Freon sniffer, etc.

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Staff member
Dec 14, 2002
New Jersey

What is the best means of "sniffing" and characterizing the freon or refrigerant in an AC system?

My 81 MB has the original receiver/dryer, compressor, etc. When I cycle it, it blows barely cold, though the compressor does engage. I do not think it cycles though.

My concern is that there is a "quick connect" fitting on the low side of the system.

I have all records for the car, but I do not have any record of a recharge. The air blew plenty cold all last year. Still, if the PO did something stupid, like topped R12 with R134, I want to make it right. If it was a matter of simplicity to use the quick connect to do R12 (which may be illegal, right?), then I want to still make it right.

I have R12 and can top that, but I want to be definite. Is there a way to DIY this, or do I need to take it to a shop? Say someone did something stupid like top off with Freeze-12 or similar, how can I be sure that I account for this too?

Now, two other quick questions:
-the car is 28 years old and all original. Does it make sense to evacuate the system and do an "oil change" anyway?
-if I want to top it off, can I use my R134a gauges for R12? I know the fittings are correct to do this. Is there a set of best practices to purge either out prior to switching to the other refrigerant?

I would have the system evacuated, the compressor oil dumped out and refilled with new oil, a new dryer installed and the system converted to 134a.

It will cost you a few dollars but your A/C system will thank you and you will be rewarded by nice cool dependable air-conditioning.

Plus in the process of evacuating the system you can test for leaks by leaving the system under vacuum for 1/2 hour before recharging.

That system being so old most likely the dryer is somewhat plugged up and needs replacement. Not to mention there is probably moisture and a bit of air in the system.

Good luck...
My FIL worked in the AC profession most of his life. He helped me diagnose a problem that two shops couldn't find. I have a 21 year old van with R-12, nothing cools like R-12 did.

Here is what I remember. Before you do anything check for leaks. You can check for leaks with a flame or soap bubbles. Look for dirty and oily residue at all connections. If a hose has become porous they are harder to detect. That was my problem. You can introduce dye into the system and look for leaks with a black light. It worked for me, I had a pin hole in the high pressure hose no one could find! R-12 is expensive so you want to be certain there are no leaks before topping it off. I don't think anyone topped your system up with 134 because the fittings are completely different, they could have used something like Freeze 12 which in theory is compatiable with R12.

Once you are certain there are no leaks you can have the system evacuated, then pull a vacuum and charge it up with R-12. To be on the safe side you can add 1/2 ounce to 1 ounce of the proper R12 oil. There is really no way to change the oil out of the system w/o really getting into some heavy duty disassembly etc. The 134 gauges should work, but you might need adapters to mate it to the R-12 Schrader valves. When adding oil don't over do it! There is a tool and a method for adding the oil do that when you charge up the system. The longer you pull a vacuum the better it is for getting moisture out of the system. If you can't hold vacuum you have a leak! It is not uncommon to have more than one leak, so if you find a leak that doesn't mean you're home free, especially in an old car! I would change the accumulator you can add the oil into it, its probably NG if it was never replaced and the system had a leak.

Sorry for all the fragmented info, I am going by memory and its been a while. I'm sure if a pro weighs in he'll clear up the fragments. HTH
This site has a wealth of good info. It's sponsored by a seller, but don't let that turn you off. Using info gathered there, I completely rescued my AC system from black death. It wasn't cheap, but it was worth the time and money. I was able to keep R12 in the system too.

I would rate the knowledge base of the AC forum as one of the better places for sound advise and info.
It is from the above-posted forum and the one below where I got all my education on automotive a/c repair.

The quick-disconnect fitting on the low side suggests that someone did a bare minimum, or hack, job just to get some cold out of the system. It's revolting to think of why only the low side was changed over. It may be a good bet that you have a mix of r12, r134, and air in the system, or some combination there of.

Unless you can narrow down some history or information of what was done, this is the plan of attack I would use:

You'll need a way to pull vacuum (must have), a set of manifold gauges (stongly advise), and UV dye (optional).

