A/C ?? Should you replace the R/D everytime?

Hugh R · 02-28-2008, 03:53 PM

I put an R-134a R/D in the system two years ago. I just had the compressor rebuilt. Should I put a new one in just because I had the system open?

Hester · 02-28-2008, 04:08 PM

Yes.

911quest · 02-28-2008, 04:10 PM

Technically yes....but I don't think the A/C police will hunt you down if you don't Do you have a way to run a vacum on the system??

billybek · billybek's Garage

How long was it open? If it was a short time (hours) no worries.
Always a good idea to replace the drier every time a change is made. They also filter as well as dry (remove moisture).
It is surprising how much moisture can remain in a system even after a good vacuum has been pulled on it. It takes time to boil the moisture out.
What are you doing for lubrication (oil) in the system?

rusnak · 05:40 PM

You didn't say if the system has been running, has been airtight the whole time, or what the status of the compressor was at the time you rebuilt it.

I think if the reason you rebuilt the compressor was that it ran out of oil and seized, then you might have bits of metal still in the system, even after a good flush. If the system has not worked for awhile, and has been open to the air, then you might have caked bits of old oil and dirt in the system. After the compressor has seized, the bits of metal can clog both the receiver dryer and the expansion valve in short order. If you go and put a receiver dryer on with a brand new or rebuilt compressor, you might immediately clog it and need to buy a new one, in other words.

I'm going to play devil's advocate here and suggest that you might want to keep your old receiver dryer on until you know that everything is working, then replace JUST the receiver dryer after having the system evacuated. If you are running R12, then I take back everything I just said.

PatrickB · PatrickB's Garage

If you decide to replace, get a ProCooler. You'll be amazed at the difference!

Hugh R · 11:02 PM

It ran fine, the compressor seal leaked. I have a Jim Sims subcooler so no procooler. I think I'll put in a new R/d as it seems to need all the help it can get. Thanks.

T77911S · 02-29-2008, 08:28 AM

replace it! if you had to have the compressor rebuilt, you may have metal in there. also flush the lines, i learned that one the hard way. might as well replace the expansion valve and be done with it. also, some warranties require a new dryer. i did reuse my dryer on my truck. the new expansion valve i put in was bad so i capped the dryer while replacing it.

Hugh R · 02-29-2008, 09:08 AM

Expansion valve, huh? I can just go to an A/C shop and say give me an expansion valve for an 84 911?

rusnak · 02-29-2008, 09:56 AM

You can go to Pelican and say give me two expansion valves.

I have a feeling you'll be replacing the first one in short order, along with the new R/D. It's no big deal to replace them yourself if you have a vacuum pump and a valve set.

stevemfr · 02-29-2008, 12:30 PM

Depends a lot on where the A/C has lived (humid climes, etc) as well as age. If the R/D was replaced 2 years ago and the system gets a good long (1/2hr) vacuum, I prob wouldn't bother in SoCal. I did most of my A/C work in Germany (very dry climate) and def. wouldn't have bothered. On the other hand, you do NOT need an OEM R/D and universals are cheap. Its usually just a PITA getting the hoses back out of the way (I'm assuming this is for the '84).

stevemfr · 12:57 PM

Quote:

Originally Posted by Hugh R

Expansion valve, huh? I can just go to an A/C shop and say give me an expansion valve for an 84 911?

A/C / refrigeration parts are pretty much universal. The '84's had an L-type expansion valve w/a capillary tube that was used all over. No clue as to what they cost these days but back about 15 years ago when I did this sort of work they didn't cost more than $15.00 from a refrig. shop.

Edit: just checked. Wayne sells 2 of the same units?! for a different price.
Expansion Valves
After the hot refrigerant gas is condensed into liquid by the condenser, it gets sent to the expansion valve. The expansion valve is used to bleed off the pressure from the hot refrigerant liquid. When the pressure drops, the result is that the refrigerant cools to the point where it can now enter the evaporator and complete the process of getting cold air in your car on a hot day. If you have trouble keeping the A/C temperature cold, and there is plenty of refrigerant in the system, it may be possible that the expansion valve is damaged or faulty.

