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Hot Rod Air Conditioning recharge pressures... need expected values

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  • Hot Rod Air Conditioning recharge pressures... need expected values

    Greetings all,
    About 12 years ago I installed Hot Rod Air Conditioning in my car. Well, it appears to need a recharge because it's only cooling to about 45°F on and 80°F day. I have the old installation documentation and it says the system holds 1.8 lbs (~28.8 oz of R-134a). The system has worked well and it comes with a Sanden compressor. However, since the company is out of business I cannot get the appropriate and/or expected pressures (Low and High side) to appropriately recharge this.
    Does someone here have an idea and/or expectation of what would be a good value to shoot for? I can always evacuate and start over, but it still has pressure within.
    Many thanks.

  • #2
    Welcome. First off need to know just what this thing is now and type of A/C would help having an expansion valve set up or accumulator.

    Pressures can tell you that you are within the limits for the temps you are dealing with at what RPM also mind you. There are not exact pressures that will tell you how much is in any system just that it's essentially OK where it is.

    So: Your old info should still be good and having leaked some over 12 years but still blowing 45F at vents could be normal but suggests you were used to lower outputs for a similar ambient temp.

    The pro approach IMO involved is to see where it's leaking and how fast. All connections, compressor's shaft seal and even service ports but anywhere is no good. I like to use sniffers but use all detection methods. Oil on things where it doesn't belong run finger under suspect spots. Evaporator drain however set up should be oil free nice if you can rule that one out.

    Soapy water along hoses and lines like checking for a leaking tire can work.

    Dye can be added if not in it now so you can see with UV light spots that shouldn't be there.

    Once you are convince nothing is overly fast the best way is remove all refrigerant and see how close it was to the known charge last time known seems you do. Note that and charge to the known weight and see if performance is back as you expect so that's ruled out.

    Now for pressures assorted factors will matter. Low if reflecting evap pressure not too low or stay too low and high about 2.5 x the air temp coming in thru grille (F. in front at a raised RPM about right.

    #1 should have said earlier is fan blowing as it should if using a fan clutch feeling warm air blowing towards engine well? Or whatever set up working properly. Condenser must be clean and clean in between it and radiator as well.

    Give us some more details and pressure at what temp when checked now helps not a conclusion but what is off is so,

    MetroWest, Boston


    • #3
      This a/c system is on a 1968 corvette. I doubt that I would be able to locate and identify a leak if it took 12 years for performance to decay from 38°F to 45°F at the discharge vent. It was charged with "dyed" R-134a and I have NOT seen any stains. (I'm sure it leaked out but it was probably extremely slowly over a dozen years). I suppose I could "snoop" it some, but I have my doubts about locating such a slow decay.
      The ambient temperature was approximately 80°F (official weather wise)-- and approximately 90°F at the air inlet to the a/c condenser. This temperature was the same both at the initial conditions(12 years ago), and at the conditions which were read about 2 days ago.
      The engine cooling fan is a Lincoln Mark VIII and it runs continuously when the A/C is energized. The engine cooling fan is working quite well. Readily obvious since when I run the A/C around town (official ambient temp ~80°F) the engine stays at 170-175 degrees.
      If the only solution is to empty the system out and recharge, then I would consider installing and even larger condenser-- since I have some room for growth.
      I guess I'll use the generic press vs temp charts for R-134a (you know, the 2.5X air temp equivalent) and see how it reads. I found a set of readings for another system that holds ~1.8 lbm of R-134a.
      Thanks for the pointers.
      Last edited by silvernblack; 07-10-2017, 11:35 PM. Reason: correct punctuation


      • #4
        OK - 68 Vette. 1st year for a Mako (sp?) body with the highly tilted radiator and however your condenser fits in?

        Here's why you charge to known amount and also a leak can happen like dropping a light bulb not for just time. It worked (still don't know what set up you have unless you say) better than any Vette I know of always ran hot, overheated on hot weather if not A/C at all just not adequate air could get thru by OE design of those.

        Don't look for pressures to tell you much that's very old hat. You don't have a dipstick to know amount of charge is correct without starting from empty - a full well held vacuum for the silly less than 2lbs would just do that and use new product not recycled or recovered just IMO take contamination out of the picture.

