88-94 5.7 Suburban A/C Diagnosis and Tuning

[Wildblue19]

Here's The performance chart for the enhanced SD7:

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And for the regular SD7H15:

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At my 5.7's idle (650 rpm), the 4805 spinning at my ratio of 1.7:1 should be 1,100 RPM (8" crank pulley, 4.685" compressor pulley).

If 1 kW = 3412 BTU/hour, then it looks like the compressor should theoretically output roughly 11,000 BTU/Hr at idle and on the highway 34,000 BTU/Hr (2,000 RPM engine / 3400 compressor).

I wonder what the factory, dual air displacement R4 was able to do.

 

[L31MaxExpress]

Here's The performance chart for the enhanced SD7:

You must be registered for see images attach

And for the regular SD7H15:

You must be registered for see images attach

At my 5.7's idle (650 rpm), the 4805 spinning at my ratio of 1.7:1 should be 1,100 RPM (8" crank pulley, 4.685" compressor pulley).

If 1 kW = 3412 BTU/hour, then it looks like the compressor should theoretically output roughly 11,000 BTU/Hr at idle and on the highway 34,000 BTU/Hr (2,000 RPM engine / 3400 compressor).

I wonder what the factory, dual air displacement R4 was able to do.

A6 could move 42,000 btu, R4 was a bit smaller displacement, so I would say a little less capacity. Like 12.6 cid for the A6 and 10.2 cid for the R4.

 

[cjcb1100]

Here's The performance chart for the enhanced SD7:

You must be registered for see images attach

And for the regular SD7H15:

You must be registered for see images attach

At my 5.7's idle (650 rpm), the 4805 spinning at my ratio of 1.7:1 should be 1,100 RPM (8" crank pulley, 4.685" compressor pulley).

If 1 kW = 3412 BTU/hour, then it looks like the compressor should theoretically output roughly 11,000 BTU/Hr at idle and on the highway 34,000 BTU/Hr (2,000 RPM engine / 3400 compressor).

I wonder what the factory, dual air displacement R4 was able to do.

mine is a nock off probably not that good

 

[L31MaxExpress]

New data points for mine. I had to replace a leaking high side service port. So I went ahead and changed out my rear expansion valve that has had the rear unit underperforming at times. The rear expansion valve on my aftermarket rear unit interchanged to a Four Seasons 38693, which is set for 8° superheat.

I also changed out the Variable orifice up front that I suspected was causing the compressor to cycle excessively at highway speeds. Seemed to get more restrictive as the high side pressure dropped at highway speeds. I went back to the GM specified White 0.072" tube.

I then replaced that high side cutout that was cutting the ac at 280 psi rather than the OEM specified 430 PSI. Replacement was a Four Seasons 38974. I vacuumed the system for 90 minutes.

It was 96°F and 47% humidity when I charged the system. It was definitely a sweaty, muggy job changing that expansion valve. Anyway, I put as much as I could of 48 oz of R152a in the system. The cans were very empty at the end with a small puff and I bleed the charge line from the can tap to the manifold set until it sprays liquid refrigerant everytime to ensure no air gets into the system.

At idle, the gauges read 180 psi and 42 psi with both blowers cranked up. I brought the engine up to 2,000 rpm and the pressures dropped to 157 psi and 27 psi. The air at the vents was hovering 48°F at idle and dropped to 39°F at 2,000 rpm.

Then I put away all my tools, took the van for a quick drive on the highway. The front vents got down to 34°F and very little cycling of the clutch. I will take it out in the heat of the day tomorrow and see how it does with full sun load on it. I did not measure the temps of the rear unit, but when I parked the van, I reached over my head to the front rear vents and they felt very cold too. Probably 40ish and those vents have 10+ feet of ducting to them.

I do not drive around town nearly as much as I did, so the GM White tube may stay in it, because it seems to do a better job at 70+ mph. The red tube definitely gives colder air, but the compressor cycles 2x as often and on a very hot day it will warmup to about 50-60°F while the compressor is off. With the white tube in it, I drove ~5 miles at 70 mph, exited, made the U under the highway and got back on the highway. I only noticed the compressor cycle 3-4 times in 10 miles with the blowers on both end at full speed. After 10 miles the interior was an ice box.

 

[L31MaxExpress]

95° and 48% humidity where I am already this morning. Just ran to Sonic and grabbed breakfast, then jumped on the highway headed back. Got down to 38°F out of the front and stayed there. At idle it is cooling ~40-44°F depending on how long I am sitting. Stuck a thermometer in that forward most rear vent while I sat and ate my food. It was at ~55°F with sun on the roof. Even though it is not blowing quite as cool in the front, the interior cools off noticeably quicker with the rear ac working properly and the compressor running more. A Blue orifice might drop the temps a bit and allow the compressor to run near equal time before cycling, but it is cooling well enough I am just going to leave it alone for now.

