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#1
06-16-2006, 11:36 AM
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Charging by system pressure-what specs for 85 300D?
I know some techs ardently argue agianst charging AC by pressure--instead charging by ounces of R12 or R134 a is common--but I'm just curious--what are the charging specs for an 85 300D; ie low side and high side pressure?
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#2
06-16-2006, 11:40 AM
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I do not know MB specs, but I do remember on R12 the low or suction side was 40 psi, while the compressor is operating. That was for GM, Ford, Dodge, Nissan, Toyota, etc. The high side would vary between 90 and 100 psi if memory serves me correctly. I do like the old sight glass setup. Then you could read the guages and see if the sightglass cleared up while charging.
Last edited by rrgrassi; 06-16-2006 at 12:10 PM.
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#3
06-16-2006, 11:44 AM
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http://www.ackits.com/index.cfm?fuseaction=Chart
The pressure varies greatly with engine speed. This chart makes no mention of engine rpm, but others that I have seen specify "high idle" or about 1500 rpm. Last edited by tangofox007; 06-16-2006 at 12:11 PM.
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#4
06-16-2006, 12:08 PM
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Good info, there Tango!
It should also be noted, that on the car, with engine and compressor off, the low and high sides should have a balanced reading due to pressure equalization. You are always better off by going by pressure instead of ounces. Based on the state ofthe current charge, you can either under or over charge the system. If the system has been opened and flushed, then you can go by ounces after vacuum pumpdown. As you near the number of ounces, be sure to read the guages. Before you operate the compressor, make sure you have added the recommended amount of oil first. Also rememberm the oil charge also contains an ounce or so of freon that needs to be included if you are just going by ounces. Last edited by rrgrassi; 06-16-2006 at 12:14 PM.
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#5
06-16-2006, 02:51 PM
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You can't charge by pressure alone because the pressure changes with engine speed and with temperature. If you want to charge by pressure and temperature read about subcool and superheat. You charge a system with an expansion valve by subcool and a system with an orifice by superheat. An expansion valve is a superheat regulator an orifice is just a little hole.
The idea behind this is based on an understanding of where your refrigerant is and what it is doing. You need liquid from the condenser through the metering device and gas through the compressor. There is a lot more to the story, this is just to get you started. While you are at it read about sensible heat and the latent heat of evaporization, then you will know more theory than many AC techs. I recently posted about cleaning my condenser, it made a huge difference. Climate Control Ramblings Last edited by TwitchKitty; 06-16-2006 at 03:08 PM.
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#6
06-16-2006, 02:54 PM
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Larry Bible has posted his suggestions on this ....in the archives...
Can't find the numbers in my MB AC manual.. my ' Modern Refrigeration and Air Conditioning' book in the automobile section.. for R-12 suggests the low side will be about 28 psi and the high from 130 on cool days to 200 on hot days... While you do need an accurate measure of the weight of refrigerant going into the system... you need to monitor the pressures so that if something is clogged or became clogged you would recognize it without blowing up everything. Remember also that your system can work at several ounces less than full capacity MUCH better than ANY over fill...
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#7
06-16-2006, 03:15 PM
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Quote:
The chart only mentions ambient temperature. To see basic system function you need vent temps, temp entering evaporator and the temp entering the compressor. Combined with the pressure readings you can see where you have liquid and where you have vapor.
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#8
06-16-2006, 03:46 PM
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Remember that vent temps are taken with the blower on low... so you are seeing the lowest temp you are getting...
"If you get the low side too low it will freeze the evaporator and shut off the air flow." You are not the first to say something like that.. but remember that our TXvalves are really block combo valves which incorporate the throttle suctioning valve into the equation... its purpose is to work with the txvalve to keep the evaporator from going below 32 degrees F... ( set a little higher for leaway since water only freezes at 32 on a standard barometric day at the prescribed humidity level )...
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#9
06-16-2006, 04:03 PM
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you mentioned the sight glass. That can be used for charging with r-12 but for r-134 it really has no purpose. The oil does something funny in r-134 (can't remember what) and causes the sight glass to look like there are bubbles all the time. If you ever got the sight glass to be clear in a r-134 system it would probably be way over full.
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#10
06-16-2006, 04:10 PM
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"If you ever got the sight glass to be clear in a r-134 system it would probably be way over full."--omegaman
AMEN However, even with R-12 the sight glass is only partially useful.... there are conditions where it can be fully charged and still see bubbles... so the rule is weigh the refrigerant accurately as it is going in AND monitor pressures. I get this from the MB AC manual...
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#11
06-16-2006, 04:54 PM
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Quote:
 The point I wanted to make is that this is not a Tim Taylor, more is better situation. Lower pressures are only better to a point. Evaporator temps below freezing are a problem to be avoided. Last edited by TwitchKitty; 06-16-2006 at 05:00 PM.
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#12
06-16-2006, 04:58 PM
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Quote:
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