        This one body I could never make very cold even OE set ups with R-12 the lines ran right next to red hot exhaust parts just interfered with it.

        Reapeat. Just evac and charge to known weight and forget it. It's already blowing colder if accurate than OE would have so if your set up can do better if spot on and want it to go for it.

        At this point this is a "toy" car to me not a daily driver or commuter nor the family grocery getter so just take care of it and always know it's full - the only way is starting from empty if you have to do that each year if you store this for a time period that alone can cause leaks you'll never find,
        MetroWest, Boston


        • #5
          Thanks. Makes sense. From your response I guess i should be happy to be able to attain 38°F at the center outlet to begin with! Many thanks.


          • #6
            Just FYI - I doubt any OE set up is meant to really see that at a vent temp as it's just pushing it for freezing up around there. Small cabin area recirculating you could or tweaking out a system that has adjustments.

            Can't say diddle about your set up now aftermarket no problem with me as OE stunk as said only car I'd fight for the 40s the other was the air cooled VW bus or Vanagon - hopeless - compressor out back and the condenser squished up front with hoses and fittings underside the VW was a treat.

            May be considered old news and haven't seen it recently but vehicles used to say expect to lose ''gas'' over "X" number of years and accommodated that with super high capacity systems didn't care up or down a pound!

            If you lack equipment to count down ounces out and back in is handy info on loss rate. IMO any can catch a rock in a condenser on your way home new the rest may or may not have problems.

            My own vehicles are bordering in ancient now neither like being outdoors and took me a while to figure out that shaft seals of compressors and the seal do NOT expand and contract exactly with temps where I am can go last Winter a windy -14.5F doesn't harm a thing (collectible vehicles - some) except flipping compressor's shaft as said. Had to be as I couldn't find a leak rate of a gram per year when warm drove me nuts for a while to figure out why.

            So in short you have a small capacity system so tweaking it from where you are now would take a LOT of time at the same temperatures bumping up and down an ounce, watching pressures and temps everywhere all at once it can be done I do it. Not suggested that way for anyone really too easy to mess up and ruin systems.

            Yes - they did list out pressure charts for assorted makes and models of vehicles as a way to charge a system! No longer as you need all conditions you are in known all at once which is just impractical up to you don't want to own that much stuff and don't blame you!

            Good luck with what you choose to do the suggestion was put it back to known weight first upon thinking more on this it can't be leaking that much unless it just happened,
            MetroWest, Boston


            • #7
              Sorry to bug you, just some trivia on Model Year 1968. A couple laws went into effect that model year. Powered side marker lighting not just reflectors. Near sure also all passenger vehicles (unsure on trucks) had to exchange interior air at a known rate that might be every 40 seconds or so! GMs in particular the blower off still stayed on if OE annoyed people a lot as you could hear it with everything OFF.

              Just know that so "recirculated" air is just better to re-cool it not constantly getting colder and colder like a refrigerator for return air.

              Don't defeat that over this. You'll see vents in door jams that are letting air out and that's what those are for. If plugged off or missing it may have been altered?

              People did suffocate in some vehicles, multiple passengers really could use up all the oxygen so on came the law. 99% of vehicles were never that air tight so wasn't a problem for all the years before in a big way.

              Gettin' old friend, I remember all these cars and things when new!
              Last edited by Tom Greenleaf; 07-11-2017, 02:14 PM.
              MetroWest, Boston


              • #8
                Haha.. thanks, but I'd have a hard time suffocating in that thing. It's a convertible and I hardly ever have the top up....and when I do have the top up - it's not like it seals that well. It came with something called "astro ventilation" which means it let hot air in all the time. When I installed the aftermarket Hot Rod A/C I sealed all the vents and inleakage paths at the front. They really were letting in hot engine air, one path (2 ducts) was via the wiper tray area, and that let that let outside air in all the time(could not close off nor isolate). It was horrible. The 2 doors at floor level I just RTVd shut. Even when closed they would let hot air in. The rear "astro ventilation" was just some vents to let air out in the back. They're still there.

