Carpokes

Echoing Tom's concern, 60ºF vent temp tells me there's still air in the system. It doesn't take much air to trash system performance. Air takes up compressor volume/capacity. Most importantly it doesn't condense in the condenser or evaporate in the evaporator.

My casual method leaves the car on a vacuum pump for 24 hours, with a good electronic vacuum gauge that reads to tors level. The mechanical gauge on the charging manifold reads to yards when you really need to read thousandths. All the hoses are connected, so they get evacuated with the system.

Do everything you possibly can to avoid having even the tiniest amount of air in the system, and there's a good chance you'll end up with cold air from the evaporator at the vent.

----
I found a spiffy three-stage lab-quality vacuum pump at an industrial auction. The oil gets changed for every serious use, something that's critical to getting serious vacuum in the system. The seals at the hoses are changed regularly. Schraeder valves in the charge ports get renewed when needed. I have a new four-valve charging manifold that allows the two hoses to the car, a hose to the refrigerant tank, and a hose to the vacuum pump. No need to risk a hose full of air when swapping hoses if you don't need to swap hoses. Most common one- or two-stage pumps are sliding-vane pumps, which might satisfy the gauge on the manifold, but only at sea level and only with a really wide needle in the gauge. If your vacuum pump doesn't have oil in it, you might want to keep shopping if you want seriously cold air.

Under 20º F (with R-134a):

Measuring to the torr level -- why would I expect anything less. While you are off measuring in millimeters of mercury, my gauge set measures more in needle widths of mercury.

I've never had the nerve to leave the pump on over night, as it clangs and smokes when filled with the recommended level of oil. In fact, when it stops smoking, that usually means the oil is low.... But watching it hiss and spit like that doesn't instill enough confidence to sleep while it's running. Maybe that's just a Harbor Freight things?

I'm confused by your 20F vent temps, and suddenly feeling inadequate with my relatively balmy 38F temps. That's cold enough to cause actual frostbite. Shouldn't our previous a/c rabbit hole -- i.e., the freeze-over capillary tube switch -- prevent the temps from getting that low?

The freeze switch had failed (closed) at that point, and in the relatively dry L.A. basin there wasn't a lot of incentive to replace it. On longer trips, after maybe an hour or two, I'd have to remember to shut off the compressor for a bit to let it defrost. Ditto here in the high desert, but I don't need the AC almost all year as we did in SoCal. The cold vent temps are indeed enough to be uncomfortable after a while. But for a black car in sunny L.A., it did a pretty impressive job of chilling a heat-soaked cabin. Plus it silenced the naysayers who were sure that an R-134a 'converted' car just wasn't cold enough.

I have the new adjustable freeze switch installed in my car but not wired in; I don't think the contacts are rated for use directly in the clutch circuit. Original switch certainly wasn't. I think it needs a relay, similar to the way your 944 has the switch before the relay coil. I have penciled a solution but need an adjustable delay-on timer to make it work correctly. I could fit a normal relay and call it a day, but what's the fun in that?

Meanwhile, if your system is cycling off at 38 vent temp, it's working perfectly. It's quite likely that you are close to freezing at the evaporator coils, about as cold as you want to run it without risking ice-up. Read the low-side temp on the pressure gauge to see the internal evap temp, assuming there are no 'non-condensables' like air in the system. A small mass of air gets pretty 'large' between the evaporator and the compressor inlet, and steals an amazing amount of system capacity. Fools the expansion valve too.

My pump has a muffler/silencer/droplet separator on the exhaust. Plus it has a 'ballast' valve that solves a lot of the issues you state by bypassing stages once most of the air is removed. Are you using real vacuum-pump oil in yours? Just curious.

dr bob wrote: Tue Sep 24, 2024 10:09 pm The freeze switch had failed (closed) at that point, and in the relatively dry L.A. basin there wasn't a lot of incentive to replace it. On longer trips, after maybe an hour or two, I'd have to remember to shut off the compressor for a bit to let it defrost. Ditto here in the high desert, but I don't need the AC almost all year as we did in SoCal. The cold vent temps are indeed enough to be uncomfortable after a while. But for a black car in sunny L.A., it did a pretty impressive job of chilling a heat-soaked cabin. Plus it silenced the naysayers who were sure that an R-134a 'converted' car just wasn't cold enough.

I have the new adjustable freeze switch installed in my car but not wired in; I don't think the contacts are rated for use directly in the clutch circuit. Original switch certainly wasn't. I think it needs a relay, similar to the way your 944 has the switch before the relay coil. I have penciled a solution but need an adjustable delay-on timer to make it work correctly. I could fit a normal relay and call it a day, but what's the fun in that?

Meanwhile, if your system is cycling off at 38 vent temp, it's working perfectly. It's quite likely that you are close to freezing at the evaporator coils, about as cold as you want to run it without risking ice-up. Read the low-side temp on the pressure gauge to see the internal evap temp, assuming there are no 'non-condensables' like air in the system. A small mass of air gets pretty 'large' between the evaporator and the compressor inlet, and steals an amazing amount of system capacity. Fools the expansion valve too.

