R134a pressures getting too high?
 

I've been trying to get the a/c in my 86.5 reinstated since it's frequently over 100f and humid here in the summer (hovering right at 100f and 35% humidity here at this moment).

Not knowing the history of this car and seeing that my ac compressor had oil all over it, I went ahead and resealed the compressor, replaced the expansion valve with a new Egelhoff unit (it was marked r134 on it when it came), replaced the drier, replaced every oring I could get at, and replaced the oil with ester oil (measured what came out of the compressor and added a bit extra for the new drier and what had leaked).

Based on what I've found I should be filling it with about 85% of the R12 volume in R134. Being that this car doesn't have rear ac that looks to be 1050g = 37oz * 0.85 = 31.5oz. After vacuuming the system down I started filling with R134. I put in 8.5 oz from a 12oz can I already had used part of a couple weeks ago then started filling with a second 12oz can. Maybe about halfway through that can the high side pressure was getting to what I considered fairly high. After a bit longer it was up to 350psi on the high side, so I stopped filling. I'd guess that at this point I have 15-20oz total in the system. The dash vent temps are about 70f, so it is doing something.

Here are the pressures at idle after I stopped charging. At this point I was measuring 220f on the inlet side of the condenser and 180f on the outlet. Not sure if this is normal or not either.
Attachment 1346497


And at ~2k rpm.
Attachment 1346498


I understand how an ac system works, but by far and away am not experienced with them. The only other r134 systems I've worked on are my 99 Durango and my buddies 14 Silverado. It wasn't quite as hot as it is today when I charged those, but I don't remember the pressures ever getting that high on those systems.

Does anyone that's done an r134 conversion on an otherwise stock system have pressures this high? I know the higher pressure is putting more stress on the compressor, but are the pressures getting dangerous at this point or am I just being paranoid? It seems like I'm well over what the WSM says I should be at on both the high and the low side though.

So I weighed the second can and it looks like pretty much the whole can was charged into the system, so 20.3oz is in there now.

I did some experimenting and it seems like the high side is getting high due to lack of airflow. The clutch fan is working and engaging as it should and the electric fan is working, but it just isn't enough at idle. I didn't calcuate the flow rate or anything here, but just for comparisons sake I measured the air speed through the condenser on my Durango (which I converted to an electric puller setup years ago) and it was about 4m/s with about the same surface area as the 928s radiator. The 928 however only has 0.8 m/s going through it at idle. It's hard to measure the electric fan, especially since a lot of the air is escaping around the sides of it, but it was sucking in at around 3m/s.

I drove around the street (25mph around the circle I live on) a couple times with the gauges sitting on the hood where I could sort of see them. While moving it was sitting closer to 325psi. As soon as I'd stop it'd get to nearly 400psi though.

I'm wondering if I might need to start considering switching over to an electric fan setup if I want the a/c to survive in the summer here.

Idle isnt the test speed however..nor is driving, but that is useful data.

The second photo up there with the gauges is at 2k rpm. Granted it wasn't there for 10 minutes like the wsm says. But whether or not the a/c specs aren't to be checked at idle, the high side pressure pretty much keeps climbing while stationary unless I put a box fan or something in front of the car. My main concern is if I charge it fully and the pressures rise to 450psi or something while stationary (sitting in traffic or something) should I be concerned? I know my Durango's compressor has a relief valve that opens at 500psi as a safegaurd. I'm guessing there is some safety mechanism here too, and the pressures are getting awful high.

There is, ~500psi or so.

Hi OCD,
I'm just guessing, but I bet you have air in the system. It is very difficult to fill with 12 oz. cans, but it can be done. You need to purge the gauge set hoses thoroughly with each new can attached. You will not get 12 oz. of refrigerant out of a can, more like 10-11 oz. I would consider removing refrigerant, vacuum for a few hours, let sit in the hot garage for a while, then vacuum again. Then refill with a target of 3 12 0z cans, with purge losses, probably about 30-32 oz. Your high side with very good air flow at 100 degrees should be around 325 psi, with low side pushing 50 psi.
You might find some other ideas, but this is where I'd start, since you did not mention purging. If you really did purge well, and you did evacuate well, then I'd make sure you have excellent air flow through the condenser and see if that won't bring down your high side pressure.
Good luck,
Dave

I did crack loose the charge hose at the manifold after connecting each can for about 2-3 seconds. Maybe that wasn't enough? Maybe some air gets in the manifold between the low side valve and the charge port and I should also crack the low side valve open a bit (to the charge port) to make sure there's no air there?