                I think I'm going to have to evacuate and recharge to get back to ~38°F outlet temp.

                To answer your earlier question, Yes, the highly tilted radiator and Yes, the aftermarket condenser fit just fine. I'm pondering going to the boneyard and finding a slightly larger condenser to squish in there..... lots of modernish cars in boneyards with good condensers and before I evacuate and recharge it may be worth the effort to put in a slightly larger condenser. That would give me a slightly larger capacity for R-134a. Pondering. I don't mind fabricating. Now wondering if a/c fittings are standardized for a/c hoses? hmmm.....

                here are some pictures from when I installed it. I have liked it quite well for the dozen years I've had it. I have about 3-4 inches available on the driver's side if I can find a condenser in about the same thickness.
                Attached Files
                Last edited by silvernblack; 07-11-2017, 08:08 PM.


                • #9
                  Suggest AGAIST changing condenser making this unknown for capacity plus if you can get cool enough air with what you have what is there to gain? More volume of cool air if needed but isn't improving anything rather the opposite.

                  Reason you see 38F is it was adjusted to that or too much air "residence" time at evaporator like you get with low fan speeds or flow plus the shorter path to vents. Check thermos as much as you can also it can easily be off even 5F - just got a couple more verifying them now against others already known.

                  Funny with convertible and enough air thing. Side topic but wasn't a hard top available for this or soon around the body change of '68? IDK - I think "T" tops soon replaced or took over market for convertible for a while.

                  Of interest did a LOT of work on a '68 no A/C new and a '78 just for A/C later as I was the only one who'd look at it still R-12 remains the only R-12 car I couldn't tweak to make blow vent air below best of maybe 48F @ ~95F ambient. Low side hose blamed on that one so close to red hot exhaust almost dangerous - owner didn't care at all, it worked well enough with volumes of that temp air plenty. Will never know if I could change that owner was waiting and taking it home no matter what that day.

                  Both by chance were "salvage" cars only mattered for the '78 no listed capacity for it under hood to go by remains unknown which model year exactly it was made out of most could see the cut rear and front were different cars show quality body repair and paint.

                  Do keep in mind in the North-East BosWash Corridor = Boston thru Washington, D.C. area) more than elsewhere most folks would opt out of A/C at all up to taking it out for these cars it's just in the way,

                  MetroWest, Boston


                  • #10
                    Ok thanks.... no condenser change, I don't need extra fabrication exercises anyway. I do have a hardtop, but I haven't used it in probably over 10 years. And, yes, that does seal a bit better. Not that I think I'd suffocate.

                    That 38°F was pretty consistent over about 10 years (that is the cold air blowing with the top down too); it's just the last 2 (or so) that it slowly got worse. Thermometer is accurate, I've used it in my other cars and when checking friends cars -- and actually it's been 2 different thermometers.
                    I was reviewing the hose routing after your posts (in preparation for my previously planned condenser change) -- and since I have a better understanding, than when I installed the system, I can see that one a/c hose may be able to routed further away from the exhaust system. I will check that when I evacuate it--- see if I can move it away a bit; if not it should be ok because it's not probably as bad as the stock systems you mention.

                    I did take some pressure readings yesterday and here are the readings:
                    29# Low, 160#High. ~90°F ambient, ~50°F at center outlet vent.
                    You'd think I could just add some, but a few days ago when I did the temps actually got a little worse. I think you're right that I should evacuate and put in a known quantity.

                    Since I won't be changing condenser at this time I guess I can evacuate and do a clean refill. I really thought that would be good idea to change the condenser just for surface area increase and it seems you express disdain for small capacity systems. I took some measurements yesterday to see what the absolute max condenser I could squish in.
                    My installed condenser is 21"W x 13.5" H --- the absolute maximum I could squish in there would be 25"W x 17"H (sounds like custom size) and that would be to avoid too much fabrication, but 24"W x 16"H are readily available and it seems I could install that with minor effort.

                    Are generic condenser fittings a standard size? I'm curious (for future plans --- I have a larger fitting and a smaller fitting. I bought system about 2005 (if that matters for standardization).
                    Last edited by silvernblack; 07-12-2017, 11:58 AM.


