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Discussion Starter · #1 ·
I have a 1990 F150 with the R12 to R134 conversion. I replaced every component and both hoses, and now I have a completely brand new A/C system. The problem is the pressure numbers are way off!! My low side is low and my high side is way too high. I’ve read quite a bit and all the information points to a clogged orifice tube. Like I said, EVERYTHING is brand new, including the orifice tube. Any other ideas?? Thanks!
 

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You replaced the condensor too? What exactly are the pressures while it’s running?


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Discussion Starter · #3 ·
Yes, condenser too. Low side is around 15, high side upwards of 250.
 

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Did you install the oriface tube in the correct direction? It does sound like a restriction.


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Discussion Starter · #5 ·
Yes, it’s installed in the correct direction. I even evacuated the system to open it up and make sure it was installed correctly. There’s definitely no blockage, everything is brand new and I even ran compressed air through everything just to make sure there are no restrictions anywhere. The lower evaporator pipe actually freezes up and builds up a good layer of frost on the outside... right up until where the orifice tube sits. But after the orifice tube, nothing. The hose isn’t even cold. So it’s showing an obvious sign of a blockage, but HOW?? Wrong orifice tube perhaps?? Although, I’m using the one that came with the new evaporator. I just don’t know what else to troubleshoot. Weak compressor perhaps??
 

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How far down did you evacuate the system. It doesnt take much air in a system as small as a car's to throw it in a tizzy. Air and moisture can also cause corrosion to build and block passages.
 

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Discussion Starter · #7 ·
How far down did you evacuate the system. It doesnt take much air in a system as small as a car's to throw it in a tizzy. Air and moisture can also cause corrosion to build and block passages.
I vacuumed it down to 29-30 Hg for about an hour. It held pressure overnight (10-12 hours.) The entire system brand new (less than a week) so I doubt there’s any corrosion in there. At least not bad enough to cause a blockage. Very very frustrating!! Question, what is the brass fitting on the main hose that’s attached to the manifold fitting, where the hose attaches to the compressor?? Anyway, that brass fitting was pretty loose. Does that matter??
 

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Yes, condenser too. Low side is around 15, high side upwards of 250.
This is similar to what happened to my vehicle when the condenser was installed backwards after a front/bottom collision. The outcome of running the system like this will be a dead compressor clutch so don't do that. I would double check it's installed correctly. I know in most vehicles they very easily can be installed backwards. If everything else is new, that seems like the best place to start as they are all unidirectional (from my knowledge). In my case, nothing apparent showed which was the correct way of installation and it somehow got flipped even with the hose inputs being differently placed on each side. If you can't 100% confirm the correct way, wouldn't hurt to just flip it and see. Won't harm anything but your time. Never know with conversions, hard to confirm certain aspects.
 

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Is the desiccant in the system meant for R-134a? A lot of conversion kits don’t mention this, but the original R-12 desiccant isn’t compatible with R-134a and I’ve been told it can dissolve and plug up the orifice tube. Which conversion kit did you get?

Frost on the downstream side of the orifice tube can be a sign of low refrigerant (I won’t say for sure that’s it, it’s one symptom). The evaporator is being starved. Your low side pressure confirms this, and I think the Low Pressure Switch should be turning off the compressor before it gets this low. Are you getting any cold air out of the vents?

How much refrigerant did you put in? Did the instructions tell how much it takes?

Since your compressor is able to build a pressure differential of about 235 psi (high side pressure minus low side pressure) your compressor seems to be in great shape. Is the clutch engaging and releasing like it should?

Would you post a picture of the brass fitting? It could be a pop-off valve in case the high side pressure gets too high, but whatever it is if it’s part of the refrigerant system it shouldn’t be loose! Leak source, possibly?

As far as evacuating the A/C system goes, I was taught it should be slurped down to at least 500 microns. You’ll need a good quality electronic vacuum gauge to read this plus a couple of extra hoses. It will set you back $150 or so but that’s cheap compared to what a shop would charge (I’ve used this justification to my wife to buy tools and it works!).

One more thought on the orifice tube; it’s intended to be a restriction. I suspect what you’re seeing might be the result of the compressor running all the time, when it should be cycling on and off in response to system pressures.

Hope some of this helps.
 

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Discussion Starter · #10 ·
Wow!! That’s a lot to take in. Ok, so what exactly is the desiccant?? I’m not sure what conversion kit was used, I had a shop do it. And all I know is there are different fittings than there were before at the service ports. That’s it. The compressor is OVER active as far as kicking on and off. It kicks on and off about every 5 seconds or so. And yes, I have noticed frost below the orifice tube on the pipe going into the evaporator. As far as refrigerant, it calls for 46 oz., however, the most I’ve put in is about 40-42 oz. before the high side gets up to 300 psi, so I stopped there. The vents alternate blowing warm and cold air. Mostly warm though. The brass fitting is exactly that, and pop off valve, now that you mention it. I tightened it, no biggie. I’m pretty sure it wasn’t leaking anyway. And if so, an extremely small, unnoticeable amount.
Anyway, I hope this is enough information to get me somewhere. I’m curious about the “desiccant” you speak of.
Thank you for your help!!
 

