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Old 05-09-2010, 07:01 PM   #1
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Default How to bypass lo pressure switch for AC

Hi,
My AC compressor will not turn on when I slide the controls to AC. I just purchased this 96 XJ, and have never needed the AC until now. The previous owner said it probably needed to be recharged.

I want to see first if the compressor will even turn on though before I pump $30 of freon into the system.

I have read that you can bypass the lo pressure switch on the compressor to get it to start up....how do you do that?
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Old 05-09-2010, 08:04 PM   #2
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jumper the switch. But if it doesn't turn on now, it is probably low on r134. If you are recharging it yourself it won't take any if the charge is already full. Take it to a shop so they can pull a vaccumme to see if theres a leak. It will save u money in the long run
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Old 05-09-2010, 08:26 PM   #3
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I have the same problem with my '94. Where is the low pressure switch located?
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Old 05-09-2010, 10:00 PM   #4
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WHAT!!! it wont take any if its already full!!! You do know that there is a specific amount (lbs) thats suppose to be in the system... if you just keep sticking cans in there till it wont take any more your going to blow out every seal in the system... the switch is on one of the lines itself... use a paper clip to jump one wire to the other... BUT NOT the ground wire... and dont run your own wire from the battery... i made this $665 mistake last week on my 03 cummins...
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Old 05-19-2010, 08:41 PM   #5
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where is the lo pressure switch? What does it look like?
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Old 05-19-2010, 10:28 PM   #6
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Click the image to open in full size.

This is a generic picture but it shows it just the same. The low pressure switch is located on the accumulator up against the fire wall on passenger side (at least on mine)
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Old 05-20-2010, 01:06 AM   #7
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Quote:
Originally Posted by chrisl757 View Post
where is the lo pressure switch? What does it look like?
It looks like this on the Renix 4.0L
Click the image to open in full size.

The 2.5 and 2.8 have the same switch on the filter/drier on the side of the condenser coil. Not sure where it is on the later models. I looked at one and can't remember where it is. Same switch though.

It is that little two pronged switch. If you want to by pass it for a test, just pull the plug, bend a paper clip and stuff the ends into the contacts.

As for not taking a charge because it is full, not true. It is real easy to overcharge a system.
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Old 05-20-2010, 05:38 AM   #8
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Quote:
Originally Posted by chrisl757 View Post
Hi,
My AC compressor will not turn on when I slide the controls to AC. I just purchased this 96 XJ, and have never needed the AC until now. The previous owner said it probably needed to be recharged.

I want to see first if the compressor will even turn on though before I pump $30 of freon into the system.

I have read that you can bypass the lo pressure switch on the compressor to get it to start up....how do you do that?
IIRC, you can bridge the two leads on the LPCO atop the receiver-dryer.

NB: Do not do this for very long! About fifteen to twenty seconds should be about the lot! The refrigerant charge also carries compressor oil through the system, and a low charge means low lubricant circulation. Yes, the compressor carries most of its own oil. Most of it. The oil in the system is necessary to keep the reed valves in the compressor head from sticking, and lubricating the top piston ring. And keep the gaskets and seals wet so they don't dry out and shrink. And ...
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Old 05-20-2010, 09:29 PM   #9
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Ok, one more question:

I took the Jeep to Autozone and they checked the lines to see if I had any freon in the system. They pushed a needle nose plier set into the ball valve and nothing came out, i.e. it is completely empty.

here's my question now:

Should I still try to jumper the lo press switch if the system doesn't have anything in it? Wouldn't it be too much wear on the compressor?

Or should I just go buy the fill kit, fill it up and look for a leak?

I am guessing to just buy the freon at this point. If the compressor hasn't run in years (likely), I don't want the first time it's started up to be when there is absolutely nothing to compress.

Right?
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Old 05-20-2010, 10:13 PM   #10
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Thefirstthing you need to do is decide if you want to fix that leak. If you don't, just charge it and the compressor will come on. Charge it with the can upside down.
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Old 05-25-2010, 08:03 PM   #11
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How do you tell if the LP Switch is bad?

Mine is doing the same thing. When I jumper the HP switch on the back side of the compressor it kicks in, but when I do the same on the LP it doesn't do anything. And it's not low on r134.
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Old 05-25-2010, 08:10 PM   #12
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Is there current going to the switch? If you have a ohm meter you would check for continuity.
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Old 05-25-2010, 09:15 PM   #13
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Take it to a shop! You will just screw it up if you're not even sure of how the system works. TAKE IT TO A SHOP! You will save money in the long run and have some kind of warranty for the work done. But if you want to throw butt loads of money at it and still be sitting in a hot Cherokee, its up to you.
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Old 02-20-2011, 09:19 AM   #14
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hea guys, i'm kinda new here, but i've built 3 cherokees already and this 1988 XJ has me stumped, it has the 4.0L RENIX FI I6 and if the AC doesnt have a charge the electric fan seems to not want to kick on, if i charge the system the electric fan will come on and work fine, but it has a nasty leak, and i'm thinking that the AC isn't worth fixing, the previous owner spent a ton of money to convert the AC to R134a from the origional R12 system (In my opinion R12 works better but just my .02), i charged the system 2 days ago because it was just a little low, the AC worked for that day, and the next day didnt work again, and its back to the electric fan not kicking on, and the jeep had a previous overheat issue, i had to drive all over town to find the cap for the pressure bottle that was rated for the correct pressure... that was a nightmare in itself... <EDIT> i found the switch, the dryer and associated electrical componants were relocated by the previous owner to make room for a tow bar assy that wrecked the front right framehorn...

Last edited by Jeeper426; 02-20-2011 at 09:55 AM. Reason: found the part
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Old 02-20-2011, 11:32 AM   #15
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The pressure switch is on the accumulator located on the passenger side frame rail.

Click the image to open in full size.
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Old 02-20-2011, 11:32 AM
 
 
 
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