                    • #11
                      Again - I just have this gut feeling it's low now shows that by pressures and will be just ducky when right charge is in it for how long isn't known if something isn't obvious.

                      The only possible benefit of a more effective condenser would be faster to get cool air which you already can get so what's the point?

                      Don't forget if you purposely adjusted or overcharged a system liquid would end up returning back to compressor and lock it up if lucky or ruin it! There are already systems in it to prevent that as it doesn't go lower than 38F you say.

                      Example - you take it out when below 32F outside and turn it on or left it on - no problems it will not (if right) allow more as it's set.

                      Seriously - this is doing very well for a non GM set up IMO. However have noticed with Corvettes especially A/C at all was after they made the car what buyers wanted it for, power, speeds, handling and you should have 4 disc brakes with 4 pistons each which were a PITA back when as you didn't just swap for one already done up - laugh - made you work to redo those,
                      MetroWest, Boston


                      • #12
                        Just an update from my experiments:
                        Transferred some R-134a to 2 of my other vehicles. They seem a bit cooler and here are the final results for the Vette. It also got a “little” better (?).

                        Best Temperature at Center Duct Outlet is 39°-40°F; then cycles warm to about 46-48°F.

                        Ambient Temp at nose end: 82-85°F
                        RPM: 850-900
                        Steady State Readings Low Side: 16-17.5 psig
                        Steady State Reading High Side: 150-165
                        Secured and equalized pressure is ~40 psig
                        (this seems a bit low to me….read using low range scale for accuracy)

                        Proposed Future work plan -- (just for tests, before I decide to evacuate and do a refill):
                        1. Mark VIII Fan cycles with compressor. I think R-134a is low because it is cycling a bit too often for my tastes.
                        2. Used Manual Override Switch and actuated fan so it ran continuously. Seems to work a little cool better.
                        3. Need to transfer some R-134a back to Vette and check performance.
                        4. Suspect that R-134a in the Vette is slightly low.
                        5. The transfer of R-134a back to Vette must be done slowly while monitoring temp with Mark VIII fan running continuously.
                        6. Investigating making a shield between headers and a/c compressor cooling line outlet to Evaporator. Really hot in that area. Either that or move the hose and see if I can get around the pressurized tank.(Doesn’t look like hose is long enough). Starboard side Spark Plug leads rearranged in case I do make a shield--- looks better too.
                        I'm open to ideas and criticism so feel free to comment on my plan. I'm thinking I was slightly overcharged (I've been researching some of these little R-134a systems) and that's why it cooled a little better..... either that, or I'm hoping


                        • #13
                          One of us is misunderstanding something as your pressures and think you meant "static" pressure just isn't possible.

                          Hope chart shows also elsewhere at this site with "Charging Procedures" as a topic.

                          Temp/Pressure relationship chart.........

                          That chart is spot on so just the static pressure suggests system is exactly 45F when checked? That's the temp of the lines and gas just setting there but if the ambient temp is in the 80s that's not possible with a listed refrigerant anyway. In fact when you find just a smidge lower pressure that's correct than chart shows it suggest system is just about empty!

                          Maybe you meant you had both knobs open on the most common type gauges which is just mixing the two together which you should NOT do.

                          What are you using for gauges or what for pressures and why type(s) of thermometers?

                          Vent temp readings what fan speed and is this closed up (the top and windows) or how checking at what RPM?

                          The delayed come back for cycling IDK for sure. IMO things a just tight in a Vette so if blowing cold and it seems to always wonder what part somewhere gets invasive heat or if this "recirculates" air sensing it's too cold to stay engaged somehow unknown for now.

                          Do check your equipment and thermos used.

                          Just a few ways come to mind unless compared to multiple ones with same readings. Your body temp should be 98.6F so can just get close holding a probe under your arm or just cover a sensor with clean plastic wrap in your mouth - same as you would medically and not to be funny not your other end!

                          Lower temps? Physics 101 - water in a glass or shallow pond (this is true) will not freeze at the top until all water is the same temp top to bottom! No joke, if water was warmer it would keep rising to the top and cooler at the top drops to the bottom mixing itself all at 31-32F of course is either liquid or solid, pure distilled water tested at about sea level.