My pump has a muffler/silencer/droplet separator on the exhaust. Plus it has a 'ballast' valve that solves a lot of the issues you state by bypassing stages once most of the air is removed. Are you using real vacuum-pump oil in yours? Just curious.

Ah, that makes me feel better about my temps, thanks. I had assumed my compressor was cycling off around 38 because of the freeze-over switch, so when I saw you got to 20F, I started second guessing myself.

As for the vacuum pump oil, my HF/Pittsburgh vacuum pump came with an unmarked bottle of oil that I've been using until it runs out. I suspect only the inner circle at HF know if its 'real' vacuum pump oil or just a jar of Wesson's Pure Vegetable Oil. Consistent with its recent up-market offerings, I see that HF now sells a pump under its "Icon" label that purports to pull down to 18.75 microns, which is fairly hard to believe, but no doubt an improvement over my little clatter pump. Guess I'd need to find a micron gauge to show off my readings though.

https://www.harborfreight.com/deep-rota ... 58141.html

On a semi-related relay tangent, I'm working on a 450SL a/c project too. I can't find a parallel flow condenser for that car, and the vent temps are only ok on r134a. The car has an electric condenser fan, but by design it only comes on when the high side pressure exceeds a certain value, which is fairly rare. When the fan comes on, however, it makes a noticeable difference in vent temps. My plan is to bypass the high pressure switch and instead power the fan using a relay driven by either the a/c button on the dash or the wire powering the compressor itself.

We are straying far from the 944 condenser leak discussion. For readers, it's Ok to skip to the next posts and threads.

Meanwhile...
Tom, you could do worse than configuring a fan relay triggered by the compressor circuit. But... There are different pressure switches in the market that might solve your issue without having to make obvious wiring changes.

More: The high-side pressure reflects the temperatures of refrigerant after the condenser, pretty much the same thing the switch is detecting for the fan-run decision. The idea is that it only runs the fans for added condenser cooling when the load indicates that ambient is high enough to need it. With a warmer cabin, the whole system pressure would go up and trigger the fan operation, offering the additional cooling when it's actually needed, leave it off if it's not.

I don't know what fans are original in your SL project car. My 928 S4 came with Bosch fan motors that are common to a lot of other period European cars from Audi to Vauxhall. Mine were getting a little rumbly after a few decades of SoCal driving with the AC running most of the year. Some cleaning, new brushes and critically some new bearings did a pretty impressive job with the noises as well as the performance. I shopped for some high-precision skateboard wheel bearings to replace the lowest-cost-available bearings the motor manufacturer had originally chosen for the duty. The 'fix' involved a little metal reformation to get the motors open, carefully drilling out a dozen spot-welds (and replacing them with rivets on assembly), and soldering in the new brushes. I wanted to keep the original look-and-feel so stayed with the original motors in the refurb effort.

Meanwhile, cooling fan design and materials have changed just ever so slightly in the months since our cars were originally produced. A popular fan retrofit for the 928 is a 1990's Ford assembly that offers a much better and shallower shroud, with more efficient (and durable) fans and motors in an all-but-drop-in package. Suggest that you look hard at similar options for your SL. Ford seems to have focused (sorry...) more effort on the fans and flow than other US manufacturers like GM, so my research would probably start there were I looking for a serious performance improvement in the SL. I know there are some high-spec aftermarket fan units floating around, but none seem to do as well in actual service as the fully engineered units with baffles and shrouds, and 100k+ expected life. I'd probably buy a used factory set from a boneyard before buying an aftermarket crash-repair set new, to get that original durability. Even if I needed to rebuild the fan motors.

Especially with R134a retrofits, managing high-side pressures on hot days is absolutely critical. There's a very noticeable bump in the pressure vs. temperature profile vs. original R12, and that happens at around 90ºF ambient and up. Folks who charge to the sight glass often find that spike and damage compressors with the extra compressor head. System performance drops off along the way as the compressor struggles to get enough pressure to actually condense the refrigerant. I wouldn't be at all afraid of shopping a modern higher-efficiency condenser with about the same height/width, then adapting the plumbing to make it work in your SL.

When I did the R134a conversion in my 928 very shortly after the car came to me, I needed to replace the hoses and all the o-rings to tolerate the different oil, and it would not have been too difficult to use different ends had I needed to change the condenser. I did a 928 AC tech session at [the late] Devek shop sort of near you in IIRC Redwood City back in the day, and co-presented with their local AC experts at San Carlos Radiator. I'd have to scrounge presentation notes (from 30 years ago...) to find the guy's name, but he seemed pretty much on top of the tech side as well as some of the fab needed to make things work. His part of the tech session was pretty generic, following the 'just bring it to us and we'll take care of it' messaging, rather than my 'here's how it works and why it works, and what you can do to your own car to do the conversion successfully'. The Big Deal at the time was the 'new' recovery machines, avoiding just venting systems, and the licensing requirements in Cali.

