Hey all, I'm attempting to recharge a 1987 300D with R12 - the car fortunately has not been converted and I do NOT plan on doing so! Anyhow, I got my hands on a manifold gauge set. This gauge is a bit different from others I've seen (it has two extra knobs labelled VAC and REF) and I'm not quite sure what those do. My understanding so far is:

1. Connect the low and high side hoses accordingly
2. Attach the can of R12 to the yellow hose
3. With the engine on and AC controls switched on high, crank open the low pressure knob of the manifold gauge
4. When low pressure approaches ~30psi and high pressure is ~250, AC compressor should engage. Close valve.

To be sure I'm not *over* charging the system, what should the low side pressure be with the compressor off? Does the REF knob in addition to the low pressure knob need to be cranked open in order for R12 to enter the system? Thanks in advance!

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You can NOT be sure of not overcharging by using the pressures alone.
The ONLY legit method of determining the amount which you have installed in a system is by weighing the can as you install.

Also, it is good to find a drawing or picture of the inside of ac manifold valves before you start... they seem kinda tricky to me...since they have two off positions and other combinations.. and that is on the standard ones... have no idea which combos to use on yours...
Check the name on them and then Google or email the manufacturer... or go to Aircondition.com and ask there... you want to be really clear before you get pressurized....

Would not be a problem except for the cost of the R12 if you lose it due to wrong settings... or you can look like you are open to one route...and not be connected.......

Greg is correct in saying that you can only be sure of the EXACT amount or refrigerant if you charge by volume with a cylinder or by weight. However, with an R12 system this is not near as important as with 134.

You can charge a 12 system by watching pressures and sight glass. If you get a little overcharged and the system is still cooling well, the extra charge will just accumulate in the r/d.

In a 134 system you can't operate that way because a slight overcharge will allow the high side pressure to run away which risks system damage.

One other thing. On these cars the high side and low side fittings are the same size, so make SURE that you are connected correctly and do not attempt to charge through the high side. This is dangerous.

Charge until you have about 190 or so on the high side and then let the system stabilize for five minutes at 1500 RPM. If your sight glass isn't clear, add a little and watch the sight glass. Once the sight glass is clear, don't add any more. If you can place a hefty fan in front of the car to simulate ram air this will help.

Also it sounded from what you wrote like you don't expect the compressor to kick in until high pressure. You can't have high pressure without the compressor kicked in. Charging into the vacuum as much as it will take without starting the engine will give you enough pressure to kick in the clutch. I assume that you are evacuating and charging an empty system.

If there is any refrigerant at all in the system, static pressure will be about 80.

Proceed slowly and carefully.

Good luck,

One of my first AC experience DIY was about 1971 and I hooked the can up to the high side... with the ac running..
I did not have goggles on... I accidentally felt of the can... because you can feel the ' cool ' line... it was HOT... it did not explode... but It easily could have .... and blinded me...
When we say read the cautions, and use eye and face protection it is not because we are being overlycautious just because we are old guys...

Thanks everyone, for the input thus far. The car has *some* R12 in it - low side reads 10psi (compressor off).

Larry, did you mean that the low side should read ~80psi when the compressor is off? The reason I ask is because I don't want to overcharge the system in case there is something else not causing the compressor to engage.

This I agree with 100%

This I take issue with. If you measure subcool and superheat and know that you have liquid where you need liquid and vapor where you need vapor and have acceptable temperature out of the vents, you are legit.

Before you run the system both the high side and the low side will read about the same. Does the compressor engage on this car?

That is a nice set of gauges, I picked up a set like that at a pawn shop but have not learned the details of the valving. That set will allow you to hook up a vacuum pump, recovery machine and refrigerant bottle all at the same time, you may need one other valve. If the hoses are not in great shape they may leak.

"This I take issue with. If you measure subcool and superheat and know that you have liquid where you need liquid and vapor where you need vapor and have acceptable temperature out of the vents, you are legit"

The people who KNOW what subcool and superheat are.. and know how to determine that they have vapor/liquids at the proper places are NOT MY AUDIENCE... LOL

Those are PROS... with training... and PRO equipment.....

I am talking the the above average DIY who owns a Mercedes....and wants the leaste number of ways to mess it up... and for it to last as long as possible with normal tool investment... If I am wrong on this description of ' my audience' I hope they will stand up and Hollar.... LOL

Larry, I am one of your audience members, sitting at the front row.

Frosty is my only technical term.

Think beer mug......

__________________
Enough about me, how are you doing?

All it takes are gauges, a temperature probe and a vapor-pressure chart, it is too easy to not do it. This is the fundamental function of an AC system, you are nowhere without it.

I think if you tested all of the "AC techs" in the country you would find that many do not understand the basics.

Subcool, Superheat, latent heat, sensible heat. Read and learn or spend your time and money to find a trustworthy shop to do the work for you.

"I think if you tested all of the "AC techs" in the country you would find that many do not understand the basics."

That is VERY true... unfortunately...

I try to read a learn that stuff... but I can't make others do that... so I just try to boil the stuff down to absolute essentials in terms of what NOT to do...and try to keep people from hurting themselves or others...

Plantman... did you mean Larry ... or ME ? LOL

speedy,

A system that has ANY refrigerant at all, will read about 80 PSI static pressure on both sides if compressor has been off long enough to stabilize.

The pressure will be about 80 whether the system has 1 oz. or 3 pounds. The gas that comes off the refrigerant will always have the same pressure as long as there is some of the element present.

If that 80 is present, then that is sufficient to activate the low pressure switch to allow the clutch to kick in.

So, that is the first thing you check when connecting gauges. If you have the eighty and all is well, the clutch should kick in and then you pick up the process from there.

Good luck,

Unfortunately many people find the concept of vapor pressure very difficult to understand. This is partially because they were long ago able to sort out the concepts of hydraulic pressure or air pressure, ie how a hydraulic floor jack or an air compressor works. In those cases mass or volume of the working substance is often equated to the resultant pressure, and that relationship seems to stick in their minds. Very different for vapor pressure, which is such an important concept in HVAC, auto A/C, and distillation.

__________________
1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150

[QUOTE]Originally posted by TwitchKitty
[B]All it takes are gauges, a temperature probe and a vapor-pressure chart, it is too easy to not do it. This is the fundamental function of an AC system, you are nowhere without it.

I will be the first to admit that I am in over my head but I don't see A/C being that difficult...The wifes Volvo was not cooling...knowing that the system had not been serviced in at least 2 years and it still had a charge...(compressor running normal) we stuck a 12 oz can of r-12 in it with a can tap. no gauges, no probes, no flow charts...its blowing cold now...

And before anybody takes me to task for my actions I have a defense that is undisputable, uncoruptable, and nothing you can say will change it ...The Wife is HAPPY...AND she "thinks" I'm a brilliant mechanic!!!

There is always that element of LUCK.
If you encountered a situation where the amount lost from your cooling system allowed you to put back in 12oz... without it overfilling it then you look GREAT...
But it could just as easily been a different outcome and you would be asking about the symptoms and cures for it....
We have to mention the potential pitfalls of such " throwing R12 at the problem" so others who want to take a more conservative route will know what to look for...

Remember Greg, his is an R12 system. If it is slightly overcharged it won't hurt anything. If it were a 134 system, overcharging even a little will often result in runaway high side pressure.

I'm not enough of a chemist to understand what makes 134 this way as opposed to the behavior of R12, but I do know that it is so.

You are right, he could have been lucky. As he says, as long as the Mrs. is happy life can continue to be somewhat pleasant. At least for a few weeks or so.

Have a great day,