                          I can't think of any practical other ways to test your thermos with checking with others already known.

                          Warm where I am today (92F in shade outside) and in a kitchen. Infrared, touchless thermos just read home A/C vent metal @ 50F and the ceiling of the room was just 77F just what is expected for me.

                          Sorry for the novel but let's both understand and know what's accurate - it isn't making sense but working?
                          Last edited by Tom Greenleaf; 07-16-2017, 04:40 PM.
                          MetroWest, Boston


                          • #14
                            Charged my Hot Rod A/C system after having replaced the condenser with the largest one I could squish in the stock location without too much modification. I was able to install a 16 x 24" condenser. Fits like a dream. When I first installed Hot Rod A/C in my 68 it cooled to 37°F and I was quite the happy camper. After about 12 years it would cool down to 40°F, but then not cool, and then after a while cool again. I figured I was going to have to evacuate and recharge since by now I didn't know the status of the how much R-134a I had charged and/or lost and/or recharged.......

                            Ambient temperature today: 70°F in the shade. Vehicle was in the shade during charging. Ambient temperature did not deviate more that 1°F during the charging and monitoring evolution.

                            The Hot Rod A/C system originally took 1.8 lbm (28.8 oz) of R-134a, which is approximately 2.4-12oz cans (round that off to 2 1/2 cans). My new condenser is significantly larger than the condenser that came with the system --- so I was guessing that I was going to have to add a little more than 2 1/2 12-ounce cans of R-134a.

                            1. After charging the first can the temperature at the outlet of the center duct was already 32°F...hmmm.... well, I know I have to add more R-134a.
                            2. After charging the second can the temperature is still at 32°F at the center duct outlet.....now I'm getting concerned that I may do something silly and make it worse.... but I waited a while at this stage and the system seemed to be cycling excessively so I figured I was going to have to add that last 1/2+ can.
                            3. Started charging the 3rd can ..... and dang if it didn't suck it right in! Oh well....don't have to worry about guessing what "1/2 can" feels like.

                            Anyway... about 32 degrees. Never quit cooling. I ran it for like 30 minutes. I'll test more later and see how it does in the hot sun.

                            32°F at center outlet consistently.
                            Low pressure side ~19-20 psig
                            High pressure side ~120-125 psig.
                            Charged with 36 oz of R-134a (note: larger condenser than original Hot Rod A/C system. Originally only took 28.8 oz).
                            Last edited by silvernblack; 10-15-2018, 10:24 AM.


                            • #15
                              OK - Pressure and temps aren't matching up quite right or just being unusual? Most certainly a larger capacity condenser will take more refrigerant unless (doubt this) was stated how much from a known is guessing. Small cans and like them many times can be great and work fine do need exact tare weights taken how you are using them just FYI I can run those thru real gauges known accurate.

                              One problem right away is done at 70F is a rough guess of air temp the system knows only what temp air is flowing thru grilled. Since it was blowing cold already with 1 12oz can that can just be explained as the can alone newly shot in was reading that not stable yet but still performance just can't be 32F at a vent inside or for long or you'll see the frost and ice on low side line under the hood which is icing up evap - not a guess it is. Notice it wasn't dripping water called "condensate" if your therno was correct seriously doubt it now can get those in digital for less than $10 bucks have more than one I don't trust that unless you did see frost or frozen lines.

                              High side is not 2.5 could be due to better performance or the condition at the time are on the low side if you totally buy into the 2.5X ambient temp at a raised idle like 1,500 or the performance and pressures are not adding up to me is all so far. That's too cold for vehicle A/C to ever stay at never aim for that if you could fudge it or force one to,
                              MetroWest, Boston


                              • #16
                                Ran it in the middle of the day. About 80 in the shade and about 90 out in the sun. Ran 36°F to 44°F and it was toasty in the sun. Note that this is a convertible with the top down and a/c blowing out into the open. Temp taken at the center outlet.
                                Another thing I did while system was empty was to relocate the a/c hose away from the exhaust manifold....so it's no longer within a few inches of it...now about a foot away.
                                And, YES, it does drip condensate as expected.
                                I will try a different temperature gauge, but I suspect that's fairly accurate from using on my other vehicles.
                                I added a picture of the relative condenser sizes just for info.
                                Last edited by silvernblack; 10-15-2018, 11:35 AM.


