88-94 5.7 Suburban A/C Diagnosis and Tuning

[L31MaxExpress]

I added a little more refrigerant in the system yesterday morning because it was still short cycling a bit out on the road. I wrapped the temperature probe on the low-side suction line in a rag to help insulate it from the engine compartment heat. My superheat reading had dropped under 10°F.

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[L31MaxExpress]

It has been so hot here and I am mostly idling around town. I decided to try another experiment. I put the stock diameter crankshaft pulley and stock length belt back on. I just re-tested the system again prior to the van heat soaking in the sun. The concrete in the parking lot is still over 100°F, sucked to lay on changing out the pulley even before the sun came up. The ambient air is 96°F and an actual wet bulb is showing 61% humidity in the shop parking lot. In this weather the stock crank pulley absolutely performs better as expected.

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The engine fan definitely moves more air at lower engine rpm compared to the 25% underdrive. Notice the big reduction in pressures, evaporator temperature as well as suction line temperatures.

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[L31MaxExpress]

Finally I tested where the system would cycle the compressor. At ~2,000 ish it cycles at ~28°F.

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[cjcb1100]

im dealing with similar issues on my 90 trying to get cooler than 60 when its 102 outside trying a diff orifice tube but after reading this my not be the issue .... i too have a nice fan aux fan and hd clutch also 95 parts and a sd7h15 the dry sweats but never gets frosty, using 134a and i have the 119mm clutch dia but my high side seems low i think getting 50/255-265 @ 102F thats why im trying a smaller orifice tube i believe maybe my high side is not quite liquid when it hits the orifice ?? possible idk im not expert i did have good temps with the r4 and a propane fill lol just got tired of replacing it

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[cjcb1100]

if its 80 deg outside it gets down to 40 at the vents noticed that this morning

 

[cjcb1100]

this is my readings with temperature just now notice vent temp was good any temps over 95 and the vent temp climbs up to 60f at 102 f ambient

this is idling in these conditions witch i'm ok with its hotter temps that i'm not seeing what i want
52% humidity

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at idle

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revving about 2k by ear

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[Wildblue19]

@L31MaxExpress Always coming through with excellent data and above average results, that system is performing really well!

What are you running for an engine fan?

I've got a heavy duty clutch (an upgrade from standard duty that was on it) that's about 2 years old. Fan is the newer model 11 blade composite variety. This combo will suck a microfiber cloth to the grille at idle with no issues. Shroud is in good condition as well and the fan fits in it correctly.

@cjcb1100 looks like your performance is similar to mine.

Side note, our discussion about the TXV's got me thinking about how these systems changed over the years. This chart is K1500 suburban specific, but I found it interesting to see what GM did to develop and tune the R12 to 134 conversion process.

Ultimately, cooling of the condenser isn't my limiting factor. I can soak down the condenser with mist and drop the high side way down and it'll still be around 45-50 psi on the low side.

What drives low side pressure? In my view, it's a combination of (1) cooling through the condenser, (2) suction of the compressor, (3) the metering of the refrigerant through front and rear valves, (4) heat load on the evaporator, and (5) refrigerant charge in the system... anything else?

 

[L31MaxExpress]

@L31MaxExpress Always coming through with excellent data and above average results, that system is performing really well!


I've got a heavy duty clutch (an upgrade from standard duty that was on it) that's about 2 years old. Fan is the newer model 11 blade composite variety. This combo will suck a microfiber cloth to the grille at idle with no issues. Shroud is in good condition as well and the fan fits in it correctly.

@cjcb1100 looks like your performance is similar to mine.

Side note, our discussion about the TXV's got me thinking about how these systems changed over the years. This chart is K1500 suburban specific, but I found it interesting to see what GM did to develop and tune the R12 to 134 conversion process.

Ultimately, cooling of the condenser isn't my limiting factor. I can soak down the condenser with mist and drop the high side way down and it'll still be around 45-50 psi on the low side.

What drives low side pressure? In my view, it's a combination of (1) cooling through the condenser, (2) suction of the compressor, (3) the metering of the refrigerant through front and rear valves, (4) heat load on the evaporator, and (5) refrigerant charge in the system... anything else?

I think you need to look at #5 given your superheat readings and the fact the evaporator outlets are warm.

 

[Schurkey]

And air-door operation/sealing.

High duct temps with a "perfect" A/C system could be a temperature-blend door that doesn't shut properly.

 

[L31MaxExpress]

And air-door operation/sealing.

High duct temps with a "perfect" A/C system could be a temperature-blend door that doesn't shut properly.

Except his suction lines off the evaporator are both warm.