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A/C Bypass Question

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So, on my build sheet one of the order options listed is the 4XA Air Conditioning Bypass. However, under the hood, it seems like all the pieces for AC are hooked up and working, (the face of the compressor spins correctly when active, and the ECU even adjusts idle speed when the compressor is on), and all parts seem to be in good condition. There is no bypass pulley (obviously, haha) and no other obvious signs that AC should not work.


There is no refrigerant in the system however, so the air comes out hot. I was thinking of trying to do the R12 to R134a conversion before an EVAC and recharge, but I'm a little confused now.



I have a few questions:

1) I thought that for XJ's and MJ's the AC bypass just meant none of the parts were installed to begin with, however after running over some diagrams, this seems to be an original system, fully installed. Am I incorrect in that understanding?


2) Is it possible that the original owner (I am the second) had an AC system installed by a separate party after purchasing the truck?

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The fact that it seems to have all the "normal" stuff for an AC equipped vehicle suggests a former owner did a conversion (or maybe some sort of dealer conversion was done, but I don't know if they offered such back in 1992)


If the AC compressor comes on, and the ECU bumps the idle speed up, there must be SOME refrigerant in the system.  (There is a low pressure switch that will prevent the compressor from rotating if there is little or no refrigerant in the system.)  Since it sounds like you have low refrigerant, you have two choices:


1.  find some R12 (which is pricey), and get it refilled with that.  (And finding someone other than a DIY who will recharge with R12 is tough.)


2.  convert to R134a


I would lean towards the latter approach, mainly because in a vehicle as old as ours, the parts of the system WILL fail sooner, rather than later, and refilling with the expensive R12 will make you go broke faster each time you replace one or more pieces that have failed.


So a couple of questions:   how knowledgeable are you about AC systems?  Do you have any tools for charging a system, or do you have a friend who does?


I can write up a long list of stuff you will need to do, but before I bore you, tell me about your experience/knowledge and or tools you have available.

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I know very little about AC systems in general, all I know I've researched in the past few weeks since buying the truck haha


I've watched videos and read write-ups from others who have done the conversion, but I've never done anything like it myself.. Your help would be greatly appreciated! 👍

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We always say, take the build sheets with a grain of salt.

When you convert to 134 you will need to find all the leaks of the r12 system

You will also need to completly flush the system and replace the orings

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I have done several conversions from R12 to R134a, and have repaired multiple R134a systems, so what I will tell you is based on what I have studied, and what I have done.


Lubricating oil (to lube the compressor) circulates with the refrigerant in an automotive AC system.   R12 refrigerant uses a different lubricating oil than that for R134a.  You cannot use R134a with the old lubricant for R12.   You MUST use R134a compatible oil.


The good news is that, although your existing system probably has plenty of R12 oil in it, once you introduce R134a (and it’s oil), the old oil will NOT circulate in the system.  Since it does not dissolve in R134a, it just sits in low spots in the system, mostly in the “accumulator”, (also known as the “receiver-drier”) and in the condenser.   It remains there, harming nothing.


Now for the bad news:   The R134a oil is not compatible with the o-rings that seal all the connections in the AC system.   They must all be replaced with new, R134a compatible type o-rings.   They are not expensive, but there are a bunch.  Basically wherever two pieces of the AC system join, there will be an o-ring.   That means every joint will need to be disassembled, and the oil o-ring replaced with a new one (using the new R134a compatible oil to lubricate it during installation).


The old hoses that are part of the R12 system also have a quirk about them.  The new R134a refrigerant molecule is MUCH smaller than the old R12 molecule.  That means an R12 hose will likely leak R134a right through the pores of the rubber hose.   EXCEPT:  if the R12 hose is old (original), it is probably soaked on the inside with R12 lubricating oil, and that oil acts as a barrier to keep the R134a from seeping out.   (New hoses used in R134a systems have an added barrier layer to keep the sneaky little R134a molecules from escaping.)


I give you all this background, so that you will know why you are doing certain things in your conversion.   I will explain the conversion in a separate reply.

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I am assuming you don’t have access to two critical tools you will need to COMPLETELY convert the system, and that is a vaccuum pump and a charging manifold.  That’s OK because I can guide you to the point where you need to recharge the system, and then you can take it to a professional to do that part.


STEP ONE  --- You must discharge the system to remove all the existing R12.   It is against Federal law to just let the R12 escape into the atmosphere, so if you are wanting to be totally compliant with that law, you would need a professional shop to remove all the existing R12.   Of course, sometimes just working on an old R12 system will cause a leak that will discharge all the refrigerant anyhow, if you get my drift.


STEP TWO  ---  Once all the R12 is gone, now it’s time to replace all the o-rings with the new R134a compatible ones.  This is simple enough—just disconnect each joint one and a time, and remove the old o-ring, and install a new one.   Be sure to use proper oil to lubriate the new ring when installing.   The newer XJ’s with R134a used PAG46 oil, so I recommend that oil for your lubricant.


There will be two parts of the system you will want to actually remove while replacing the o-rings:   the accumulator and the compressor.   I will explain below.


Part of your conversion will be installing a NEW accumulator.  The part, aside from storing a bunch of the old R12 oil, also has a dessicant to remove moisture.  On a system as old as yours, it’s probably worn out.  New accumulators are cheap, and help assure a proper conversion.  (All accumulators are compatible with both R12 and R134a, as long as you make sure they have the R134a o-rings on their fittings.)


