1988 Chevy Suburban r12-r134a retrofit problems

[LUTHER4130]

I have not had the time yet to empty and recharge the system with 36 oz of refrigerant but today I had the time to replace the LPCOS and it is now turning the compressor off at 26-27 psi and turns it back on at 40 psi.
Then I removed the duct that attaches to the rear of the heater-A/C housing under the dash so I could check the air mixing shutters to see if all the air was flowing correctly. It looked and felt like all the air is flowing through the evaporator. I stuck a long thermometer in there to get a reading right behind the evaporator. With the engine running @ 670 rpm and the blower on high,the pressure was 30 low and 180 high, the right side of the evap was 58* and on the far left side it was 80*. Looking at the back of the evap there is just enough condensation on the far right 1-1/2" to see it and none on the rest of it. It looks like only 1/4 of the evap passages are getting cold at all. I cut open the old evap and all its capillary passages are connected to a large plenum on the bottom where the refrigerant flows in from the orifice tube giving them all basically the same amount of flow . This one is getting cold on the end farthest from the orifice tube, it seems like the first 4"-5" of capillaries are completely plugged .This seems really bizarre to me but this looks like it could be the main reason the system is not working correctly. Are there any suggestions on how to diagnose this without taking the whole housing out again and replacing the evap again?
I want to thank all you guys again for your help.
Luther4130

[tbirdtbird]

Any chance we can get a pic of the guts of the old evap....

[DetroitAC]

Sounds like you are just starving most of the evaporator, either undercharged or the OT is too small, is the evaporator an aftermarket? If you could link us to a picture of the evap so we can see how it's circuited?

[LUTHER4130]

Here are a couple of PICS of the original evaporator. The one I replaced it with is a Fourseasons 54275.
If its under charged and starving the evaporator would the over all temp of the evaporator just be warmer and not have a 22* difference from one side to the other? It also seem like the path of least resistance for the refrigerant to flow would be on the left side of the evaporator not the right where it seems to be flowing ,or is the velocity high enough that the majority of the flow it getting to the right side where it can only do a 90 degree turn and go up or do a 180 to get back to the warmer passages? I trust your opinions on this way more than my little bit of experience and the pressures do seem a little low so I am happy to add some refrigerant and see if this temp difference goes away. How does one determine if the orifice tube is too small?
Thanks again.
Luther4130

[LUTHER4130]

Here is another PIC showing the inside a little better. For some reason I could not attach both images in one message.

[DetroitAC]

Single pass evap, I never worked on stuff that old, would not have guessed it. They are not great at any loads, but there is also not much that can go wrong.

What size, color orifice tube are you using? Is this a front only system?

[LUTHER4130]

This is a front only system and it has a white orifice tube

[Tim]

Do I have this correct? Single evap retro trying to cool a Suburban!

[Dougflas]

I do not use Four Seasons anything. Their compressors did not hold up and that was enough for me. I do not even try their evaps or condensers. Did this vehicle at one time have a dual evap system?

[DetroitAC]

Pretty sure you are just undercharged, white OT is a big one, 0.072"

A newly rigged system takes some patience to figure out the charge. Hopefully you live somewhere warm, set to max blower, put an assistant in the drivers seat to hold the engine RPM to something like 1500 to 2000, doesn't matter that much just keep it consistent.

With a tank or cans on a scale, add R-134a in small increments, keep track of how much you have added. When you are undercharged the pipes exiting the evaporator will be warmer than the pipes after the Orifice (before the evaporator). Add in small increments until you get the evaporator exit pipe to be pretty much the same temperature as the inlet pipe. At this point, there is liquid exiting the evaporator (which is how it should be working), you then add in the charge reserve, this is excess refrigerant that basically just partially fills the accumulator and this "reserve" lets you keep good performance for many years before you need a recharge. I don't have any idea about the size of reserve back in '88, personally I would just guess 6 or 8 ounces, add this 6 or 8 ounces more in. Performance should not change and you'll notice the accumulator will be cold and sweaty and the suction line to compressor should also be cold.

If you keep track of how much you are adding, you can easily get back to that point if you need to fix some other leaks or want to try a different orifice size. If you are just winging it, that's OK, but you'll do this work all over again next time.