901-573-915-00-M23

A/C Expansion Valve, 911/911 Turbo/911SC (1974-83), Each
Brand: Egelhof [Photo] [view in parts diagrams]

$28.75

911-573-925-00-M23

A/C Expansion Valve, 911 Carrera/911 Turbo (1984-89), Each
Brand: Egelhof [Photo] [view in parts diagrams]

$32.50

Yours is the more expensive.

I just saw that JC Whitney sells a R/D custom fit for a 911 for $15.99 (Whitney, I know, but my experience in A/C parts is they're all the same - once you've made them fit)

T77911S · 02-29-2008, 01:27 PM

i get all my a/c stuff here: http://polarbearinc.com/
great people to deal with and very knowledgeable. they will even help you troubleshoot over the phone. expansion valve should be around $30. you may want to take it out first to check the type of fittings.

Hugh R · 02-29-2008, 02:05 PM

I bought a universal R/D today for $30 with a high-low switch. I'll pull the expansion valve and look at it.

Hester · 02:49 PM

Quote:

Originally Posted by stevemfr

After the hot refrigerant gas is condensed into liquid by the condenser, it gets sent to the expansion valve. The expansion valve is used to bleed off the pressure from the hot refrigerant liquid. When the pressure drops, the result is that the refrigerant cools to the point where it can now enter the evaporator and complete the process of getting cold air in your car on a hot day. If you have trouble keeping the A/C temperature cold, and there is plenty of refrigerant in the system, it may be possible that the expansion valve is damaged or faulty.

The way I understand it, air conditioning (refrigeration) is all about the exchange of heat that happens when a liquid evaporates into a gas and when gas condenses into a liquid. I’m no physicist but these are the applicable rules:

(A) Refrigerant (R134, R12, or whatever) has a very low boiling point. I don't know what that is but let's say it's 0 degrees Celsius.

(B) Pressure raises the boiling point of a substance, which is why propane, which is a liquid when it is in the propane bottle under pressure, comes out as a gas when you release the pressure by opening the valve.

(C) A gas releases heat as it turns into liquid (condensation) and a liquid absorbs heat as it turns into gas (evaporation). That is why sweating or putting a wet towel on your head makes you cooler. It is also why a glass of ice water sweats on a hot summer day (the moisture in the air condenses as it's heat is absorbed by the cooler water). It is also why water drips out your car when the A/C is on and why the A/C helps defog the windows (the moisture in the air condenses down into liquid when the heat is absorbed by the evaporating refrigerant in the evaporator).

The major parts of an air conditioning system are (1) the compressor, (2) the condensor, (3) the evaporator. Automotive air conditioning systems usually have a receiver/drier (which is a filter with a desiccant to trap moisture).

Starting at the (1) the compressor:

(a) the hot, low-pressure refrigerant gas is sucked into (1) the compressor through a low pressure line and is compressed into a high-pressure gas.

(b) The hot, high-pressure gas then leaves the compressor through a high pressure line and passes through the receiver/drier where it is filtered and moisture is trapped.

(c) From there, the hot high-pressure gas travels to (2) the condenser where it is releases heat to the atmosphere and condenses down into a low pressure liquid. The pressure is lower but there is still pressure which keeps it from turning back into a gas (i.e., the pressure has made the boiling point higher).

(d) The low pressure liquid then flows out to an expansion valve that meters the low pressure liquid and sprays it into (3) the evaporator where it has room to expand thus reducing the pressure to just about atmospheric pressure, and therefore, the boiling point.

(e) Because the low pressure liquid refrigerant has room to expand in the evaporator and because it is warm enough for it to turn back into a gas, it does. The process of turning back into a low pressure gas takes heat and it gets that heat from the interior of your 911.

(f) The hot, low pressure gas then gets sucked back into the compressor through the low pressure line (taking the heat from the interior of your 911 with it) and the process starts all over again.

The refrigerant is circulated through the system by the force of the compressor sucking in the hot refrigerant gas and forcing it back out as compressed gas that is looking to relieve the pressure.

