88-94 5.7 Suburban A/C Diagnosis and Tuning

[RCFallis]

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.

Next Idea. Mostly all rear AC units have 1 to 2 rear blend doors. (1) Will blend AC Evaporator cold air and heater core hot air. In the winter this dehumidifies the air so your windows do not fog up. (2) will control air flow direction. Ceiling, side and floor vents. The plastic gear in the electric blend servos will break (random clicking sound from the vehicle rear)
And if the blend door is not closed the heater core will kill your cold air temp. All year round because heater cores always have flow. The blend door is the temp control.

 

[L31MaxExpress]

Next Idea. Mostly all rear AC units have 1 to 2 rear blend doors. (1) Will blend AC Evaporator cold air and heater core hot air. In the winter this dehumidifies the air so your windows do not fog up. (2) will control air flow direction. Ceiling, side and floor vents. The plastic gear in the electric blend servos will break (random clicking sound from the vehicle rear)
And if the blend door is not closed the heater core will kill your cold air temp. All year round because heater cores always have flow. The blend door is the temp control.

I put a ball valve in my heater hoses on my vehicles. No need for heat to enter the passenger compartment when when we see 40 days or more of 100F and beyond often. As soon as it stops being cool enough to need a heater in the AM, that ball valve closes until we need heat again.

 

[L31MaxExpress]

This was a startup with the gauges connected and a heat soaked interior, immediately prior to evacuating the system.

Orifice changed out, waiting on the vacuum pump to do its thing now.

No real sure on the source of the little green shards of silicone on the orifice tube except maybe off the seals on my manifold set. I saw one little speck of metal as well. Perhaps something left behind that did not flush out from one of the two older compressors or something left behind in the manufacture of the new manifold set, accumulator or condenser. I added about 3oz of oil to replace what I lost recovering the refrigerant.

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[L31MaxExpress]

My vacuum pump is a ~12 year old Harbor Freight US General brand and it still pulls a good deep vacuum after 100s of hours of use. This stuff does not have to be $$$ to work well. I open both manifold valves when vacuuming, especially on a system with the volume of the vans system and hose length. I figure it makes it easier for air and boiled moisture to find its way out. Close both valves, then remove the vacuum pump, then hook up the refrigerant supply, then purge the charge hose at the manifold. I then open both valves and charge the system until it reaches equilibrium with the refrigerant supply, I close off the high side valve and start the engine and turn on the ac and continue charging.

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[L31MaxExpress]

Got it charged and drove it around. 106°F and 26% RH. With the 0.057 things really did not change all that much. After it cooled down, the front dash managed to dip down to 37°F at highway speeds. At idle it stayed around 50ish. The rear is about the same as it was prior. My pressures are about 275 psi and 55 psi now at idle. Best guess is I need to research and find a R12 rear expansion valve or one with a lower BTU rating. I think the one I installed is rated at 24K and I feel more like an 8-10K would suffice. I blew through the R134a unit and it flows a ton through it. With the R134a rear expansion valve the system cannot pull down the low side pressures nearly enough. With the rear unit off and the front on low fan speed, it dropped down to 42 psi and the rear coils were completely covered in ice with the fan off. Refrigerant flow seems too high with the R134a unit.

 

[Wildblue19]

I follow the same process you outlined @L31MaxExpress. I also always use fresh synthetic oil.

My pump has a 30min duty cycle, so i do two full cycles with a cool down period. I had a very slow vacuum leak when I pulled one on my truck last week. However, I bought a sniffer and haven't found any leaks anywhere. I also did the simple soap method and no bubbles. Time will tell.

I also agree that with what I've seen, low side pressure management is paramount to performance. I am currently running a blue 0.067 in the truck. I tested an orange myself and had poor performance with it, but that was before the rear txv swap as well. I may try to see if a smaller red or orange is a better option to lower my low side px.

I've also been toying with the idea of making my own lines for a H block style expansion valve up front with adjustment in it to stop opening the system so often. Coldhose is a shop that will make custom lines I've used before.

 

[L31MaxExpress]

I follow the same process you outlined @L31MaxExpress. I also always use fresh synthetic oil.

My pump has a 30min duty cycle, so i do two full cycles with a cool down period. I had a very slow vacuum leak when I pulled one on my truck last week. However, I bought a sniffer and haven't found any leaks anywhere. I also did the simple soap method and no bubbles. Time will tell.

I also agree that with what I've seen, low side pressure management is paramount to performance. I am currently running a blue 0.067 in the truck. I tested an orange myself and had poor performance with it, but that was before the rear txv swap as well. I may try to see if a smaller red or orange is a better option to lower my low side px.

