merchauser

soon to be vacuuming down my AC system and wonder if I have to leave the key on to make certain that
the rear AC solenoid is energized? if there is no power to solenoid, that line won't be open and might impede draw down?

FredR

Makes no difference to pulling a vacuum as the rear a/c is open to both sides of the compressor. Whether or not some residual lube oil may get stagnated in there is another matter.but should be of little consequence. The solenoid simply prevents circulation around the rear sub circuit.

merchauser

guess I am not comprehending the solenoid function. I thought with the solenoid disengaged one side would not be open to compressor?

Speedtoys

ONE side wont.

But the other side is.

Both sides are "separate" as far as function, but are connected all the same.

With that valve closed, they are still connected via the compressor.

Do you have a WSM?

FredR

The rear a/c system is simply an additional parallel path to an evaporator- a single block valve in that circuit renders it inoperative. The system performance is limited by condenser capacity so when the rear a/c is working one gets less performance from the front evaporator when the system is on its limit which roughly speaking occurs when the ambient temperature is in the region of 38C to 40C thus why I rarely use the rear a/c.

merchauser

^^^got it

Christopher Zach

However the rear AC is an incredible way to defog the rear hatch and/or the rest of the car quickly, so there is that.

Speedtoys

Keeps my neck cool too.

GregBBRD

In a perfect 928, there is no need to open the rear control valve by turning on the key and the rear A/C.

Unfortunately, I very rarely get to see these perfect cars. As a matter of fact, these "perfect cars" are few and far between!

Most of the cars I get to see have had the A/C system filled 10-15 times, over the years, with most "techs" adding some oil...."just in case" the system is low on oil.

Consequently, most every system I see is way overfilled with oil....and systems with excessive oil don't cool very well.

The quick way to tell if the system is significantly overfilled with oil is to remove the two lines, under the car, to the rear A/C (because you need to change the O-rings, anyway.) If these two lines are full of oil there is too much oil in the system....way, way too much.

The other thing which occurs when the lines are full of oil is that the rear part of the A/C system doesn't get evacuated....'cause the lines are blocked with oil.

So, I always pull the lines off, turn the rear system on with the key on, and let things "drain". And when I evacuate, I always evacuate with the rear A/C turned on.....at least part of the time.