88-94 5.7 Suburban A/C Diagnosis and Tuning

[Wildblue19]

I suppose I'll admit defeat for today - No change in performance yet again.

With the help of @L31MaxExpress, I identified and pulled out old o-tube, it was clean without any buildup in the screen. Replaced with the new VOV, vacuumed down to 29" for over an hour, and added 4# R-134. Saw pressures 55/245 at idle. 92° ambient, got to 65° at idle and 60° on the highway, recirc off. Recirc on, front and rear air hovering around 55°. Seems like the truck just won't do better than about 30-35 delta T.

For reference, other things I've done are check the evaporator for debris and rinsed it out, wasnt much in there. Replaced the metal 5 blade fan with the newer body style plastic blade fan along with a heavy duty fan clutch, and added insulation to the reciever drier as pictured.

I'll check that the shutoff valve is doing it's thing and for any leaks.

 

[L31MaxExpress]

I suppose I'll admit defeat for today - No change in performance yet again.

With the help of @L31MaxExpress, I identified and pulled out old o-tube, it was clean without any buildup in the screen. Replaced with the new VOV, vacuumed down to 29" for over an hour, and added 4# R-134. Saw pressures 55/245 at idle. 92° ambient, got to 65° at idle and 60° on the highway, recirc off. Recirc on, front and rear air hovering around 55°. Seems like the truck just won't do better than about 30-35 delta T.

For reference, other things I've done are check the evaporator for debris and rinsed it out, wasnt much in there. Replaced the metal 5 blade fan with the newer body style plastic blade fan along with a heavy duty fan clutch, and added insulation to the reciever drier as pictured.

I'll check that the shutoff valve is doing it's thing and for any leaks.

Have you tried the R134a specific cycling switch? The R12 ones are calibrated at a higher pressure. I have also seen alot of failures where they will cycle the compressor off way too early. My buddys 2004 1500HD got to where it would not cool and it only had a bad cycling switch.

30-35° delta from exterior temp is far lower than I typically can achieve even at idle. I usually get more like 40-50°F at idle and often as much as 60°F going down the road. I should mention that is on recirculated air, 30-35° delta across the core once the interior is cooled. I see something like, 105°F outside, 70°F interior, 35°F at the vents.

You want 4 Seasons 36675. The R12 switch cuts the compressor off 10 psi earlier than the R134a switch. Won't make much of a difference at idle, but once you get going down the road and the pressures drop with airflow across the condenser it should make a good difference. Using the R134a pressure/temp chart, the 20 psi switch cuts the compressor at 22°F. The 30 psi switch cuts it at 35°F that is the boiling point of the refrigerant at that pressure. The evaporator surface will be a bit warmer and the discharge air a bit warmer than that.

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This is the factory R12 switch

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[L31MaxExpress]

I forgot to ask something last night. Was this a genuine Sanden or a knock off? Your low side pressure seems high. At 55 psi R134a boils at 58°F. Get some gauge readings at 1,500-2,000 rpm.

Also what condenser do you have? The factory serpentine style cannot be flushed and is not a great design compared to the newer and thinner parallel flow design. Every vehicle I have swapped to a modern parallel flow cools alot better. I have found the parallel flow one of the best upgrades one can do and they are generally under $100. The Express van, the G20 van and Tahoe all have parallel flow. My first parallel flow upgrade was the old 83 G20 in about 2006. It knocked nearly 50 psi off the high side pressure in 100°F ambients. Went from 325 to 275 psi. The low side dropped 10-15 psi as well. Vent temps dropped from ~50°F to about 42°F at idle.

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[L31MaxExpress]

One other thing, I just remembered your 93 should be R12 from the factory and you stated 4 lbs in the system. The 68 oz R12charge should be about 80-85% of that on R134a. I would reduce the charge to 3.5 lbs. Overcharge could explain the high low side pressure and poor cooling.

Reduce the charge, get a R134a switch, then get some performance numbers and gauge readings at both idle and 1,500-2,000 and report back.

 

[L31MaxExpress]

I always use a dual temperature probe with clamps to charge retrofits. I monitor both the high and low side temperature along with the pressures. The R134a pressure temperature chart will show you the saturation temperature of the Evaporator and Condenser. Look at the temperature that corresponds with the given pressure. From the temperature readings the Subcool and Superheat are easy to calculate. Generally speaking there should be no real superheat on a FOT evaporator at idle. I measure the temperature of the suction line at the outlet of the evaporator and the liquid line just in front of the orifice tube. I charge until the suction line before the accumulator is 0-2° within the calculated saturation temperature. Once the evaporator is fully saturated with superheat near non existant the system is fully charged. At that point both the accumulator and suction lines should also be cool to the touch. Just an example, at 55 psi, the saturation temp is 58°F, you would want to see no more than 60°F at the suction line off the evaporator. Subcooling is what it is in an automotive application. Its really does not have a meaningful use other than a measure of condenser efficiency. The more subcooling the better the system will function. 245 psi would be a condenser saturation temperature of 145°F. If the liquid line was 135°F that would be 10° of subcooling.

