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Sanden SD7V16 (variable displacement) new to me, I have a few questions

MXHOWES on Sun June 12, 2011 6:25 PM User is offline

Year: 1996
Make: VW
Model: Golf
Engine Size: 2.0
Refrigerant Type: R-134A
Ambient Temp: 62
Pressure Low: 26
Pressure High: 133
Country of Origin: United States

Last summer this AC system was "rebuilt" by the Previous Owner at a "qualified"AC repair facility (I have no paper work), I do see a new Sanden 4252 (SD7V16 compressor) and new looking accumulator. I picked up the car in November and of course the AC felt cold. It finally got hot enough around here to turn on the AC and I only got minimal cooling, vent temp of 48, ambient around 68. To shorten this story, I finally figured out I had a variable displacement compressor which I need to learn more about. When I first put on the gauges I got a constant 29 Low and 120 high at 62 degrees. I added about 3 oz of R-134A and got 26 Low, 133 High and vent about 45, still not very cool.
Sanden's web page says the displacement is "internally controlled by a pressure valve"

So..... I will assume the following, please tell me where I am going wrong or direct me to some more info on how to dial in this type of compressor.

Once the high side is set at optimum pressure ...(2.2 X ambient) will the compressor try to maintain a constant differential between the low and high side or just what factors are considered by this "pressure valve"

I think my problem is that my low and high side readings are always constant, right now it is at 26/133. When you first start the AC or increase blower speed it just stays the same. I would think that the low side would be down around or below 20 when first activated and blower set on high.

Any information on helping me understand this would be appreciated.

mk378 on Sun June 12, 2011 8:18 PM User is offline

That's the whole point of a variable compressor, it never pumps the low side below the setpoint of about 25. Vents of 45 may be what it's going to do. Check for reheating from the heater core. No auto A/C system goes much below 40 F at the vents because the evaporator will frost. You should test in hot weather first before doing more. Charging really has to be done by weight on variable systems because you can't tell much from pressure.

MXHOWES on Sun June 12, 2011 11:01 PM User is offline

Originally posted by: mk378

Charging really has to be done by weight on variable systems because you can't tell much from pressure.

"by weight" you mean evacuate and add correct charge?

I am also going to check out the heater mixing flap.

Just to satisfy my curiosity ..... what would the gauge readings show on an overcharged variable system?

tomw on Mon June 13, 2011 3:23 PM User is offlineView users profile

Without delving into the mental gymnastics, I think that an overcharged variable displacement compressor would have very short cycle time, if it were to cycle, as the overpressure would get high quickly. The low would still 'try' to stay around the 25 set point. It just wouldn't run if overcharged if the HPCO switch were functional, or it would run at minimum displacement cause it couldn't cram more R134a into the condenser... no room.
Ya think?

simplificate and add lightness

ice-n-tropics on Mon June 13, 2011 4:29 PM User is offline

The SD7V10 displacement is controlled by a internal Mass Flow Compensation Valve (MSCV). Unlike other variable displacement comps (VDCs) the pressure regulating valve has a small piston exposed to the discharge chamber so that the move to decrease piston stroke can be influenced/delayed if the discharge pressure (Pd) is higher. This MFCV smart valve knows that the driver is not yet comfortable if the Pd remains high, so in that case the suction pressure can go lower than normal to get the louvers colder.
This VW/Sanden system requires a accurate charge amount to work properly at all ambients.

Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy. How To Air Condition Your Hot Rod

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