Refilling R134 - some questions
 

I need to do a "top up" on my 2001 Mercedes. It's never had the freon or AC system touched or refilled! Over 17 years on the original R134.

It is finally now starting to not get quite as cold (although still not bad).

I have the gauges and R134, and have added freon to other cars in the past (R12, a while ago), but I don't quite remember the procedure.

I hooked up the gauges and both the high side and low side read low, so I think it's just a matter of adding 134.

Pretty sure I just keep adding until it gets to the high and low pressures on the chart below, correct?

Also, my biggest question, are those numbers on the chart at idle, or at elevated idle, like 2000 rpm? Because of course the numbers change fairly significantly at idle v. higher rpm.

Thanks!

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Idle is fine. Usually when I'm putting in freon, it is ALWAYS 95F+.

Have a tub of warm water to warm the freon can.

Also, only add the gas to the low pressure side. Close off the high side when adding the gas.

Purge your hoses before introducing new refrigerant.
Add to the low side.
Undercharge= Bad
Overcharge= Bad
Add very slowly.

Shake the can well, it has oil in it. If it doesn't get some. Don't slug the system by turning the can over and introducing liquid freon, but do tilt the can back and forth as you fill.

Do not use that pressure chart! All cars are different. Charge to a desired vent temperature at idle. I always put the fan on speed 2, recirculate, and shoot for anything below 40 degrees at the center vent.

I say purge the line like billybek says and put a 14 oz can in.

If cold. The end.

Read and save over the winter for a 30 pound tank and a vacuum pump.

OP did not mention he has these. Hey, he might, but not specific in info provided.

I have all of the equipment, except the vacuum pump. I had one, but it died, and it's been 10 years since I've needed to add freon to a car so I've never replaced it.

This car is almost 18 years old, on the original freon, and it was slowly starting to not cool as well, so I'm pretty sure it's low.

The sticker says it holds 750 grams (26 ounces).

I ended up basically doing what Bob Kontak suggested.

I hooked up the gauges, hooked up a 12 ounce R134 can, purged the freon line, and started adding the can to the low side line.

The gauges didn't really move much (or at all) during the adding. It took the entire 12 ounces. The air coming out of the vents became very cold.

Even though the gauges didn't move much and the pressures seem low-ish (roughly 30 on the low side and 150 on the high, at idle, 75 degrees ambient), I think I'll leave it alone for now.

Those pressures don't seem low to me. You have to remember, some cars have the high side service port before the condenser, some after. Yours is after, so the high will be lower than, for instance, a ford truck.

The goal is to end up with the required fl. oz. of oil and weight oz. R134a in the system. Why not use an extraction machine that separates the oil. See what oil comes out and add that to the required R134a weight recommended you insert.

Maybe because recovery machines start around $1000

Most everyone is going to need to evacuate the system if they plan to be successful with charging in the future.

It is all about the weight with the variable displacement compressors being used today, and pressure charts will not help as the pressure will be all over the place. System capacity has now also dropped below 12oz of refrigerant, with very little tolerance +/-.

The chart above is junk, if you follow it, you risk blowing up your system.

A friend who is a refrigeration mechanic recently re-gassed my car. He vacuumed the system for about 90 minutes then while the car was running put some gas in according to a little book he had. He then checked for bubbles using the site glass & added more refrigerant as needed. He made it look easy and the system works great now.

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I assumed he would go to a tech. that had one. They are quite common now.
In any case the point is to fill with the specified weight of R134a and not go by the pressures or bubbles in the dryer. Maybe Griff will chime in.

for a car that has very slowly lost refrig over 18 years I think slowly adding r134 and monitoring the vent temp is a good enough approximation of the right way to do it, especially considering you monitored pressures to make sure they didn't get wonky

If it leaks at a much faster rate or fails to cool going forwards, you may need to bust out a bit more science, but I imagine it will be fine for another decade or 2

The problem with going strictly by weight is fluctuation in ambient temperature and condition of the system, externally. All cars have +/- refrigerant charge fudge factor.

If you want peak efficiency you will charge to a desired vent temperature, taking note of system cycles.

Your condenser or evaporator may not be as clean as another car, or the fan clutch might not be up to par (if equipped), making the system less efficient at a specified refrigerant weight. No car is the same.

How did you do this? :cool:

Why do you need to warm the freon can? :confused:

Simplified, it's the same reason why the AC cools your car. Freon in the can is under pressure and in a liquid state. The rapid discharge of the liquid through the hose to the AC allows the pressurized liquid to expand and become a gas--it's natural state. That conversion from liquid to gas draws heat from its surroundings resulting in a cooling of anything in the immediate area--like the can or the coils in the AC. If the can is not kept from freezing, the expansion of fluid to gas within the can may slow or stop. The process in the AC system is regulated by expansion valves to keep the cooling process stable and prevent freezing of the components.

While that may be true, there's no need to warm the can. An evacuated system will suck the refrigerant in past the point where the minimum charge (pressure) has been met by the LPCO, and the compressor will pull in the rest when running. If the compressor can not pull in the remaining charge, there's something wrong.