Automotive Air Conditioning Information Forum

Hi Guys

I’m working on my 87’ GN that I restored the AC system on two years ago. Originally I charged it by weight (as close as I could guess it using cans) and the system worked great. Pressures were 34L/260h at idle on a 98* day. With the system really heat soaked I’d see 37L/296H and would get 38-39* vent temps. It was great.

Fast forward two years. System still works fine but I noticed on freeway drives, the vent temps would jump to 55-58*... and at slow speeds I’d see 45-47* in 88* ambient temps.

So I put gauges on it, static pressure was 78/80 on a warm system. Idle pressures were 32L/280H and vent temps about 51*. Ambient was 95* @ the condenser and the system was quite heat soaked, running it all day. So I thought it was a touch low on Freon. So I added about a half can of R12.

Then the static pressure jumped to 80/100 psi, idle pressures were 32L/270H, 97* ambient @ the condenser. Not bad right?

Well then I revved it to 2500 rpms and pressures went to 29L/330H ! That seems really high to me on the high side. Vent temps still about the same at 51* at idle, and about 48* at best moving.

So I can’t tell if it’s over or under charged, or do I have a restriction somewhere?

Compressor, drier, oriface tube is all new. Rest of the system is original 87’ stuff with 88k miles.

Any thoughts / info is greatly appreciated.

No one will touch this thing with R12 in it so I have to do it myself.

1. You overcharged a system that was not condensing well
2. the condenser function is likely reduced by reduced airflow.
3. Once the system equilibrates on a 90° or more day,
do the following:
Record the temp into and out of the condenser as close as possible to the condenser itself.
Grab onto the lines in and out of the evaporator, they should be nearly equally cold
Double check the function of your fans. If electric, they can weaken or not be switched on properly. Best if they come on at the same time the comp does.
The fan clutch of your mechanical fan may be weak. These often have to be replaced every 2-3 years. Fan can be too weak for AC, but not so weak that engine overheats
Report back with data from above

Thanks man!

Yeah it has a big front mount intercooler in front of the condenser so it definitely has some restriction...however when I first charged the system, it worked great like that. I did notice the high side pressures were a little high in the 260-275 psi range but...the vent temps were 38-39* so I called it good.

Oddly, I have always noticed the ac does not blow as cold on the freeway. It warms up to about 55-57*, yet slow speeds and idle it’s in the low 40s. Opposite of what you would think it would do. I’m guessing that’s from the high side pressure rising with rpms on the freeway.

Last night, I instantly noticed vent temps would come up 5-6 degrees pulling away from every stop light... definitely seems like it’s over charged just going by that.

I’m gonna take some pressure readings with a different set of gauges to verify, then remove some Freon.

The suction line gets really cold and sweats a lot. The other line seems pretty warm from what I recall.

I’m gonna mess with it some today and I’ll report back.

Thanks for the help!

The original data I have for your car specify 52 ounces (3-1/4 pounds) of R-12 and 8 fluid ounces of Mineral Oil. Is that what you have in the system?

Better to pull the charge, vacuum (with an electric motor powered vac pump only), and recharge by weight, then go from there. The location of that intercooler is not helping at all. Would be a really good idea to do everything you can to increase airflow over condenser. I assume there is a fan shroud for the mechanical fan.
Since the AC ability was better before, that adds credence to the idea of a weak fan clutch. In the world of AC, they just do not hold up.
Try misting the condenser with a garden hose (not a small hand pump house plant mister) and record your temps and pressures.
All AC testing at your shop/garage should be with RPM at 1500, and max load, meaning max fan, max cool, windows down or doors open, recirc on
Report back

oops John and I were typing at the same time

tbirdtbird wrote: ↑Sat May 21, 2022 7:27 am oops John and I were typing at the same time

Seems to happen to me all the time

Tested static pressure with two sets of gauges. Tried to post pics but the files are too large. I’ll try to resize them.

Pressures were 75/85 on my R12 gauges and 83/83 on my newer R134 gauges. Around 86* ambient.

Not sure if I should let some Freon out or not. I’m getting the car hot now and I’ll check running pressures with the newer R134 gauges. I didn’t want to cross contaminate the gauge sets but..I don’t trust the R12 gauges now.

JohnHere wrote: ↑Sat May 21, 2022 7:21 am The original data I have for your car specify 52 ounces (3-1/4 pounds) of R-12 and 8 fluid ounces of Mineral Oil. Is that what you have in the system?

Originally when I charged it 2 years ago, yes, I charged it as close to factory specs as I could get it. 3.25 lbs and as close as I could measure out on the mineral oil. I had used 3 - 12 Oz cans and 1 - 14 oz can with dye. The system worked great initially.. then over time it started to warm up some. I figured it had a slow leak.

Looking at the static pressures by temp charge for R12, the pressure is a little low, indicating under charged but.... after I added Freon yesterday, it definitely got worse.

tbirdtbird wrote: ↑Sat May 21, 2022 7:27 am Better to pull the charge, vacuum (with an electric motor powered vac pump only), and recharge by weight, then go from there. The location of that intercooler is not helping at all. Would be a really good idea to do everything you can to increase airflow over condenser. I assume there is a fan shroud for the mechanical fan.
Since the AC ability was better before, that adds credence to the idea of a weak fan clutch. In the world of AC, they just do not hold up.
Try misting the condenser with a garden hose (not a small hand pump house plant mister) and record your temps and pressures.
All AC testing at your shop/garage should be with RPM at 1500, and max load, meaning max fan, max cool, windows down or doors open, recirc on
Report back

oops John and I were typing at the same time

I wish I could find someone to recover the system and vacuum it down but....it’s R12, no one will touch it.

Originally I vacuumed it for 6 hrs, let it hold for 4 hrs, then vacuumed it another 4 hours before charging. It held a perfect vacuum for 4 hours.

It does have the original AC hoses so they could seap, I don’t see any dye on them anywhere but.. they could still leak a little.

As for the fans, I have two 12” Spal Extreme fans and an alternator that holds 14.5v at idle so... that’s about as good as I can get for this car. The intercooler blocks the entire radiator/condenser, and it’s 3” thick. So it’s a significant restriction... plus I have a trans cooler up in there too. There is nothing I can do about those items. But as mentioned, the system used to work great, even with these restrictions.. It just got gradually worse and then yesterday I think I really screwed it up.

I’ll report back with running pressures after lunch when it’s hotter outside.

Ajrothm wrote: ↑Sat May 21, 2022 7:51 am Not sure if I should let some Freon out or not. I’m getting the car hot now and I’ll check running pressures with the newer R134 gauges. I didn’t want to cross contaminate the gauge sets but..I don’t trust the R12 gauges now.

Be mindful of your terminology in a public forum that could be misinterpreted. I'm sure you meant "whether to recover some of the Freon myself" as you wouldn't want to lose any of that precious R-12. As for the R-134a gauges, you could pull a vacuum just on the Manifold Gauge Set after you're done working on the GN.