I also pulled a vacuum for about an hour then let it sit for another hour before charging. Maybe longer is needed?

I measured the weight of the can + can tap before and after charging. From the already opened can I got 8.5oz difference and from the full can 11.8oz. Not sure how much of that got purged out.

I do think airflow at idle is definitely inadequate. The mechanical fan (the clutch is engaging) doesn't pull enough air at idle speeds and the electric condenser fan's shrouding isn't good enough and doesn't seem close enough to the condenser to keep the bulk of the air going through the condenser. I think I have an electric pusher fan in the garage that I could try hooking up in place of the oe fan.

I guess if that doesn't work I should evacuate, vacuum for longer, and purge more when recharging?

Hi Matt,
It sounds like you're doing everything, maybe just not quite enough purging and vacuuming, possibly. Also if you are in a garage, with the front of the car facing a wall and the garage door open behind you, there may not be enough cool air for the fans to pull over the condenser. If you can get fresh (I know it's 100*) air movement and a fan, maybe you can get the pressure down. I have seen 50 psi pressure drops with proper auxiliary fan placement. It really sounds like you are 98% there, and just need small details to get good performance. That can be the difference in a/c work, in my experience, having made plenty of mistakes myself.
Good luck,
Dave

I had the car backed fully out into the driveway with the nose maybe a foot from the doorway. There wasn't really much breeze though, so I do suspect some heatsoak was going on. I had a box fan pointed at it for a while and it did seem to help, but it's kind of just tossing air at the cars fans rather than pushing it through anything.

I think I'll give that other fan a try tomorrow and see what that does before having to empty the system again and go from there.

Thanks for the input.

Were the cooling fans operating correctly? Hi-side pressure is directly associated with the condenser, looking it it's ability to actually condense the refrigerant and reduce its volume in the process. Pre-S4 cars are candidates for R134a conversion only iff ther fans are all (both...) operating perfectly. The mechanical fan senses the heat in the air behind the radiator, with the coupling stiffening up as temps get higher. The accessory fan in front of the radiator should come on as soon as the AC is engaged. If one of them isn't doing its full duty, the high-side pressure will go up as your gauges show. Your experiment with the box fan confirms the cause.

First steps:

Verify that all the vacuum actuators and flaps are working correctly. Based on your low-side pressure readings, you should be seeing about 35ºF evaporation temp in the evaporator if you are feeding liquid to the expansion valve.

Look at the sight glass to see if you are in fact getting mostly liquid to the expansion valve.

Fix the the fan clutch and/or the aux fan to get full air circulation across the condenser.

See if there are leaves and other crud between the radiator and condenser, blocking airflow.

Verify that your heater control valve is actually blocking coolant flow.

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Air in the system will raise pressures a max of about 1 bar (15 psi), and kill efficiencies enough that there's less heat movement available. Problems show up with higher low-side pressures rather than on the high side.

The S4+ cars have a compound pressure safety switch that opens on high high-side pressure, generally able but not guaranteed to avoid popping the blow-out plug in the drier. Good idea to fit one of those to your pre-S4 car. The port is protected by a Schraeder valve, one of the several you replaced as part of your reseal effort. (two main charge ports, the two charge ports on the compressor manifold, the safety switch port, and on S4+ cars there' one under the pressre transducer for the cooling fans). R12 schraeder valves will leak with PAG or polyolester oil.

The WSM lists oil charge amounts for the system, with breakout estimate numbers for each component. You should drain and flush the compressor with new oil a few times, then add the correct total amount for the compressor plus the at least the drier based on the list. If you didn't flush the plumbing, condenser and evaporator, you still need to add the right amount of oil for those as if they were empty, as the mineral oil that was in there is not miscible in the R134a and therefore won't get around to lubricate the compressor.

Before you reopen enverything and vacuum it again, run a garden hose of cold water over the condenser when the pressures are high. You’ll likely see the high side drop rapidly, and many times remain at or below a normal pressure. Then you can add refridgerant as required.

So previously I had been under the impression that the auxiliary electric fan was only to come on via the temperature switches on the condenser/drier, radiator, or intake manifold. Mine does come on, but only once the a/c has run for a bit and put some heat in the condenser.

