Automotive Air Conditioning Information Forum

High everyone. I've been gleaning knowledge from this site for a while, but this is my first post.
I have a 88 Chevy suburban that needed a new compressor and it was recommended to be retrofitted to use R134a at the same time. The mechanic said he had done quite a few retrofits and while they do not cool quite as well as R12 his customers who he had done them for were happy, so I had him do the conversion. It seemed to work ok , but after about 5000 it developed a leak and stopped working.The shop that did the conversion is now out of business so since I have an R134A gauge set, vacuum pump, electronic leak detector and a little knowledge about automotive A/C systems I decided to try to fix it myself.
I vacuumed the system for 1 hour then put enough r134a in to get the compressor to run. I then used the electronic leak detector and found that the evaporator had a leak. I replaced the evaporator, orifice tube, accumulator/dryer with OEM replacement parts and added a high pressure cut off switch to the compressor. I also upgraded the condenser from the stock tube and fin type to a 3/4 x 16 x 26 parallel flow 2 pass unit made by UPI. I was thinking that increasing the efficiency of the condenser may help make the system produce cooler air. I removed the compressor and drained out the residual oil. I added POE oil to the new components for a total of 6 oz. I then evacuated the system for an hour. Since the condenser is not stock I figured the recommended amount of refrigerant for this retrofit would not be correct so I charged it up until the compressor stopped cycling at idle and the pressures were at 27 and 175. The air temp coming out of the main duct was around 58*F with the ambient temp 90*F . I took it for a test drive to see if it would improve but the compressor started cycling every 2 second or so and the temp only dropped about 2 degrees. I've tried adding more refrigerant until the pressures were 34 psi and 210 psi @650 RPM and 21 psi and 250 psi @ 2300 rpm, ambient temp 90*F.
I may be mistaken but it seems like the compressor should not be cycling this quickly when driving. I am wondering if the orifice tube should have been changed to a different size or an adjustable type. I have built a custom R134A system that used an expansion valve for a military HUMVEE that worked great but this retrofit has me in over my head.
I could really use some tips or wisdom to get this system working properly.
Thanks in advance for any help with this trouble shooting.
Luther4130

Switching to a PF condenser will not change the charge amount by much at all.
Idle pressures are not helpful, we are looking for about 1800-2000 RPM.
Did you measure how much oil drained out of the compressor?
Bohica is gonna want a full load test.
These conversions are a bit over my pay grade so lets let some of the sharpshooters comment before you go further

According to my references, the specs for your vehicle are as follows: Without rear A/C - 52 ounces net weight of R-12 and 6 fluid ounces mineral oil; with rear A/C - 84 ounces net weight of R-12 and 9 fluid ounces of mineral oil. If you have an under-hood decal with differing specs, then the decal takes precedence.

You should put in about 70 percent of the R-12 amount when converting to R-134a and work up from there until you reach the lowest center vent temperature. So the approximate minimum R-134a amounts for your vehicle would be 36 ounces and 59 ounces respectively. If you're charging only by pressures, then I think that your charge amount is probably off, perhaps by quite a bit, because you can't charge by pressures alone. You'll need an accurate special-purpose scale to weigh-in the refrigerant. Of course, the mineral oil would be replaced with PAG oil of the proper viscosity.

See the tutorial titled "Minimum requirements for converting a system to R-134a" in this forum.

Johnhere.
I was thinking that by changing the type of condenser from tube and fin to parallel flow would change the amount of refrigerant needed for a correct charge.The old one looks like it has a much larger internal volume than the new one and therefore the stock specs would be off as a starting point. I was thinking that I may have over charged it. As for the compressor cycling duty cycle, What is considered acceptable? What is your opinion on variable flow orifice tubes for R-134A retrofits?
Thanks for your help.
Luther4130

You may want to re-read what I mentioned and what John mentioned.
Strongly suggest you follow John's advice as closely as possible, he will take you thru your problem step by step. As mentioned, until you obtain the correct charge (that is step 1) , all bets are off on the other parameters you would like to address.

The 4 experts (which does not include me) on here are the sharpest mobile AC guys I have ever come across. Stay with them......

The pressures indicate a reheating or blend door issue.

With evaporator pressures in the 21 to 27 psi range that is a very cold evaporator - well below freezing. Even 34 psi is 39f Yet you have 58f vents.

Air flow is likely bypassing the evaporator somehow. It is getting cold.

Cycling.
First, cycling only happens to keep the evaporator above freezing. On a hot day it may not cycle at all until the heat load drops. Sometimes cycling switches fail, or develop a lot of hysteresis - a big dead band.

Short or rapid cycling can happen with a low charge or when there is very little heat load. Using your A/C on a damp winter morning with the defroster will have the system running very short cycles because the evaporator drops below freezing very quickly.

But you seem to have enough charge. Just no heat load. And why is your cycling switch trying to make ice cubes? It should cycle off by 26 psi, and come back on around 31 psi. It really should not be running at 21 psi on the low side.

You state an ambient of 90f. How humid is it where you are?

LUTHER4130 wrote: Tue Aug 17, 2021 5:53 pm Johnhere.
I was thinking that by changing the type of condenser from tube and fin to parallel flow would change the amount of refrigerant needed for a correct charge.The old one looks like it has a much larger internal volume than the new one and therefore the stock specs would be off as a starting point. I was thinking that I may have over charged it.

The reason why going from a tube-and-fin condenser to a parallel-flow type won't materially change the system's capacity is that, yes, the tube-and-fin looks larger. And yes, the parallel-flow condenser has much smaller passages. But the PF also has more tubes in the same space. So the refrigerant capacity of both types of condensers would be about the same.

It's certainly possible that the charge is off if you didn't use a precision scale to weigh-in the refrigerant.

LUTHER4130 wrote: Tue Aug 17, 2021 4:08 pm The air temp coming out of the main duct was around 58*F with the ambient temp 90*F

Does this imply that some of the other vents were warmer? or you only measured air from the center duct? at high load, 1500 RPM max blower, are the vent temperatures roughly the same or are 1 or 2 of them warmer?

Do you have the LPCS with the adjustable screw between the prongs?

One other thing to check, since it will run as low as 25 psi on the low side...

Run it above 1500 rpm for a little bit, and look at the accumulator. Does it have a frost line? Is if half full of refrigerant?

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