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[Schurkey]

You're my A/C hero. And I'm really jealous.

None of my cars cool that well. In fact, I'm really disgusted with the way they DON'T cool. I don't think I've ever seen an R4--CCOT system that could reliably get below 45 degrees on high blower speed, and lots of 'em won't do that.

Maybe my thermometer is off. I've never checked it...but since I'm not cool enough, the thermometer is really just verifying what my body is telling me. It ain't that cool in the car.

 

[L31MaxExpress]

You're my A/C hero. And I'm really jealous.

None of my cars cool that well. In fact, I'm really disgusted with the way they DON'T cool. I don't think I've ever seen an R4--CCOT system that could reliably get below 45 degrees on high blower speed, and lots of 'em won't do that.

Maybe my thermometer is off. I've never checked it...but since I'm not cool enough, the thermometer is really just verifying what my body is telling me. It ain't that cool in the car.

The R4 system in my 1980 Vette was ok temperature wise, but airflow was poor in those C3s and you sweated anyway especially since mine had glass t-tops. I usually had the ac on and the t-tops off in that car. those cars ran so hot I often had to switch the ac off in traffic jams to keep the engine under 250°F. Ditto for my 1987 S15 Blazer 2.8L I had. It was cold, but very little blow and with the AC on I was always in that gutless 2.8L hard enough accelerating to activate the WOT AC cut-off relay. 0-80 mph was like 2 minutes.

My old G20 with the A6 would reliably get into the high 30s on the highway and had decent idle cooling as well considering it still had the tube and fin R12 condenser at the time.

I feel the R4 has solid cooling performance but it does not have good reliability on R134a, even new ones are lucky to last 3-4 years with R134a. My 1988 Fleetwood stayed R12 while I had it, but that was close to 20 years ago and not very practical today. I put a new GM Genuine A6 using older 350 Olds brackets when it was not cost prohibitive on that car and it would freeze you out of the car. When I changed the compressor with a 403 swap in that car in 2005 it still had the OE R4 compressor, but was moaning, rattling and not cooling the way it should right before it started leaking. I had barrier hoses built and used a late R12 Caprice condenser. Actually I used the whole cooling stack, 9C1 radiator, trans cooler, power steering cooler and condenser in the Cadillac. I had barrier hoses added to the 70s Oldsmobile A6 manifold setup after thoroughly flusing the hard lines. The compressor end was 70s olds 88/98 and the condenser and accumulator end was 1988 Fleetwood. I ran that car 6 years until the 200-4R gave out behind the 403 MPFI and never had to touch the R12 charge. The thermostatic cycling switch would cut the compressor at about 30°F at the vents.

 

[L31MaxExpress]

Well I spoke too soon, warmed up to 106F and 28% RH where I am today. The GM white orifice is definitely the wrong one for extreme heat. At idle the best it could do with both blower fans on high was about 62°F althought it did get down to about 42°F on a 20 minute highway drive. I have an Orange Ford tube 0.057 kicking around. About to evacuate the system and replace the White with the opposite extreme the Orange. I know the results of the Red and it was not as good as the Variable at idle and slow speeds and cycled a bit at highway speeds, but that was with a malfunctioning rear expansion valve that was hanging partially closed. The variable ended up malfunctioning after a year and was hanging in the most closed off position, causing frequent cycling at highway speeds and subsequent loss of cooling. Fingers crossed that a fully flowing rear expansion valve and 0.057" front tube put the system at a good point of balance to where it cools well at idle again and the frequent cycling at highway speeds is reduced. Now I am thinking I should have used the R12 version of the rear expansion valve with its smaller metering area. The high side pressure is low and the low side pressure is high. The more I look at data, the more I realize the low side pressure is the most important number to how well the system cools. The lower the low side pressure, the colder the refrigerant in the evaporator(s) boils and thus the colder the air from the vents. My high side pressure runs extremely low, thus I have no qualms increasing the restriction even if it increases the pressures a bit. I think the rear expansion valve will automatically close off some when I restrict the flow to the front and the suction side pressure makes the low side colder, atleast that is my theory I am about to test. The refrigerant should boil colder in the rear, causing the rear unit to also use less refrigerant flow. I want the low side pressure back into the 30s and ideally able to drop into the 20s at highway speeds.

 

[RCFallis]

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.

 

[RCFallis]

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.

Retired AC Tech: The rear AC units on nearly all GM units do not use an orifice on the rear evaporator. They have an Expansion Valve located in the evaporator case.
Type into youtube 1993 Chevy suburban rear AC expansion valve location.