As for the clutch fan, I haven't done any testing on it beyond seeing if it doesn't freewheel, it would only allow maybe 1/8th turn if pushed after shutting off the hot engine. It does sound like it's moving a lot of air when you move the throttle. I have an anemometer so I should be able to check it's performance before and after turning on the a/c to see if it's engaging more.

I didn't see a test procedure for the fan in the wsm (unless I just missed it). So far it's seemed to have no problem keeping the motor cool in the heat though.


I do still need to verify the flap positions and actuators. I do believe that the mixing flap is working properly as my vent temps have been exactly the same as the discharge side of the expansion valve (70 degrees). I also pulled the rubber boot off of the blower and got a temperature reading from the evaporator itself and also got 70 degrees. I couldn't reach the whole way to the far side by the expansion valve, but the parts I could touch seemed pretty consistent in temperature.


There are still bubbles. The system is not fully charged yet though due to the high side being so high.

I'm going to try switching out the auxilary fan today to see if that helps things out here.


The condenser itself seems pretty clean. I pressure washed it, picked out some rocks, and straightened some fins on it. The radiator is very clean and there doesn't appear to be anything between it and the condenser. The top of the condenser is sitting about 3/4" off of the radiator, so I tried adding some foam tape between it and the radiator to make sure the air is being pulled through it and not around it.


I replaced this part a couple months ago and it was working properly at that time. I will try and measure the temp of the hose before and after it again. I am getting the same vent temp as I measured on the evaporator itself though.

I did purge the lines, but I do wonder if some air trapped in the manifold set itself could have gotten in.

Where was the safety switch port? I don't recall seeing any schraeder valves other than the service ports on the lines and the ones on the compressor manifold. I definitely think getting one of these switches rigged up is a must given how high these pressures are getting.

I added 8oz of ester oil to the system. I think that's about right based on what I read. If not I can definitely add more. I didn't flush anything.


I did see the pressures drop when I put the box fan in front of the nose of the car. I also pointed a mister hose at the condenser at one point which did bring it down more. Take those away and it went back up.




I really do appreciate the input guys. I'll see what I can manage with the different fan today. I'll also see about checking the mixing flaps (I found pirtle's instructions for those).

My major concern here is that if I can't get it to the point where the car can keep the pressures down while sitting at idle, I don't think the r134 is going to be a very practical option with as hot as it is here. I'm hoping adding a high pressure shutoff switch could be a good safeguard at least, but hopefully not one that needs called up on often.

Far be it from me to disagree with Dr. Bob, but I'm pretty sure in stock form, the aux fan does NOT come on when AC is engaged. Coolant temp and AC freon temp, I think, are the two primary triggers. In fact, I've heard that a common modification is to wire it to come on with AC engagement.

For what it's worth, in South FL where the temps aren't quite as hot as you're seeing, but the humidity is often worse, my high side pressures were not nearly as high as you're seeing, and I'm getting very good performance (finally!) out of my AC, despite a less-than-perfect fan clutch (I have a new one I need to put on, just haven't got around to it). So I do think something is wrong with your system, and the suggestion of air in there is a good one IMO.

Edit: Forgot to add that I too am running R-134a. That itself is not your problem.

So my plan was to borrow the extra 16" fan I put between the condenser and radiator of my durango as an emergency backup fan to try on the 928. I forgot how much of a pain that was to get in there, so I think I'm going to just have to order another fan for that purpose.

I'm going to try getting the factory aux fan to seal more tightly against the condenser to see if it helps with airflow through it. I have a couple electric fans sitting in the shed that probably won't fit well, but I might be able to set in place just to see if the extra airflow helps. If that doesn't yield any results it sounds like I'll be evacuating and pulling a vacuum for longer then recharging and doing more purging in the process.

So it's not quite as hot yet today, 93 at the moment.

When I first started the car after sitting all night, the pressures were 20 low and 175 high.

As it sits and the engine warmed up it's sitting around 29 low and 300 or so high. It's currently creeping up from 300 towards 325 the longer it sits.

Spraying the condenser with the water hose does bring it down to where it was when first started. I'm currently getting a 55 degree temp drop across the condenser though (222 to 167) so I'm not sure that it not getting enough cooling really is the problem.


And again, just numbers for comparison's sake. At idle immediately after startup I was measuring 1.3m/s airspeed from the clutch fan with the anemometer held on the throttle bracket and about 1.2 sitting on the front of the condenser. After the engine was up to temp am measuring 1.2m/s on the throttle bracket again and only 0.8 on the face of the condenser (measured in the same spot both times and both times with the electric fan off).