RPM for AC Charging - PeachParts Mercedes-Benz Forum

shertex · #1 08-25-2021, 08:49 AM

The W124 has a very slow AC leak. I add refrigerant once or twice a year and am happy to continue to do that.

My method has always to charge with AC on, max cool, max fan, and engine idling and then charge till the pressure is in the green range. However I noticed a few folks recently say that engine ought to be at 2000 rpm for charging. Poking my nose around I see a wide range of opinions.

Happy for any insights anyone has. Perhaps engine speed doesn't matter a whole lot.

123boy · #2 08-25-2021, 09:05 AM

I'd say ( and I'm certainly no expert ), if what you're doing is working for you, keep on keepin on. You are correct in your methodology. Not gonna hurt anything. You have a TINY leak if you're only adding freon "once or twice a year"!! If the system is suckin in freon, and you're gettin a proper indication the charge is good, and, it's cold in the car, what else is there? If you're happy, I'd "leave well enough alone". Anything else and you'd have to "get into it" a lot deeper ($$) Gettin through the hot season, with only a couple squirts of gas, is something tons of people would be ecstatic about! Cheers!

leathermang · #3 08-26-2021, 11:08 AM

I think the FSM says to charge at ' high idle ' ...
If you have any leak you can figure you have sucked some air / moisture / into your system. That moisture mixes with your refrigerant and oil and creates acid.
That acid can eat through important parts of your system you would never want to have to replace... like your evaporator under your dash.
There is a long AC thread that used to be a sticky..... if you search for ' flush' using my name in the advanced search it should show up. I think 80,000 something views.

ah-kay · #4 08-26-2021, 11:49 AM

Quote:

Originally Posted by leathermang

I think the FSM says to charge at ' high idle ' ...
If you have any leak you can figure you have sucked some air / moisture / into your system. That moisture mixes with your refrigerant and oil and creates acid.
That acid can eat through important parts of your system you would never want to have to replace... like your evaporator under your dash.
There is a long AC thread that used to be a sticky..... if you search for ' flush' using my name in the advanced search it should show up. I think 80,000 something views.

Please define "high idle'.

No moisture will be sucked into the system when charging due to the higher pressure in the system than atmospheric. Moisture would only get in when the system has a hole and was depleted. Whatever moisture was there will still be there. That is why you need to vacuum the A/C for a new or opened system.

Any rpm would be OK. I don't think it makes any difference except in time. Do whatever you been doing if it works.

leathermang · #5 08-26-2021, 05:48 PM

I did not say moisture would get in during charging ...
I said if he has a leak then he can figure he is getting moisture into the system.

vwnate1 · #6 08-26-2021, 07:25 PM

You're correct in thinking the system will draw in the freon faster with a raised idle ~ it's easy to add too much this way so add a little and allow the engine to return to idle and watch the gauge for a few minutes as it takes time to normalize the system .

I was careless doing this and blew a hose on my 300TD a wile back .

As far as how much to raise the engine speed, look at the gauge whilst you're gently revveing the engine, the pressure will visibly drop as the pump begins to suck the freon in...

BillGrissom · #7 08-27-2021, 05:18 PM

I would just charge at idle until the can is almost empty, then rev the engine for the last 20 sec as you get the last out and to suck as much from the can as you can by lowering the suction side pressure, close the fill valve, then return to idle. Helps to put the can in a pot of hot water to help boil off the liquid, and replenish if the water gets cold.

If still using Freon, refills are getting pricey. I just sold a can of R-12 I had sitting on the shelf for over a decade, plus the fill hose, for ~$35 on ebay. Some ask as much as $100 per can. I use Duracool in all my cars. It works slightly better than R-12. I also use PAO 68 oil, which doesn't absorb moisture from the air, so no acid formation to worry about.

123boy · #8 08-27-2021, 08:57 PM

Quote:

Originally Posted by leathermang

I did not say moisture would get in during charging ...
I said if he has a leak then he can figure he is getting moisture into the system.

This is, %100, an accurate statement. It all hinges on the vapor pressure of the atmosphere as opposed to the vapor pressure of the refrigerant. High ambient temps with high humidity are the conditions most likely for this to occur. Moisture in the air will mix with fluorine and create hydrofluoric acid. The most corrosive of all acids. The thin aluminum walls of the evaporator, aluminum lines, etc. will, eventually, fall victim to this.

Many variables, of course. And, how long before this will actually happen is impossible to know. If you want your AC system to last, have the leak found and repaired (could be as simple as an o-ring or union, etc). Install a new filter/dryer, pump it down, add the proper charge and "off you go"!

Or, you could continue as you have been and the system may outlast the car. Who knows? It all depends what your level of comfort is and what you want to do.

BillGrissom · #9 09-02-2021, 05:31 PM

As long as the refrigerant pressure at idle stays above atmospheric, you shouldn't get any moisture in. On a 70 F day, the base pressure should be ~70 psig at both high and low ports (engine off for 30 min). When running, the low port should drop to 25 to 50 psig. The compressor won't even actuate if <15 psig, which protects sucking a vacuum. If still on R-12 w/ mineral oil, the oil absorbs moisture just slightly. If R-134A w/ PAG oil, the oil strongly absorbs moisture, to form acids which corrode aluminum parts internally, like the AC evaporator.

PAO 68 might be the best oil since it works in all refrigerants and doesn't absorb moisture, plus tends to stay put in the compressor better for more efficiency (less coating of condenser and evaporator tubes), at least that is the claim by Hella and Duracool (their "Oil Chill"). One VW owner left his R-134A system uncharged for a year, then filled with Duracool and noticed a leak from a corroded hole in the aluminum evaporator. He blamed Duracool for the corrosion, though it is just hydrocarbons which aren't known to cause corrosion (ex. oil and kerosene). He got mad after Duracool tech support said his comments were idiotic and he began flaming them all over the internet. More likely, the leak allowed refrigerant pressure to drop to 1 atm, and then it can actually go negative as the gas expands and contracts with daily temperature swings, to draw in outside air and moisture. That likely formed corrosive acids with the PAG oil. Others who were similarly negligent and then refilled with R-134A also found leaks from corroded holes in aluminum tubing.

leathermang · #10 09-11-2021, 11:28 AM

https://vasa.org.au/us-hydrocarbon-refrigerant-company-cops-300k-fine/