the compressor by bypassing this pressure switch (jump the 2 pins with a wire) and the compressor should kick on and run normally... dont do this for more than a minute or so though. You'll have to get a nissan guy to tell ya exactly where that pressure switch is on a Nissan... on GM vehicles its a oval shaped plug on the side of the freon tank with 2 prongs in it, cross the 2 prongs and the compressor thinks theres enough freon in the system and runs.

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No, the compressor wont run if theres no freon, the freon is what lubricates/cools the compressor, without it, it would burn up quickly. The system has a pressure switch on it, if that switch doesnt detect a near full load of freon, the compressor wont kick on.

for VERY short periods of time you can check the compressor by bypassing this pressure switch (jump the 2 pins with a wire) and the compressor should kick on and run normally... dont do this for more than a minute or so though. You'll have to get a nissan guy to tell ya exactly where that pressure switch is on a Nissan... on GM vehicles its a oval shaped plug on the side of the freon tank with 2 prongs in it, cross the 2 prongs and the compressor thinks theres enough freon in the system and runs.

Do what cowboy says, and the dual pressure switch is located on the reciever drier (liquid tank) by driver's side radiator. Have engine running, turn a/c on, disconnect dual pressure switch and short the 2 wires together. If it comes on you need freon and find the leak. Don't leave the wires shorted together for more then a couple of seconds. If it doesn't come on, you have an electrical problem.

Good luck!

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my mechanic friend told me that if there's not enough Freon in the A/C system then the compressor will not kick on. I'm fairly confident that it's simply too low on Freon. My buddy is going to pressure test it next week. Until then...

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Good chance your 91 uses R12, I think the change to R134 was done around 95. If so, good luck tryin to find R12.

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Good chance your 91 uses R12, I think the change to R134 was done around 95. If so, good luck tryin to find R12.

or being able to afford it when you do find it.

Change a couple O rings and you can switch to R135 for next to nothin... walmart sells the recharge kit and cans for 20 bucks or so.

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Good chance your 91 uses R12, I think the change to R134 was done around 95. If so, good luck tryin to find R12.

R12 has actually gotten cheaper over the last couple of years. Unfortunately you can't just buy it anymore. You need to pass an EPA test to get the "license" to buy it. While the wholesale price has gone down the shops still charge top dollar for it. If your system isn't screwed up, like a bad compressor, I would just try and get it recharged. It may just be a hose or fitting that is leaking.

or being able to afford it when you do find it.

Change a couple O rings and you can switch to R135 for next to nothin... walmart sells the recharge kit and cans for 20 bucks or so.

This is a sure method to having a fried compressor. The oil used for R12 is not soluble in R134a. At least you can buy R134a. What happens is that all the oil gets stuck in the evaporator and the compressor fries. I would not reccomend changing to R134 unless you swap out the compressor, flush the rest of the system, and go with a good ester or PAO oil. To properly fill an opened system requires a good vacuum pump.

There is a forum just for A/C systems, with good info and helpful people. It's at http://www.autoacforum.com/

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This is a sure method to having a fried compressor. The oil used for R12 is not soluble in R134a. At least you can buy R134a. What happens is that all the oil gets stuck in the evaporator and the compressor fries. I would not reccomend changing to R134 unless you swap out the compressor, flush the rest of the system, and go with a good ester or PAO oil. To properly fill an opened system requires a good vacuum pump.

There is a forum just for A/C systems, with good info and helpful people. It's at http://www.autoacforum.com/

I've swapped several vehicles, including 3 of my own and never had a problem with compressors. The thing about having the vac out the system to fill it is just a myth put out there by AC shops to keep people from doing thier own work (and not paying the AC shop)

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I've swapped several vehicles, including 3 of my own and never had a problem with compressors. The thing about having the vac out the system to fill it is just a myth put out there by AC shops to keep people from doing thier own work (and not paying the AC shop)

I can tell you firsthand, that sucking down the AC system with a good vac pump and replacing the reciever/dryer anytime you open the system is only good sense-these systems do not appreciate moisture, condensation, or improper oil in them, or they like to go :applause:

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I can tell you firsthand, that sucking down the AC system with a good vac pump and replacing the reciever/dryer anytime you open the system is only good sense-these systems do not appreciate moisture, condensation, or improper oil in them, or they like to go :applause:

All I can tell ya is I've been filling AC systems at home for years (home and car) and never had a problem. Sure seems like these horror stories about compressors locking up or tanks rusting out is mostly bunk.

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To change to R134: Remove the compressor and let all the oil drain out (won't be too much as most is in drier). Replace drier putting 3/4ths of the required system oil in it first. Replace all o-rings on system coating them with A/C oil first. Put the remaining oil into the compressor intake (low side) hose (usually a couple of ounces). Leak test the system using nitrogen, a small dose of R-134 and a sniffer (vacuum leaks tests are ineffective). If the system has a leak, for example around the compressor shaft, evacuate the system using a recovery device then fix the system or replace the bad component. It is against the law to knowingly discharge any refrigerant into the air. Fines are stiff. If the system tests OK, charge it, substituting the specified amount of R-134 for R-12. Label the underhood A/C sticker so future owners will know it has been converted. Cross your fingers and enjoy the A/C. Remember that R-134 engineered systems have larger condensers and evaporators than R-12 systems because R-134 has a higher boiling point. Your system will be 5% - 10% less efficient and you will run higher head pressure which has the potential of destroying the compressor.

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I converted my R12 system today to R134a with no problems. I also changed out the drier while I had the system open. I ran some flush through the lines before I added the new freon/oil. I was only showing about 30 lb's in the system...that's why my compressor wasn't kicking on. She's charged up and all is well now. Thanks for all of the tips!

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