88-94 5.7 Suburban A/C Diagnosis and Tuning

[Wildblue19]

Hey everyone. Unique A/C question on orifice tubes for my 1993 GMC 5.7 4x4 Suburban. TLDR, AC sucks and I took off my condensor lines and there was no orifice tube - straight up empty pipe. Put one in and negligible changes in vent temps. Is having no orifice tube intentional for rear air trucks?

Backstory: In south Texas, it's been over 100 for about a month straight. Prior to moving down here, I had pre-emptively rebuilt the entire A/C system and converted to 134 from R12 which was not working. I made custom lines, flushed all the hoses, completely took apart and cleaned the rear air system and TXV, installed a sanden compressor, new parallel flow condensor up front, and changed all O rings. Vacuumed down for an hour, checked for leaks and charged with 4# of 134 and about 8oz (don't recall exact amount) of oil. Worked fine for Colorado summers.

Fast forward to living in Texas, I felt that the A/C in my truck was lacking, with vent temps for a 105° day with 30% humidity around 75°. Tested high/low side pressures, and before taking the system apart I saw 50/245psi. This seemed reasonable, but dissatisfied with this performance I opted to install a VOV to replace whatever factory one came in the condensor.

Took the condensor lines off today and to my surprise, the condensor had no orifice tube. I thought the condensor came with them installed, and didn't check on initial install (I know, shame on me). I put the VOV in today and vacuumed down, checked leaks, and charged with 4# again. Vent temps are pretty much the same, perhaps 2-4* cooler, for both front and rear air. Pressures changed slightly to 55/240 for a 102* 20% day.

My questions:
1) for rear air trucks is the orifice tube left out? Perhaps the rear TXV handles the front and rear systems?
2) is a delta T of 30 degrees or so the limit for these trucks? With front VOV and rear TXV the best I've seen is about 32* Delta T.

Maybe an e fan is the answer, but I don't see much change from idle to highway, maybe 4 degrees. Any input is appreciated to include data points on other suburban drivers vent temps and pressures. Thank you!

WB.

Edit, been looking around and local expert @L31MaxExpress may be able to shine some light upon the situation. My sanden unit is part # 4864 and the VOV is 38905.

 

[Carlaisle]

I'll be watching this with interest because I also have an AC system that is functioning better than yours but which is still inadequate. It should blow 40 degree air going down the highway even in 100 degree outside temperatures. Mine did that before the compressor seal failed. Since replacing it, it will only hit about 65 at idle and 55 on the highway. Pressures are good, charge is correct, condenser is clear, so there's obviously at least one additional problem yet to be identified. Hope you get your problem sorted out.

 

[Schurkey]

There is no orifice tube in the rear system. That's the job of the thermostatic expansion valve. Remove the orifice tube from the rear.

There is no orifice tube in the CONDENSER. There will be an orifice tube somewhere in the high-side liquid line between the condenser and the front-system evaporator.

Back in the R12 days, we were told to expect high-side pressures of roughly three times ambient temp. If it was 100 degrees out, high side pressure would be somewhere around 300 psi. I don't know what the rule-of-thumb for R134a is, but I bet it's more than three times ambient. Your high side seems low to me; and the low side seems high.

When it's me, I vacuum for WAY more than an hour. Maybe I'm wasting time.

Even if the A/C system is working perfectly, if the air doors don't seal, you'll have high duct temps.

I have NO idea what a VOV is.

 

[L31MaxExpress]

I saw some information that some year suburbans with rear air used an orifice tube at the condenser but I do not see how that could work at all. My G20 van, my Express van and my Tahoe all have an orifice tube after the Y for the rear unit ahead of the evaporator and a TXV in the back.

My 99 Tahoe has a front orifice exactly as shown in this clip.

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[Nad_Yvalhosert]

I have NO idea what a VOV is.

"Variable Oriface Valve:
Basically a orifice tube assy that will change size automatically.

