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How much freon?

bigwingboy on Mon August 27, 2007 7:47 PM User is offlineView users profile

Year: 1990
Make: BMW
Model: 325i
Engine Size: 2.5
Refrigerant Type: R12
Ambient Temp: 105
Pressure Low: 40
Pressure High: 250

Hello all! I replaced the AC system in my 1990 BMW. New evap, expansion block, receiver, compressor (Denso), and condenser. Fan clutch is new, and the electric fan operates properly. After finishing the install I charged the system to factory specs. It called for 2.15 lbs. of R12. The pressures were too high (lo. 75, high 275). The system performed poorly and appeared overcharged. I started investigating and found out that the condenser I received was a serpentine instead of the old tube and fin. Thanks Tim! This is a better condenser, but it appears the system does not need the factory charge. I recovered freon to bring the low side pressure down to 35 and the system performed much better. However, when the engine is at idle the vent temps rise 10 to 20 degrees. I' thinking that the charge is not fine tuned, but I don't know how to determine the proper charge with the different condenser. I tried adding a small amount of R12 and the low side pressure began rising. Any suggestions on determining the proper charge for a modified system?


TRB on Tue August 28, 2007 12:51 AM User is offlineView users profile

You will always see an increase at idle compared to higher RPM and additional air flow across the condenser. These vehicles are not the best candidates for conversions. What kind of idle and hi-idles pressures do you have? What are the vent temperatures?


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mk378 on Tue August 28, 2007 10:34 AM User is offline

Since you're using R12, you can watch the sight glass to evaluate the charge level. I would suspect the serpentine has less volume inside than the old fin and tube, so you're on the right track to use less than the factory amount.

bigwingboy on Wed August 29, 2007 4:42 AM User is offlineView users profile

I tried the sight glass. It didn't clear until the system was well overcharged. Not good! A clear sight glass yielded around 75psi on the low side! I'll try and get some numbers. Perhaps the system isn't that efficient since the car is a 1990. Also, the ambient temps have been brutal lately. The OBC showed 115 when I was on the road yesterday. Maybe I'm asking too much of an old design, but when I stop at a light it's like I don't have AC.


Edited: Wed August 29, 2007 at 4:45 AM by bigwingboy

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