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I think I'm overcharged

dhaz0221 on Thu April 17, 2008 6:12 PM User is offline

Year: 1999
Make: BMW
Model: M3
Engine Size: 3.2
Refrigerant Type: R134A
Ambient Temp: 90
Pressure Low: 26
Pressure High: 240
Country of Origin: United States

I think I overcharged my system-but not sure. I replaced the condensor and evacuated. I downloaded the capacity chart on here and it said 29oz for a flat tube condensor and 36oz for a round tube. Well I replaced mine with a round tube. I recharged it at just a hair under 36oz. (the sticker on the car says 29oz).After the system has run for about 10 mins at idle I am getting 26 low and 240 high at 90 degress ambient (termometer 2" from condensor). At this point the compressor starts to cycle about every 30secs or so. I am concerned because when I rev up the engine to about 2K it cycles off the compressor and the readings are about 20 low and 250 high. The vent temps are about 38 degress at idle and driving down the road. However, I can feel the compressor cycle on and off quite a bit while driving. Maybe I am ok just not sure- trying to get this fixed before the Phoenix heat hits. Please let me know if this sounds normal.

cnick on Thu April 17, 2008 9:51 PM User is offlineView users profile

I would say that you are overcharged. If your name plate says to use 29oz you should go with that. As you changed the style of your condenser to one with more tube surface area and may require a larger charge you should let the system run for a few minutes allow the system to settle, then charge another Oz or 2 into it. Your low side suction pressure should read between 35-45PSI. Once you have a ball park in that range stop adding gas and you should be good to go

2nd period Commercial AC Heating and Refrigeration Apprentice

mhamilton on Thu April 17, 2008 9:52 PM User is offlineView users profile

Rapid cycling usually indicates an undercharge, but your pressures look in line with a full charge.

I don't really know the difference between those condensers, are they both designed for R134?

dhaz0221 on Thu April 17, 2008 10:03 PM User is offline

I guess BMW used two different tyle condensers a flat tube and round tube- I assume the round tube holds more liquid. The compressor cycles based on an evaporator temperature sensor that shuts down the compressor at a specific temperature which seems to be about 37-38 degrees. So it is definately getting cold-I am just afraid that at 90 degrees ambient my high is 240 what will happen when it gets 115 degrees here in phoenix I don't want have a really high side.

TRB on Thu April 17, 2008 10:05 PM User is offlineView users profile

I have not checked yet. Does this system cycle by pressure or probe?


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TRB on Thu April 17, 2008 10:36 PM User is offlineView users profile

Looks to be controlled by pressures and the climate control module.

Using that data I think you are just a little under charged. 2000RPM with a high side of 250PSI and an ambient of 90 is not too bad. 2000RPM and a low side of 20PSI could cause the system to cycle early. My suggestion is to add some refrigerant in .5 ounces at a time Letting the system have enough time to react to the change. You're not going to need very much more to slow down that cycling in my opinion. maybe only another .5 to an ounce.


When considering your next auto A/C purchase, please consider the site that supports you:

mk378 on Fri April 18, 2008 9:57 AM User is offline

If you have vent temp of 38 at idle, that's about the best it will do. The compressor is going to cycle on evaporator temp like it should. If it didn't cycle off your evaporator would frost up. Wait for hotter weather and check again.

If your new condenser is the exact same as the old one was you should go by what the sticker says.

NickD on Fri April 18, 2008 10:49 AM User is offline

A vehicle that cycles in a 90*F ambient, no mention of RH that is equally important, at 2,000 rpm, doors wide open, AC on, with blower at max either has a way over capacity AC system or there is a problem, let's talk about the problem, BMW's are not noted over over capacity systems.

Not sure about the control mechanism either a cycling switch or a thermistor on low the low side, but with low side pressures of 20 psi, feel it's safe to assume, the controls are working properly. Key problem appears to be the low side pressure dipping down to that 20 psi, and if it does, I would expect the system to cycle.

So let's discuss why it's dipping down, would expect pressures more in the 35-40 psi range at 90*F and even greater if the RH is high. Can be a low charge, but your high pressure doesn't indicate that, but would expect it to be more in the 250-270 range or higher if it's really damp outside, but would still expect to see the low side higher than 20.

Does this vehicle have an expansion valve? What do I know, I don't own a BMW, but would have a manual if I did, but it sure sounds like if it has an expansion valve, it's not closing properly. If it has a fixed orifice, was that changed? With a 99, we are not talking about a conversion, this car came with R-134a, but is another reason why that low side dips.

I am not a stickler with the nameplate weights nor charging to those precise weights, vehicles with that charging procedure have vent and pressures all over the ballpark. And logically slow, is it really possible to have identical system volumes in a mass produced vehicle? All those minor production tolerances can add up so how can anyone specify a fixed weight? The only manual that I have read that makes sense is for my 88 Supra where adjustments are made in the charge for optimum performance with complete P-T charts given for peak performance. I have applied this principle to other vehicles as well using the mean pressures, RH, temperature, vent temperatures over that wide range of listed parameters. Makes a big difference.

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