 

[Wildblue19]

More data coming in for you guys...

When I converted to R-134 I installed a pressure switch from a R-134 truck that I puled from the junk yard. The switch works, as it will disengage the clutch when not hooked up to the receiver/drier. However, my pressures have never been low enough to have it cycle the compressor while running.

The condenser is a spectra (?) parallel flow unit from local parts store. Mine is painted black, yours looks aluminum. Looks very similar though with the pipes running down the edges.

I have that foam grommet installed and there's no obvious cold air leaks I've found.

I opted to put in 4# of refrigerant based on research that showed 4# was the R-134 amount used in the later year Suburbans that came with 134 factory. I know the R4 compressor internal volume is different vs the Sanden but I figured I would start with a higher fill amount and try to adhere to factory specs.

The compressor is a genuine Sanden with all Sanden markings and part numbers as best as I can tell.

With the system charged at 4#, I gathered this data today:

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First test done with the truck cold and at idle, I quickly took these readings before it got too warmed up.

Second test accomplished while the truck had begun to warm up but before reaching full operating temp. Readings were pretty transient here, may have been a factor of the oil in the system heating and the VOV cycling? The peaks were rather high for a few RPM ranges but settled down as the truck got hotter.

Third test done after the previous two, and by now it was slightly warmer than a normal idle operating temp. Readings were fairly stable as depicted.

Static PX was measured PSI of the system while engine was off.

I may lower the amount of refrigerant slowly and see what the gauges do. I'm just not sure at this point what these numbers are telling me. Thanks again.

Edit, going to try and put some temp probes on the lines and see what else I can find.

 

[L31MaxExpress]

More data coming in for you guys...

When I converted to R-134 I installed a pressure switch from a R-134 truck that I puled from the junk yard. The switch works, as it will disengage the clutch when not hooked up to the receiver/drier. However, my pressures have never been low enough to have it cycle the compressor while running.

The condenser is a spectra (?) parallel flow unit from local parts store. Mine is painted black, yours looks aluminum. Looks very similar though with the pipes running down the edges.

I have that foam grommet installed and there's no obvious cold air leaks I've found.

I opted to put in 4# of refrigerant based on research that showed 4# was the R-134 amount used in the later year Suburbans that came with 134 factory. I know the R4 compressor internal volume is different vs the Sanden but I figured I would start with a higher fill amount and try to adhere to factory specs.

The compressor is a genuine Sanden with all Sanden markings and part numbers as best as I can tell.

With the system charged at 4#, I gathered this data today:

You must be registered for see images attach

First test done with the truck cold and at idle, I quickly took these readings before it got too warmed up.

Second test accomplished while the truck had begun to warm up but before reaching full operating temp. Readings were pretty transient here, may have been a factor of the oil in the system heating and the VOV cycling? The peaks were rather high for a few RPM ranges but settled down as the truck got hotter.

Third test done after the previous two, and by now it was slightly warmer than a normal idle operating temp. Readings were fairly stable as depicted.

Static PX was measured PSI of the system while engine was off.

I may lower the amount of refrigerant slowly and see what the gauges do. I'm just not sure at this point what these numbers are telling me. Thanks again.

Edit, going to try and put some temp probes on the lines and see what else I can find.

Definately overcharged, high side pressure is through the roof. The parallel flow and sanden will both reduce the required charge volume.

 

[PlayingWithTBI]

A question, what high pressure shut off switch are you using? Shouldn't it be closer to ~430 PSI but, I see 435 up to 475. How accurate are your gauges?

 

[L31MaxExpress]

A question, what high pressure shut off switch are you using? Shouldn't it be closer to ~430 PSI but, I see 435 up to 475. How accurate are your gauges?

Stock switch should cut at ~430 and the Sanden relief valve should open around ~450. I think his high side gauge is a bit off.

 

[Wildblue19]

New changes:

Reduced gas volume until the pressures below were stable at operating temp at idle:

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Then, I checked the temp with a probe just prior to the orifice tube and after the "Y" as pictured:

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Probe was measuring 130°F at this point. I moved the probe to the outlet from the evaporator and prior to the receiver/drier as pictured:

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Temp reading here was 121°F. Vent temps were now ambient. It is possible the gauges are bad, I can go rent a set from a local store and compare if that seems like the root cause.