Confused - recharge freon by weight or by pressure - PeachParts Mercedes-Benz Forum

tyl604 · #1 09-18-2009, 11:15 AM

Guys - I am now confused. My indy said the best way to add freon (81 300SD) is by weight not pressure. However a forum member Leathermang, who seems to know what he is talking about, reported it should be done by pressure and mentioned a part of the FSM.

Is my indy just wrong - and have I been doing it wrong all these years by topping it off when I see bubbles in the sight glass?

Graplr · #2 09-18-2009, 12:15 PM

If you know EXACTLY how much refrigerant you are putting in by using a charging machine then it is easier by weight (assuming a stock system with a known refrigerant amount). But if you don't have a dial to put the exact weight in from a refrigerant jug, than I would keep a close eye on the pressures.

I have a charge machine that I hook a 30lb jug up to. All I have to do is dial in the amout of refrigerant going in and a minute later it is done with the correct charge.

If you are using cans, estimate the weight and go by pressure on the dials.

leathermang · #3 09-18-2009, 07:54 PM

OK , that was a pretty close description of what I said...
When I mentioned the FSM I was talking about the warning as to using the sight glass as the first and/ or reliable method for determining whether to add freon to a system.
The MB FSM on AC warns that under certain conditions it may show a false ' add' indication. And since overcharging is VERY SERIOUS.... excessively working the AC system from that point forward the warning should be followed by utilizing other more reliable indicator before adding freon.

Grapir stated the situation correctly....

but remember that on that other thread the person was talking about ADDING to his closed system.... so that leaves pressure as the only way to check to see if he has the correct amount added ( in combination with vent temperature ).

leathermang · #4 09-18-2009, 08:09 PM

Quote:

Originally Posted by tyl604

.....and have I been doing it wrong all these years by topping it off when I see bubbles in the sight glass?

Here is the situation... and I do not know if you have an accurate way to measure the amount you add or not... I am assuming you are adding a 12 oz can when you ' top off' ....(?)

IF you are careful AND GO SLOW... and your vent temperatures were NOT what they should be...over a period of time... several days of various types of driving would be my criteria...

and if you have forced air across the condensor, and the engine rpm's at high idle....

and still have bubbles...

AND you add small amounts at a time.. like shoot for a couple of ounces... then go drive the car.... always checking for changes in your vent temperatures... ( keeping in mind that checking low vent temperature is done on SLOW fan ) ....

If you follow these very conservative items... not leaving out any... you should be able to SAFELY add refrigerant to your car's system...
This would be the type of addition which was meant to replace the ENGINEERED LEAK at the compressor which is used to lubricate the front seal on the compressor... NOT TO ADD BECAUSE YOU HAVE A LEAK THAT NEEDS FIXING....

The main point is to add very little at a time and be sure you have given it time and conditions to assimulate itself into the system and show the effect in the vent temperatures....

Remember, your system can work JUST FINE with several ounces less than max.... but one ounce too much overworks the system ALL THE TIME and affects cooling in addition...

Checking for low temp at low fan speed is not what you do when adding and stabilizing the system.. that is at high fan speed, max air with windows open.

#5 09-18-2009, 09:44 PM

If you are topping off a system, the only choice you have is by pressure. If you are following the factory service manual and all you have are gauges, then following the manual with gauges will get the job done.

If you have a charging scale, and you are charging an empty, evacuated system, charging by weight is very simple and foolproof.

I only recently bought a charging scale and have found it to be REALLY handy. It is beyond handy when charging a system converted to R134a because you can charge to the 70% level by volume, then go up in increments of 1 oz. This is the very best way to deal with the dreaded 134 conversions.

All that said, I have been charging a/c systems of all types with nothing more than a set of guages for about 45 years, so a scale is definitely not the only way to get the job done.

dagObx · #6 09-19-2009, 12:57 PM

This is of interest to me, as I will shortly be re-charging my empty system back to R-12.

According to the label on my '87 124, the system needs 2.4 lbs. Does this sound right? I have gauges, so I guess that's the way I'll have to go. Any tips on charge pressures?