                                • #17
                                  Dislike page 2 or more but here goes. It's working well, better than any Vette with that basic body I could ever make just poor air flow, angled radiator and condenser on if not the same an OE one had the low side metal line so close to glowing hot exhaust figured best that was going to happen.

                                  Anyhow all my checks - so far mind you is air temps from real evap temps gain at least 4-5* F between it and the center vent even so you are really close but seeing it drip can work then freeze some back and forth right there.

                                  Which system is this now CCOT or TXV set up? Matters as either can have temp or pressure slow downs or cycling set to prevent it from getting too cold just for this reason and some will do that on purpose if no other side effects let it be but it's NOT intended to really get that low and stay there.

                                  The same therno is sold and works from Harbor Freight or other places is digital read out and fast enough. Faster I a wired thermos like you might use at home toss the wire out a window and read both in and out temps can be used to hang sensor for "out" in front of grille and other end on dash you can see both and what changes at what RPM and even test it with water over evap (beats any air for cooling evaps) see the maxes out efficiency and what is quits at and must quit.

                                  If you can't make this quit you risk choking the compressor when cooler if it's all hooked up right just defog or defrost window when way too cold it could compress but shut down flow or disengage clutch just so you didn't hydro-lock up a compressor as a default or can wouldn't work and you could be walking over it never mind A/C at all. That's the point to prevent that.

                                  Enough. I'm near sure your thermos isn't on the mark and not just me but check them with each other have dozens of assorted ones,

                                  Last note on why A/C can come on off season is the mostly lie that it dries the air so when heated it works much faster is foolishness IMO as air can't be wet if super cold out. Other is to let some oil flow thru off season is important. I can hear either of my own if just parked with a warm engine (both V8s) the heat of it cooling down heats refrigerant so it's static pressure says it can operate just for a few seconds sometimes even when down in the teens.

                                  Yes, you didn't ask but the two vehicles I use I have thermos in front for incoming air and down a center vent all year long so I can see just what it's up to anytime under what conditions driving along both now older vehicles same game though but watching so far I've avoided any repairs to my own if not where expected quick disable compressors so they don't run when low (would be why) and burn up for lack of lube. So far so good for decades of doing that on my own keepers of way too many vehicles (classics to antique ones) over the years,

                                  MetroWest, Boston


                                  • #18
                                    My experience in going with a bigger condenser is it doesn't change the charge amount hardly if at all. Many time the tubes are smaller on the newer condensers so the surface area is greater but the volume doesn't change much. I'd go back with the spec and try it.


                                    • #19
                                      If you went larger for a real '68 Vette condenser the new one will be for 134a and may not use as much as the smaller one. Tough to know specs now if not plain said so I've done some OE R-12 swaps stayed R-12 said reduce the known charge by an exact known amount with the new part no other alterations best guess is you are just taking stabs at how much. If a Vetter already said I'm near sure that is at a high angle not just up and down and channeled thru as best as could be done for air brand new these cars overheated on hot days never mind if A/C or not was the first year of that body.

                                      Still I can't see how charge amount is producing the low temps seen. You could verify my thoughts on that by just running vent only, no A/C and watch air temp coming in thru now cowl up by wipers see it get warmer than that at the vents there plain is engine heat involved at least some gets thru firewall to dash,
                                      MetroWest, Boston


                                      • #20
                                        I went and got another 2 thermometers per suggestions since the temps I was reporting did not make sense. Does 40° to 44°F sound more reasonable and in-line? Ambient temperature again was about 80 in the shade and about 90 in the sun. Convertible top down and running around.
                                        I will get a digital gauge one of these days just for funsies. I'm just glad it's working quite well. I'll keep an eye one it. Thanks all for all the pointers.
                                        Last edited by silvernblack; 10-15-2018, 11:35 PM.