AC oil

[Robert Squires]

* What's the best way to add refrigeration oil to an AC system? Assume that the system has been flushed and holds a good vacuum, and that the compressor, receiver/dryer, and expansion valve were replaced; i.e., the system is empty and clean. The compressor, which doesn't have an oil crankcase (like the old 450s), was installed with about 3 oz of oil in it (any more oil would have run out during installation). Once installed, the compressor was turned, by hand, through a dozen revolutions to displace some of the oil and prevent "slugging" (can't compress liquids). What's the best way to put in another 4 oz of oil?
* Don't own one of the big recovery/recycle units. I've watched them pump oil and a full measured charge into the discharge side with the engine off. But that option isn't open to me (when I get a spare $4K maybe I'll buy one). A vacuum pump, a standard 30lb HFC tank (half full), and a manifold gauge set with an oil infuser attachment is what I have to work with. This is a long post. Thanks for taking the time.

[Kestas]

Is the unit under pressure or still in the pre-charging state? r12 or r134?

[wscheffer]

I put mine in the line leading from the dryer to the expansion valve. I disconnected the line at the dryer and added it there. This worked out fine on my replacement of the evaporator, expansion valve and dryer.

Of course the system was empty when I did this.

Hope This Helps!

[Robert Squires]

* Kestas, the system is in vacuum, approx 27inHg. Guess you would call that the "pre-charging" state.
* The question was a general procedure question, without particular regard to refrigerant type. For a given system, you would add the oil for either refrigerant the same way, despite the fact that the type of oil may differ.

[Robert Squires]

* WScheffer, your procedure, if I understand correctly, is to pour the oil (with the help of a small funnel no doubt) directly into the discharge hose which you have removed from the receiver/drier. The system will be open to the atmosphere. Filling the discharge hose would keep the oil from flowing directly into the compressor, and possibly damaging it, when the system was closed, charged, and the compressor turned on. Since the compressor already was installed with a few ounces of oil in it, you wouldn't be starting it dry (i.e., until the oil you've poured in makes it through the exp valve, the evaporator, and back around to the compressor). I suppose you could add oil to the receiver/drier instead, following the same fill idea (don't know if a rec/drier will hold 4 oz of oil).
* When the oil and charge are added to the discharge side, with the engine off, and with the help of a recycling unit, the end result, for the oil, is the same. It ends up in the rec/drier and the discharge hose.
* Wonder what the downside is for adding oil manually with the system open? (other than the mess, since, in the MB, the rec/drier lives at a lower elevation than the discharge hose).

[Kestas]

Usually oil is added just before everything is buttoned back up. The needed charge is spread throughout the system... 2 ounces in the condenser, 2 ounces in the receiver/dryer, 4 ounces in the compressor (which you say is already charged). This gives a better distribution during startup of a/c operation.

[Robert Squires]

* Kestas, I've never seen a professional shop add AC oil that way (and I've serviced cars professionally for 30 years). You're suggesting to add oil to each component and then close, "button-up", all the connection points before vacating and charging. Seems like that procedure would avoid compressor damage, but it's a lot of extra work. The oil quantities listed in the tech specs are allocation amounts. The oil is put in in one place and then circulates with the freon throughout the system when the compressor is turned on, each component capturing approximately the allocated amount.
* The procedural question came up in the shop and I thought I would put it before a larger audience. After a system is flushed (generally a good idea plus most compressor manufacturers require flushing to validate the warranty) oil has to be added. A load of bubble-free oil through a running compressor can damage the compressor. So I guess it comes down to installing the oil in such a way that it is "foamed" or mixed with the refrigerant before it gets to the compressor.

[Kestas]

My method is typically done after a flush when the components are all disconnected, so it shouldn't be a whole lot of extra work. I learned this method from the pros on autoacforum.com and aircondition.com.

Conversely, if the components were not all disconnected, why would the whole system need an oil recharge? Just replace the oil in the component that was removed.

It's not critical to use my method. I'm sure your method will work. But why not give the system a good start during a complete oil recharge?

[Robert Squires]

Kestas, commenting on your last points:
* A flush isn't done with all the components disconnected. Google an AC flush machine manufacturer/distributor and look at the instructions for hooking the machine to the car. The flush is done in 2 circuits, one through the evaporator, exp valve, and interconnecting hosing. The other circuit is through the condenser and interconnecting hosing/lines. Flushing in the reverse direction is good. The only components which are disconnected and out of either flush circuit are the compressor and the rec/drier. The rec/drier always gets replaced.
* There are other reasons than component replacement to add oil. A system which hasn't been opened up, component-by-component, and has been flushed prior to, for instance, an R12-to-134 retrofit, will need an oil recharge.
* We have 2 trade schools locally for auto and truck maintenance. I called and spoke with the AC instructor at one of the schools. His comments boil down to one idea. Put the oil into the discharge side somewhere after the compressor, whether you pour it in to an open, disconnected hose or whether you use an oil injector and let vacuum draw it in (vacuum pump assist, engine off) via the high-side valve to a closed system. He preferred adding to a closed system under vacuum (but that takes a vac pump).
* Think I've worked this thread to death. But it's all been helpful to me to arrive at procedure I can believe in. Thanks.

[TOPGUN]

I Have Always Used The Vacuum Draw In Method. You Will Have To Repump The System. Be Careful Not To Put Too Much Oil In.you Can't Get It Back. Try For A Tighter Vacuum Then 27". Goodluck.