The other thing you will want to do is to drain the compressor of as much of the old R12 compatible oil as you can.   You can do this by disconnecting the refrigerant lines from the compressor, and unbolting the compressor from it’s mount.  Then orient the compressor so the inlet/outlet is facing down, and rotate the central shaft of the compressor so it will pump out all of the old oil.  There will probably be an ounce or two in there.  Catch and measure how much comes out, as you will neeed to know that for the next step.


More to follow.....

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STEP THREE -- Now you need to start adding the new oil to the system.   My 1988 shop manual lists the following amounts that need to be added to various parts of the system:


CONDENSER – before making the final connections with the new o-rings, using one of the lines that goes into the condenser, add in ONE OUNCE of new oil.


EVAPORATOR – again, before making the final connections with the new o-rings, using one of the lines that goes into the evaporator, add in ONE OUNCE of new oil.


COMPRESSOR – before making the final connections with the new o-rings on the compressor, you will want to add the oil.  The shop manual says to add seven ounces of oil to the compressor.  I recommend you add the amount that came out of the compressor when you drained it (and measured it, remember?), PLUS ONE OUNCE.  (You add the oil in though the suction port, while turning the compressor shaft by hand.)  What ever portion of the seven ounces is left after doing this, add that to the accumulator as described below.


ACCUMULATOR – add whatever was left from the compressor fill above plus TWO OUNCES.


STEP FOUR – now it’s time to have a professional fill (charge) the system.  First, check to make sure all of your fittings are tight, of course.  Then head to your favorite AC shop.


Tell the shop what you did, and tell them you have prepared the vehicle to be converted to R134a, and you need them to check to make sure the system is evacuated of all remaining R12 and air, and that it will hold pressure.  


Then tell them you want it filled with the appropriate amount of R134a.  (The general guideline is that you fill with 80% of what the original fill of R12 would have been.)


Hopefully the shop finds no leaks, and they can fill sucessfully.  That’s it.  


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^Listen to this guy. He knows what he is talking about. There is much "hackjob-ery" in the world of A/C, but you are being given good advice here.


Photos of your truck's A/C setup would be extremely helpful, just so we can be sure we're not dealing with some backyard Frankenstein's monster of a system.


Do you know that the system contains no refrigerant?


There is also every possibility that the low-pressure cutoff switch has been bypassed, is defective, or disabled in one way or another. This is an unfortunately common hack way of getting a misbehaving A/C system to "work" and sometimes does work in the short term. This can be a VERY BAD thing because it can result in the compressor running without meaningful lubrication. Imagine what happens if you run an engine with no oil OR oil filter, so any metal bearing material that shakes loose is recirculated into the system to make things even worse. As you may imagine, this will ruin a compressor very quickly. So don't turn the system on again until you know for sure that it contains refrigerant and the low-pressure safety switch (that's its real purpose on an expansion-valve type system - to protect the compressor from oil starvation) is working as intended.


(assuming it's the OEM setup or approximates the OEM setup) To check to see if the low-pressure switch has been bypassed, follow the smaller of the two lines that comes out of the expansion block on the firewall until you get to the receiver-drier. There should be a switch threaded into the side that faces the passenger side shock tower, with a two-pin electrical connector plugged into it. Check that this is plugged in and hasn't been bridged with a paperclip or something.

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Here are some general pictures of what I found:


1 and 2 are both sides of the compressor, old, but in relatively good shape. 


3 and 4 are the low pressure switch, with the two pins like you said. It all looks in good condition. I have see no reason to think that the system doesn't work, however the compressor never turns off like it should if it's low on freon, so I'm not sure what that would mean. Maybe the switch itself is bad/never triggering?


One obvious sign is the stain on the hose coming out of the top of the receiver, which most likely needs to be replaced. All the hoses should probably be replaced when I convert anyway. Other than that, connections on all the hoses are solid.


Any thoughts?





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OK, this doesn't look like a '91-'92 factory install to me. Can you take a "zoomed out" pic of that whole side of the engine bay, getting as many of the A/C parts in it as you can?


I'll try to study the pics in more detail later today. It may just be the perspective but it looks way different from my truck's original setup.

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Here is a picture of the left side of the engine bay. You can see near the bottom center the rear of the compressor, below that is the top of the receiver-drier. 


I'm planning on cleaning the engine bay here in the next couple days too, it's pretty dirty haha :laugh:


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This looks like an aftermarket installation for sure. All bets are completely off. The factory expansion valve is notably absent. You have one of a completely different style.


You need to get someone to put gauges on it, verify that it contains no refrigerant, and then perform a vacuum test to see if there's any possibility that it can hold refrigerant. Do not run the system anymore. If the compressor turns on without refrigerant in the system, there is a wiring defect somewhere that NEEDS to be fixed before you can have a viable system again.


I would almost be inclined to take it all out and install an OEM system if the wiring isn't all hacked up. You can at least get most of the parts for the OEM system still.

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Jeepmjga, that sounds great.


Minuit, do you have a picture of the OEM setup? I'd love to see it to compare so I know what to look for. 


With any luck, the system was installed after R12 was made illegal, and is already setup for R134a! haha

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It is an aftermarket setup, but the one thing I can't answer is if it's a dealer install setup. That compressor does appear to be a Chrysler-style one, similar to the one Joe and I installed. The way to tell if a setup was a dealer unit is to look for the four-wire connector in the engine compartment, driver's side, next to the ground attachment (C102?) The factory units are not given that connector. Joe and I went with factory parts, and the only one I had to modify was the A/C power harness.

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