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Discussion Starter · #11 ·
This is similar to what happened to my vehicle when the condenser was installed backwards after a front/bottom collision. The outcome of running the system like this will be a dead compressor clutch so don't do that. I would double check it's installed correctly. I know in most vehicles they very easily can be installed backwards. If everything else is new, that seems like the best place to start as they are all unidirectional (from my knowledge). In my case, nothing apparent showed which was the correct way of installation and it somehow got flipped even with the hose inputs being differently placed on each side. If you can't 100% confirm the correct way, wouldn't hurt to just flip it and see. Won't harm anything but your time. Never know with conversions, hard to confirm certain aspects.
Actually, the problem I’m having was happening before I replaced the condenser. I thought changing the condenser might fix it, but still doing the same thing. So it is installed correctly. I guess unless the factory one was installed backwards, but I highly doubt that.
 

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It sounds like you have R-134a adaptor fittings. If one was loose it could have leaked. Is it oily around the fitting that was leaking? Ok so when you put gauges on it the low side drops and the high side climbs and is not steady right? It sounds to me like it’s low on refrigerant. You can jump the low pressure switch to get the clutch to engage continuously to get a more accurate reading. You can’t get a good reading with the clutch short cycling like that.


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Discussion Starter · #13 ·
Yes, they are R134 fittings. Neither one was loose or leaking. I did jump the low pressure switch while servicing the system so I could get accurate readings. Both gauges are steady, they don’t fluctuate.
Speaking of low refrigerant, it calls for 46 oz. but the high pressure gauge steadily rises and gets up to near 300, so I stop at around 40 oz. or so. Should I just go ahead put the full 46 oz. in and see if it levels out??
 

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So you don’t have adaptor fittings? What was loose that you tightened?

When it says 40 oz is that for R12 or R134a? Also how are you adding refrigerant? Are you using small cans?


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Discussion Starter · #15 ·
Yes, I have the conversion fittings for R134. The brass fitting that I tightened was just a high pressure relief fitting or something for the hose assembly where it attaches to the compressor. It calls for 52 oz. for R12, and I was told to use around 90% for R134, which would be around 46 oz. Yes, using small cans through a hose manifold gauge set using the yellow hose.
 

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Do you have a picture of this valve? Usually there is a valve on or near there for the pressure cutoff switch and a low pressure fitting/port.

When you put the cans in do you purge the lines for air? Does the can get cold? As far as percentages go I was told around 70% but that’s just a ballpark. The best way to fill refrigerant is to read the gauges not the amount of refrigerant. Also if you used a compressor that had ester oil in it that could explain the clog.

Another note, I converted my 87 Grand Marquis about a year and a half ago and I initially had issues with the adaptors not sealing and the cutoff switch leaking around the threads. A little Teflon tape and hvac sealer goop on the threads and it’s been good ever since. It did take some work to dial it in though.


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Discussion Starter · #17 ·
While filling it, I do look at the gauges, not the amount of refrigerant. And you’re right, I only get about 60% of the recommended amount it the system and my readings are already all out of whack. Here is a picture of the brass fitting I’m talking about that was only hand tight but I tightened it and it didn’t change anything.
53415
 

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Hmm maybe that’s the high pressure relief, I’m not sure. In any case there is a blockage somewhere and my money is on the orifice tube. What color was the orifice tube anyway? There are two that are available for automotive ac systems. At this point I don’t know if there’s anything that can be done with the system charged. The system may have the correct charge but there’s a blockage somewhere, probably the orifice tube, and I don’t think it can be cleared with the system charged.


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Discussion Starter · #19 ·
Do you have a picture of this valve? Usually there is a valve on or near there for the pressure cutoff switch and a low pressure fitting/port.

When you put the cans in do you purge the lines for air? Does the can get cold? As far as percentages go I was told around 70% but that’s just a ballpark. The best way to fill refrigerant is to read the gauges not the amount of refrigerant. Also if you used a compressor that had ester oil in it that could explain the


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Oh, and also, are you asking if there was ester in the new compressor that I installed??
Hmm maybe that’s the high pressure relief, I’m not sure. In any case there is a blockage somewhere and my money is on the orifice tube. What color was the orifice tube anyway? There are two that are available for automotive ac systems. At this point I don’t know if there’s anything that can be done with the system charged. The system may have the correct charge but there’s a blockage somewhere, probably the orifice tube, and I don’t think it can be cleared with the system charged.


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Ok, so I evacuated the system, checked/changed all of the o-rings, tightened everything down, and now I have a good vacuum. I changed the orifice tube from a red variable orifice tube to a blue, fixed orifice tube. I’m hoping that may fix the issue. Maybe the Variable tube was too restrictive?? I will find out when it’s time to charge the system.
 

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Yes but it would’ve boiled off when you vacuumed down the system anyway.

From the little research I’ve done the variable orifice tubes are good but apparently they can stick closed. There’s not much info from what I’ve seen so take that with a grain of salt. Hopefully that did the trick. Don’t fill it too fast, make sure it’s going in as a gas and not a liquid and don’t forget the pag oil. Let us know how it comes out.


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