Fixed Orifice Tube Diagnosis - AC Repair

To troubleshoot a CCOT automotive air conditoning system take the FOT fixed orifice tube and VOV variable orifice tube into consideration. There's a difference between the two - a fixed orifice tube and a variable orifice tube.

www.freeasestudyguides.com

 

[L31MaxExpress]

There is no orifice tube in the rear system. That's the job of the thermostatic expansion valve. Remove the orifice tube from the rear.

There is no orifice tube in the CONDENSER. There will be an orifice tube somewhere in the high-side liquid line between the condenser and the front-system evaporator.

Back in the R12 days, we were told to expect high-side pressures of roughly three times ambient temp. If it was 100 degrees out, high side pressure would be somewhere around 300 psi. I don't know what the rule-of-thumb for R134a is, but I bet it's more than three times ambient. Your high side seems low to me; and the low side seems high.

When it's me, I vacuum for WAY more than an hour. Maybe I'm wasting time.

Even if the A/C system is working perfectly, if the air doors don't seal, you'll have high duct temps.

I have NO idea what a VOV is.

As long as it is over 85°F and I can get down close to 29 in/hg I vacuum 90 minutes.

VOV stands for Variable Orifice Tube. Its basically an orifice designed by the designer of the original F.O.T designed to reduce flow at idle and then open up at highway speeds.

 

[L31MaxExpress]

What I am getting at idle currently. It was 102F when I tested this. Both front and rear blowers on high fan speeds. Recirculate up front and the rear lacks an external air inlet so it is recirculated 100% of the time.

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I added a 16" puller fan behind the radiator with the mechanical duramax fan blade in place. What I get when it is switched on. Measured at the left vent this time. Center vent sits at 38°F.

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At 1,250 rpm it will drop to 33°F and cycle the compressor. Going down the road it will get down to about 29-30°F before the compressor cycles. The suction line off the accumulator will start to pickup frost before the compressor cycles.

I have a ball valve in the heater hose coming off the intake tha positively shuts off the coolant flow through the heater cores. The plastic GM valves do a poor job and heat still enters the cores and heater boxes.

 

[Wildblue19]

Thank you all. @L31MaxExpress , that video helped a great deal. I will pay penance for my transgressions and blow the truck apart tonight and report findings.

Did you fill to 4# then add / remove refrigerant to tune pressures or is that straight up?

I have a wired in board I made that reads the potentiometer for the blend door that compares against a reference voltage. Once at full cold, it triggers a relay that shuts the heater core off via a solenoid at max cold, pictured below.

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Thanks again.

 

[L31MaxExpress]

Thank you all. @L31MaxExpress , that video helped a great deal. I will pay penance for my transgressions and blow the truck apart tonight and report findings.

Did you fill to 4# then add / remove refrigerant to tune pressures or is that straight up?

I have a wired in board I made that reads the potentiometer for the blend door that compares against a reference voltage. Once at full cold, it triggers a relay that shuts the heater core off via a solenoid at max cold, pictured below.

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Thanks again.

On my 97 van, I put straight up 60%ish of the 4.25 lbs R134a charge as per molecular weight difference. It may or may not be charged with R152a if you catch my drift. I put as much as I could get out of 4 x 10 oz cans. At that point pressures looked good, superheat temperature was good and the a/c was ice cold.

Ps, I would still verify the hot water is positively shut off. I have found many of those valves do not seal well and it almost seems by design as some even have a small hole drilled in the butterfly.

I also discovered last night that my 97 has a pretty large cold air leak out of the evap box under the hood near the accumulator. Its not in a place easy to see, so I plan to poke around with my inspection camera. Either the box is cracked or the seal on the suction line of the evaporator is missing. Either way for me I am lucky as GM still uses the same evaporator case even on a 2022. The case itself as well as the seal is fairly cheap and still readily available.

 

[L31MaxExpress]

Well found my air leak. Foam seal must have come apart and blown away into the wind. Evap case has a 1.5" x 2.5" opening straight into the engine compartment. Luckily an easy cheap fix.

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