While GM's orifice tube system is not the same as the more complex thermal expansion valve used in our Benz's, I thought you might enjoy reading about the performance I can get from a R134 retrofit system.

Today, I helped a friend by retrofitting his 1988 Chevrolet 1 ton to R134. The weather at the time of completion was 95°F and 30% humidity.

After removing the lines, we found evidence to suggest the Delphi H6 compressor needed replacement. We found one from a local supplier for $312.00. The stock condensor was in good shape and quite large, so we retained it.

We flused the system with (3) quarts of NAPA #409889 A/C flush until all debris was removed.

The stock system uses 8.0 ounces of oil and 44.0 ounces of refrigerant. I added 2.0 ounces of PAG oil to the H6 and installed it. We installed a new drier and a NAPA #207883 adjustable clutch cycling switch.

I modified an orifice tube with a .049" restriction (stock Gm is a .072" orifice) and installed it in the evaporator inlet.

After installing the drier, I insulated all the tubing on the inlet and outlet of the evaporator with NAPA #290491 drip tape. I pumped the system down for 90 minutes at 28/30 in/Hg while I did other things.

I started with a 2.02 lbs charge of R134a which gave 65 psi static on the gauges. The big 454 has a huge engine driven clutch fan, but no electric fan.

Once started, I allowed the system to stabilize for 3 minutes. At idle, interior vent temps were 48°F with 20 psi low and 190 psi high on the gauges. At 2000 rpm, vent temps dropped to 45°F. I added 6 additional ounces of refrigerant and the vent temps dropped to 38°F.

At this point, I set the compressor cut off at 24 psi low and it kicked back on at 32 psi.

At 200 rpm with 38°F to 40°F (fluctuating) vent temps the low side read 28 psi and 218 on the high side with 95° ambient and only the trucks monster clutch fan providing the cooling. The final static charge was only 85 psi.

So, keep this in mind when you are working on A/C systems, as more is not always better and airflow is the most important factor in getting adequate cooling and when you see horrific high side pressures and little cooling, something isn't right...FWIW

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Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership

I thought 88-95 Chevy trucks used an r4?

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----Adam

1985 300SD 322,000

So the smaller restrictor helped make the system cooler?

From what I have read it seems like CCOT systems are less sensitive to getting the charge absolutely dead on, whereas TX valve systems require it to be nearly perfect to cool the best. Moreso when you have a TX valve / variable displacement compressor.

-J

nevermind, I see the vbelt now. Gmt100 body style.

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----Adam

1985 300SD 322,000

This is a one ton tow truck with a multi-rib for the water pump and V belt for the A/C.

__________________
Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership

Optimum vent temps seem to be a + or - 2° affair, regardless of the compressor or type of restriction.

The CCOT is so much easier to work on. On my E34 BMW I made an aluminum black that replaces the expansion valve with a simple restriction and threaded port for the cycling switch...

__________________
Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership

I guess I should ask this then: Why did you change the size of the restrictor?

-J

Reducing the size of the orifice drops the low pressure side of the unit to a lower temperature. The only difference between a refrigerator and a freezer is the pressures that they operate at.

__________________
Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership

Another point to remember is that unlike your home HVAC unit, when an undercharged condition will ice the suction line, your car's A/C system is different.

So many people seem to think that an overcharge on your car's A/C system will cause icing and this is simply untrue.

Overcharging any vehicle's A/C system simply raises high and low pressure sides. A low pressure side that's supposed to have approximately 35psi all the time will now have a higher pressure from overfilling.

Correlating the temperature/pressure chart for R134a shows that 35psi would relate to roughly 38°F vent temps while, say 45 psi (on an overcharged system) would show 50°F vent temps.

Simply raising pressure, as a result of overcharging, raises evaporator temps, period. Look at the pressures on a R134 freezer - low side pressures are near zero!

Expansion valves operate by continuing to adjust pressure (according to temperatures) to keep evaporator temps above freezing. This pressure is approximately 30psi+ so freezing doesn't take place in a normal system.

Some evaporators have a very dense fin count and will ice faster than others. I am yet to ice any evaporator in California...

__________________
Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership