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kuehl kuehl is offline
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Join Date: Aug 2004
Location: Stuck in NJ
Posts: 3,066
In a perfect world, you would use a known weight to charge. However, you still have to verify pressures* . And, a weight can be developed for a modified system based on pressures.

1) The factory has a known weight published on a sticker in the engine bay or published in the shop manual. Say, 47 ounces R12. You evacuate the system, toss in 47 ounces R12. You observe the actual ambient air temp around the vehicle and its 90F on your thermometer. *You toss on your gauges and take your pressure readings at idle. Your low is 30 psi. That suggests your evaporator core temp is about 32F. Your high is 280 psi. That's an issue. It should be around 216 psi give or take. So you know you have an issue. So you must investigate if the issue is a condenser function (fan not running) or you have residual air in the system; examples of just some of the troubleshooting you need to do.

2) You have modified the system in some manner (more or a few condensers for example). Where do you start? Well with a logical minimum charge of refrigerant. Let's say the 47 ounce R12 condenser system is now a 1 condenser system, you deleted the front condenser and added an oil cooler there. You could start off with half of that amount, about 24 ounces. Note the ambient temp, look up the suggested high side and observe the low side. There is a bit more to it than that but its a good starting point.
1987 911 cab, modified
Old 06-12-2019, 07:27 AM
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