Back to the eBay parallel flow a/c condenser.

It cooled down to an even 100F today here at CP headquarters, after a high of 107F earlier in the week. Pretty crazy, but I took the opportunity to check the 944 a/c and was pleased to see it still brings the vent temps down to about 38F, then starts cycling the compressor to keep in that 38-40F range. Not sure if that's the high pressure switch or the freeze-over switch, but I figure if it gets the car uncomfortably cold on a 100 degree day, I'll call it a success and move on. The eBay condenser works!




Here's a shot from my truck on Tuesday or Wednesday this week. This was out driving around, not heat soaked in a parking lot. 107F in October.

If you are getting that 37º vent temp at your typical driving conditions and 100º ambient, you have done a very fine job.

For those playing along at home, California is subject to fall weather phenomena where a high-pressure zone parks over the southwest states east, and the winds blow hot and dry off the deserts. The 'dry' (low relative humidity) ambient condition seriously reduces the load on the AC system when the dew point is below the mid-30's evaporator surface temps. A pretty massive amount of heat is needed to transit water from liquid to vapor state (see: latent heat of evaporation in your high school science books), and a similar amount of heat needs to be transferred out to cause the change back to liquid on the evaporator. For folks in humid places with heat along for the ride, there's a comfort factor making the cabin air feel cooler when the water is condensed out, somewhat offsetting the loss of that capacity to the state conversion part. But with all that capacity spent on condensing the water vapor, there's less to actually reduce the sensible heat in the air flowing through.

Condensed if you will, don't expect to get the same vent temps as Tom is enjoying if you have humidity to contend with, even if your system is performing at design capacity.

Tom -- I identified the time-delay piece of the relay setup for bypassing the freeze switch for a while on system start-up. Parts and pieces are ordered for a couple demo kits. Film at 11...

dr bob wrote: Mon Oct 07, 2024 9:57 am If you are getting that 37º vent temp at your typical driving conditions and 100º ambient, you have done a very fine job.

For those playing along at home, California is subject to fall weather phenomena where a high-pressure zone parks over the southwest states east, and the winds blow hot and dry off the deserts. The 'dry' (low relative humidity) ambient condition seriously reduces the load on the AC system when the dew point is below the mid-30's evaporator surface temps. A pretty massive amount of heat is needed to transit water from liquid to vapor state (see: latent heat of evaporation in your high school science books), and a similar amount of heat needs to be transferred out to cause the change back to liquid on the evaporator. For folks in humid places with heat along for the ride, there's a comfort factor making the cabin air feel cooler when the water is condensed out, somewhat offsetting the loss of that capacity to the state conversion part. But with all that capacity spent on condensing the water vapor, there's less to actually reduce the sensible heat in the air flowing through.

Condensed if you will, don't expect to get the same vent temps as Tom is enjoying if you have humidity to contend with, even if your system is performing at design capacity.

Tom -- I identified the time-delay piece of the relay setup for bypassing the freeze switch for a while on system start-up. Parts and pieces are ordered for a couple demo kits. Film at 11...

According to the interweb, it was 102.2F ambient with humidity of 22.11% and a dew point of 53.6F yesterday afternoon around the time I checked the a/c. To your point, I think that qualifies as 'dry' weather. I do remember my days in South Florida where even a glass of tap water would sweat. No idea what the vent temps were, but turning on the a/c in a car was like turning on a faucet at the condensation drain. Even if the vent temps were 20 degrees higher, the dehumidifying effect of the a/c made all the difference in the world. Humid heat is tough on a California guy.

Looking forward to the 11 o'clock film....

Amen on the humidity. I found out young that I was allergic to heat and humidity. Dashed from the east coast to Cali at 16, in time to qualify for the 'free' UC education benefits, and that's when I figured out that the humidity and sweating was a root cause of my chronic summer rashes and breathing issues. When those went away. It all kind of slipped my mind until 30+ years later when I took on a spring project at the NC/VA border, where grass, humidity and ragweed pollen conspired to make life miserable again. Emergency room there, again here when I got back. ER doc gave me oxy to reduce the swelling so I could breathe. Who knew there were such creative ways to use it? Epi pen did nothing.

Now, back to our regularly scheduled 'cool stuff'!

dr bob wrote: Mon Oct 07, 2024 12:03 pm Amen on the humidity. I found out young that I was allergic to heat and humidity. Dashed from the east coast to Cali at 16, in time to qualify for the 'free' UC education benefits, and that's when I figured out that the humidity and sweating was a root cause of my chronic summer rashes and breathing issues. When those went away. It all kind of slipped my mind until 30+ years later when I took on a spring project at the NC/VA border, where grass, humidity and ragweed pollen conspired to make life miserable again. Emergency room there, again here when I got back. ER doc gave me oxy to reduce the swelling so I could breathe. Who knew there were such creative ways to use it? Epi pen did nothing.

Now, back to our regularly scheduled 'cool stuff'!

Go Bears! Go Aggies!