The A/C system is a closed system. For a reason that I do not know and cannot explain, it is bad to have any moisture in that system. The way to get the moisture out of the system is to turn it into gas and suck it out. The way to turn it into gas is to make it boil. One way to do that is to raise the temperature of the system to the boiling point of water. That, of course, is impractical. The other way is to reduce the boiling point of the water by reducing the pressure on it (i.e., put it under a vacuum). That is how a vacuum pump removes the moisture (most of it anyway) from the closed A/C system. The vacuum lowers the boiling point of water. That is why A/C work is best done in the heat of the summer.

The air that comes out of your A/C vents isn't cold so much as it is "not hot." The A/C system does not blow cold air into your 911. It takes the heat out of the air that is already in it and moves that heat to the exterior.

Did I get that right?

Hugh R · 02-29-2008, 02:57 PM

Uh, I hope not cause mine goes compressor > condensor > R/D > Evaporator > compressor. The reason the moisture is bad is cause it turns some refrigerant into a weak hydrochloric / hydrofluoric acid and the acid eats up the compressor shaft seal.

Hester · 02-29-2008, 03:42 PM

Okay so maybe I got the order of the R/D and Condensor backwards. Sorry. Oh, that's why moisture is bad. I get it. I am always trying to learn. I had no idea how A/C worked until it stopped working in my daily driver and I wanted to fix it myself.

stevemfr · 03-05-2008, 03:59 AM

Yes Hester, you pretty much have it right.

2 things: the receiver/drier has functions, as the name suggests. First off, it receives and collects the (mostly) liquid coolant coming from the condenser. It's basically a bottle with the inlet at the top and a dip tube leading to the outlet. The liquid collects in the bottle and the dip tube, which extends nearly to the bottom, allows only liquid refrigerant to pass on. Otherwise the expansion valve would be spitting liquid/gas and the AC wouldn't be as efficient.

Secondly, there is a desiccant in the bottle to collect any moisture - the drier part. The reason is because your assumption (A) is incorrect(google boilingpoint r134...

): R134a boils at -26degC (and R12, which is actually the better refrigerant, -30degC). As the expansion valve is basically just a restriction in the AC line (your kitchen refrig and home AC will usually just have an orifice or very small restricting tube). If there were any water in the system, it would instantly freeze at -26deg and block the system. This is why an AC must be evacuated properly after being open to the atmosphere. Water boils at ambient temp in a vacuum. After 15-20mins on a vacuum pump, nearly all the water in an AC system has turned to vapor and been sucked out.

This is also the reason why I suggested not replacing the r/d above. If worst comes to worst and there is water in the system, the high pressure switch will shut the compressor off and the AC won't work. You've done no real harm other than wasting a little time/money. The refrigerant will then need to be evacuated, the r/d replaced, and the entire system re-evacuated and recharged. In a dry climate and in my own car, this is a risk I'd be willing to take.

BTW, the evaporator in a R134a system will not have 0psi - unless you have a prob. Depending on the system/refrigerant/ambient temp, you'll usually see around 35psi.

rusnak · 03-05-2008, 10:11 AM

I'm going to print this thread out and put it in my files.

Can someone explain what happens when an R134a system is not filled enough, and what happens when it is overfilled.

What is the butter zone for the high side reading, and at what rpm, say at 80F ambient?

Thanks, hope I'm not being too selfish. BTW Hester and Steve, great posts you guys.

stevemfr · 03-05-2008, 10:32 AM

Quote:

Originally Posted by rusnak

I'm going to print this thread out and put it in my files.

BTW Hester and Steve, great posts you guys.

Awww, I'm blushing.

On an (slighter) under fill, you are getting exactly what the receiver is supposed to eliminate: despite the dip tube gaseous refrigerant is reaching the expansion valve = poor cooling.

On an overfill, above all you are exceeding the condensers capacity to cool the refrigerant = poorer cooling.

This was (is? quit doing AC work 15 yrs. ago) a prob. w R12 to 134a conversions in my experience. When filling by the sight glass (top of the r/d bottle. No bubbles = full) in the r/d, 134 a was better when gas bubbles were still present - something that indicates an r12 under fill. We got better cooling by not completely filling a converted r12 system.