As a must, you'll have to disconnect all hoses in the system, replace the o-rings, flush out the system, and replace the dryer. Flushing removes the oil (and contaminants) to give a zero starting point - for both lube chemistry and amount. The compressor will need to be removed and the oil dumped and flushed out of the system. The compressor is flushed by pouring new oil into the compressor and working it through by hand-rotating the compressor. Hang it to drip-empty.

The remaining components will need to be flushed using copious amounts of flushant and shop air. The flushant will be alcohol or lacquer thinner. You can economize by using mineral spirits, then finishing with alcohol.

After everything is done, cleaned out, buttoned up, and vacuumed, you have three options (in my opinion).
1. r12 (original to the car)
2. r134
3. A hydrocarbon (HC) refrigerant, typically a propane/butane mix, such as Duracool.

With r12, you'll need to make sure the o-rings, lubricant, and dryer are compatible. You'll need r12-specific manifold gauges and remove the quick-disconnect at the low side, so both high and low side have the schraeder (tire stem type) valves.

With r134, you'll again need to make sure the o-rings, lubricant, and dryer are compatible... and r134-specific gauges. Both ports (high and low side) need the quick-disconnects.

If you go with HC refrigerant, which is more r12-like when it comes to converting (or unconverting), it should use the same o-rings, lubricant, and dryer as an r12 system uses. Commercially, these systems use the quick-disconnect fittings for legal reasons, and most vendors make the cans compatible with r134 charging equipment. I'm not sure which manifold gauges are appropriate for HC refrigerants.

After a full job as I posted above, I sometimes don't need the pressure gauges as much as I use a vent thermometer. I charge roughly the prescribed amount needed for the system, keepng an eye on the vent temperature. I charge until the vent temperature doesn't drop anymore.

I only touched on the basics of quality a/c repair. There are many more nuances to a/c work. Spending a few hours on the automotive a/c web sites can give you much of the information you need.
That MB really needs to stay with R12. It's A/C system is marginal with R12 and won't be any good with R134a since it's not as efficient as R12.

Don't go with Hydrocarbon refrigerant. It's not legal to use it in a car in the USA
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My plan is to maintain original characteristics, which includes R12.

The AC blows plenty cool with it! Ive had three of these cars.
Kestas has pretty well covered it, except whatever you do, don't use shop air to blow anything out, including the flushing solvent, use either dehydrated air or dry nitrogen otherwise you will be introducing far too much moisture into the system.

I use a dedicated flushing solvent like R141b or an HC we have here made by ERG called UFA. I wouldn't use lacquer thinner at all.
Evacuated, refilled with R12, now very frosty. The delta was about 2 lbs on a 2.9 lb system. Works great. Compressor cycles much more gently, and when the system is running, no hissing in the lines (indicative I guess of a phase change where there is not supposed to be one due to low pressure?). Held solid vacuum for over 30 minutes.

Will overhaul the system later this summer - wanted to get on the road with cool air.

No indication of 134a, and they evacuated the system no problem. Told me that lots of mechanics just charge some 134a onto R12 because it works to some extent. Not legal or right, just what they have seen done before. Kind of like a real lousy conversion, but where some R12 is in there (I suppose to keep the oil moving).

Anyway, it works beautifully.
Yeah but no way did I want to have any trace of 134a in there, if there ever was. I figure after 28 years, the receiver.dryer is likely used up just due to diffusion and slow seepage, and the oil in the system is probably done with its buffering capacity, and may not be in spec.

I probably could have shot a bit of 134a in there and gotten some cooling too... but better to do it right, or on the right path, then to have a long term issue.

I will open and refurbish the working fluids at some point. Again, better safe than destroy a 28y.o. evaporator and have to pull the dash...
Originally Posted By: JHZR2
... I figure after 28 years, the receiver.dryer is likely used up just due to diffusion and slow seepage, ...

Can you elaborate on that? If I understand your explanation from Critic's post correctly, this is not possible since, as it still had positive pressure, nothing could be in your system except refrigerant.
I have heard that they "clog" up from the breakdown of oils, particle attrition, etc. Junk moves through the system and apparently collects there.

I may be wrong. Yes, I do not believe that water is in there, but whatever other junk may be would likely be in higher concentration in the R/D if there are solids and stuff.
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