A/C Needs Recharging

[mthomas58]

Hard to believe but its been 10 years since I last charged the system.

https://forums.pelicanparts.com/porsche-911-technical-forum/412571-mastercool-refrigerant-scale-98210-a.html

Don't drive the P-car much anymore. Took it up to the tire store yesterday for some new rubber and the A/C was not cooling. I suspect the compressor (10 year old Denso reman) may be the culprit. Will recharge this weekend.

[T77911S]

my guess is probably not comp.
I would suspect one of the coils first, but also depends on hoses you have. if you did not upgrade the hoses then the low charge would be normal after 10yrs.

[GH85Carrera]

First off, you need to drive it more! Shame on you. Porsche abuse! Porsche abuse!

I think I have seen your posts on AC before and I am pretty sure you know the AC basics. Yea, start bu checking the pressures. But if it has sat unused and unloved for a while there are lots of possible causes.

You did not say if the compressor even is running. Do the basic checklist. Add refrigerant if the pressures are low.

[mthomas58]

Pulling a vacuum now. Almost no pressure in the system so I'm sure the compressor was not running. Did a vacuum test earlier it held steady at -28 so I should be good to charge after 45 min of vacuum. Definitely need to drive it more! Hoses were replaced 10 years ago. Did full Griffiths kit with dual rear fender condensers.

Thanks!

Just because I hate threads without pics......

[speedracer92]

Mark, assume you are very happy with the dual fender condenser setup. Did you ever run it with just one rear fender well condenser and if so - did it work good, ok, or not? I live in Texas. Curious as to whether both are really needed or not.

[FrankM_]

I see you are recharging by weight. I just had it filled with 950 grams of R134a (approx 80% of R12 weight specified). What is the common agreement on weight of R134a needed for this system ?

[kuehl]

Mark,

Do you have an electronic leak detector?

Griff

[Jack Stands]

Shouldn’t the system be refilled by reading pressures rather than weight as various configurations of condensers would require different amounts? I’m learning about AC systems as I’ll need to swap out the hoses on my old Targa and am trying to finally tackle this system.

[GH85Carrera]

Originally Posted by Jack Stands

Shouldn’t the system be refilled by reading pressures rather than weight as various configurations of condensers would require different amounts? I’m learning about AC systems as I’ll need to swap out the hoses on my old Targa and am trying to finally tackle this system.

Opinions vary. If you happen to have a manufacturers recommended weight, and the right scale, that sure makes it easy. I just use pressures, since my system is modified with the 4 total condensers. I have been waiting to get a good hot summer day to do a system check of pressures before a upcoming road trip. It has been 5 or 6 years since I touched the pressures.

[kuehl]

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.

Examples:
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.

[adias]

What is the criteria to recharge? Blow temp at the AC vents at what temp?

[kuehl]

Originally Posted by adias

What is the criteria to recharge? Blow temp at the AC vents at what temp?

In the R12 factory design, the drier had a floating ball in a view glass, although not accurate it kinda gave a measure. With R134a site glasses don't give an accurate reading.

You can't go by vent temp because there are a dozen issues that cause vent temps to seem higher than normal. Say for example you had made observations after you had a perfect charge, maybe 85 ambient, vehicle in the shade, windows up, thermostat max cold, center vent temp 44F at evap fan speed 1. A year later its 95 ambient and your car was sitting in the sun for 5 hours heat soaked and you are feeling the vent temp at fan speed 3. An iced up evap core will have higher temps. A failed front condenser blower motor can cause high temps. Keeping the engine deck lid up (no air flow through the condenser coil) will cause high temps. A broken evaporator outlet air tube will cause high temps.

Best to go by pressures in the system.

Static:
Assume you knew you had a good factory charge; you just has the system serviced, everything was documented:
the ambient temp that day, low and high side pressures at idle, amount of refrigerant,
and a 'static' reading after the system had been turned off and the system pressures equalized (low side and high side are the same). If they documented the static, say for example: R134a, 90F, 114 psi. or whatever. You could use that as datum to determine if your system was near full charge same way it was the last time it was properly serviced. Compressor clutch not engaged, etc.

Running:
Ditto pretty much all of above in static, however, this time the system is running and you documented everything when the system was last properly charged. You compare apples to apples: pressures, ambients, etc.