I've also been toying with the idea of making my own lines for a H block style expansion valve up front with adjustment in it to stop opening the system so often. Coldhose is a shop that will make custom lines I've used before.

The more I look at the pressures I am seeing, the more I am starting to believe it is a pumping capacity issue with both units operating correctly. I have a good GM HT6 off the 8.1L I bought at the shop. I may put the HT6 on it for ***** and grins, because my van cooled better with the OEM HT6 and the Valeo replacement than it does with the SD7H15. I emailed Sanden to find out if they have an Enhanced version of the 4261 in an alternate part number. Then again I should probably just quit screwing around, buy a serpentine clutch A6 and, build some brackets and hoses and put it on. The A6 is a beast BTU wise and my old 83 G20 was always cold, even at idle. That sucker could pull the low side pressure down to 35 psi with both blowers on high and the engine idling. The A6 is heavy and power robbing, but it is extremely rugged and just plain works. I also have a SBC DA6 mounting bracket that would not be hard to adapt a V7 to. A V7 has more displacement than an A6 does in full stroke.

 

[L31MaxExpress]

I may have stumbled across a better compressor for the van. The Four Seasons 88947. It has the same ~4.25" pulley but its 10 cylinder design is 170cc displacement per revolution vs the Sandens 155cc. I made a mistake previously on the pulley math. The Vortec pulley is 8" OD, but the area that drives the belt is about 7.75". 7.75/4.25 = 1.7898. At my 750 rpm idle speed, the compressor spins ~1,350 rpm at idle. Negating volumetric efficiency differences the Sanden had a potential to move 209,250 cc of refrigerant in 1 minute. The 10 cylinder Four Seasons 229,250cc. That is 7.4 cfm vs 8.1 CFM. That is a 9% improvement in displacement.

I wish Four Seasons had a published effeciency chart for that unit. I saw one place that suggested it is a DKS17D gut pack in a HT6 style case. If that is the case, it outperforms the SD7H15 handily. At idle they show to be really close but as the compressor rpm increases the DKS17D blows the Sanden out of the water. 3,000 rpm compressor speed coincides with just over 1,600 rpm engine speed. The Sanden is down a solid 1KW in output at that point. I assume that gap would keep increasing and at the 2,250 rpm engine speed I cruise at it very well could approach 2kw cooling difference. The 88947 is cheap, might be worth pulling thr Sanden off and throwing the 88947 in its place. The Sanden works excellent for a single evaporator and would probably work great on the 87 G20.

Edit...It also appears that the DKS17D is the OE unit on my 2011 Pathfinder and was manufactured by Valeo. The Valeo 10000585 that was on the van for years also looks to have been a DKS17D. Starting to see why the Sanden is not performing as well as the old compressor. The DKS17D in the Pathfinder has 111K miles on it and still blows as cold as it ever did and is as quiet as new. The DKS17D does take a bit mpre power to spin, but I have plenty of power and the minor loss in fuel mileage will be worth being cooler, because I will be in less of a hurry to get places around town, thus probably saving fuel.

Also none of this became apparant until we started having 100°F and hotter days. In 85°F to about 100°F the Sanden does an excellent job, especially on the highway. Get to 106-110°F with the real feel over 110-115°F and stuck in around town traffic and it lags badly. Especially cooling off from heat soaked.

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[L31MaxExpress]

Next Idea. Mostly all rear AC units have 1 to 2 rear blend doors. (1) Will blend AC Evaporator cold air and heater core hot air. In the winter this dehumidifies the air so your windows do not fog up. (2) will control air flow direction. Ceiling, side and floor vents. The plastic gear in the electric blend servos will break (random clicking sound from the vehicle rear)
And if the blend door is not closed the heater core will kill your cold air temp. All year round because heater cores always have flow. The blend door is the temp control.

Zero blend doors in my rear unit. It is a simple plastic enclosure with a heater core then an evaporator core in line with each other. Air flows through the heater core first, then through the evaporator. Evaporator core is about 3x the size of the heater core. One of the reasons I chose to use the ball valve in the heater hose to make sure that the heater circuit was positively closed off.

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[Erik the Awful]

That would make sense that the later 88947 compressor pumps more than the earlier Sanden. R-134 requires a larger compressor to achieve the same cooling capacity as R12.

I've been following your work because I really want to put good A/C in Roscoe, and I appreciate you sharing all the part numbers and descriptions. I think I need to order an 88947 instead of the GM GENUINE 1520185 "R134 retrofit" compressor. The only problem is going to be mounting it with a TBI serpentine belt setup.