Quote:
Originally Posted by newBMWownr
with a modern BMW, it's really not possible to charge a system by pressure any more. As one knows, pressure varies by temperature and therefore just charging to a pressure (either the low side or high side) just isn't accurate enough. The issue is compounded on the e90: The tolerance band around a full charge (1.1 pounds) is less than an ounce. In my opinion there just isn't enough 'slop' for a DIYer to add refrigerant to the system. Even if one were to start with a system that had been properly evacuated, it's really hard to calculate how much refrigerant is in the system versus left in the can, the manifold gauge and the lines.
I used to think you might be able to charge an automotive air conditioning system like a home A/C unit: determine the required superheat or subcooling, record the appropriate dry and wet bulb temperatures and charge to a calculated pressure based on the refrigerant type. However, since I don't know that the superheat or subcooling is for an automotive air conditioner, I can't follow the same practice. Likewise, because all cars of a given model/year have the same A/C configuration, the system volume is known, one simply needs to charge to a given volume.
I don't mean to totally discourage folks from charging their own A/C unit. I've done many cars myself with satisfactory results. My point is to alert folks that the e90 (and probably other BMWs of the same vintage) have relatively low charges of refrigerant and very tight tolerances around what is considered full. This makes overcharging a definite risk.
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Wish I would have read here first before trying out a walmart r134a bottle using their gauge. I went from supposedly 28psi to supposedly 32psi 70f ambient. Luckily, I didn't go too crazy and shoot for 45psi like some post I have read that said our cars should be 25-45psi. Shortly after, I notice my fan was just going crazy all the time. Turn off the A/C and fan calms down to nothing. Turn a/c back on and fan goes crazy again. Worried me a bit, so I randomly bled pressure off the low side, which got the fan to stay quite with the a/c on.
Got the gauge kit at Harbor freight ($59) and read high/low
75f ambient
High= 210psi
Low= 29psi
Even though I seem to have gone full circle, it seems to cool better now, without obviously overworking the compressor?
Still though, I am probably going to go ahead and evacuate the freon, run suction on it for an hour or two and re-service with straight up r134a +PAG oil(stuff I used had fix a leak shit in it). Have everything I need minus the r134a. Suction pump is just this little $15 thing
http://www.harborfreight.com/air-vac...ors-96677.html . Just need a really good compressor which my work has.
Tag on hood says 1.3lbs +/- .2
Bently says 590grams +/- 10 (20.8oz +/-.4)
I guess there is not a gauge that meters out how much is going in by weight
Maybe after completely using one 12oz can, I can weigh (using a food scale) an empty can compared to a full can and then expel a little off the full can till it drops enough to get me in range and then fully empty that can into the a/c system.