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Any 2002 Jeep 4.7L HO AC experts out there? Advice needed....

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    Any 2002 Jeep 4.7L HO AC experts out there? Advice needed....

    Hello everyone. I'm a well seasoned commercial refrig man and I have done my share of auto ACs for friends but I got the crap end of the stick with my owm AC in this dang 2002 Grand Cherokee Ltd! This is the one with the digital AZC and has a hyrdraulic PWM radiator/condenser fan. At test of 100% output at idle it is LOUD and moving lots of air!

    When I got it 2 years ago it had already had the compressor, receiver drier, and TXV changed. It worked but this thing is Black, has a sunroof, and is in South MS where it can be 95F with 85%RH or hotter. Well the first thing that happened was the Recirc door and then the blend doors started goofing up. Then the evap developed a leak that summer. So I had the 'pleasure' of pulling the dash and HVAC box out. I put the Heater Treater kits in and while I had everything apart I cleaned and resealed everything better than the factory and put a new heater core, drier, and TXV in there while I was at it and put it all together in a weekend by myself. Yay me!!
    Worked good but vent discharge air temps were just not cutting it due to the program cycling the AC clutch off too early it seemed even though it was within factory service manual specs. I added a 1K trimmer resistor across the evap temp sensor (there is NO low pressure switch, only this sensor) and after that I dialed it in as cold as possible without freezing the evap on low fan and low ambient conditions and it has been great!

    BUT, there can be better. That hydrofan is barely running even in traffic idle conditions. It does increase and if you rev the engine to 1500 you can see the high side pressure modulate as pretty as the factory specs say. The problem is that those factory specs say 220-300PSI!! And that is at 95F! I see it modulating around 230-250psi at 1300RPM but the fan is turning only a little faster than its idle speed. This fan has plenty of power to go and my traffic idle temps would be a lot better if I could fool that discharge pressure sensor into modulating about 25psi lower than it is.

    Here's what I have found so far..... The sensor is a 3 wire transducer that sends 0-5V back to the PCM, The signal is reading 1.27 right now with the AC system On but engine not actually running. Chart says normal range is up to 4.5V and high pressure condition at 4.5-4.8V. So what I think this is telling me is that I need to add my trimmer resistor between the 5v+ and the Signal wire and slowly increase the voltage going back to the PCM to maintain the new pressure I want.
    Does that sound right? And if you have done it, what value resistor did you use? I'm planning to use a 1K. I could get a new pressure transducer but I dont think it would be much better than what this one is since its actually running within the factory specs. Of course if I could just reprogram the computer with a new target pressure, none of this would be necessary. LOL! Thanks for reading and any info you have on this system please let me know.

    I have to say, you already know more about these "computer controlled" systems than I. On one system, I was able to find that the HPCO was road speed dependent! As long as you understand the perimeters of the signal, so you don't burn something up, try it and take lots of notes on what you do, so you don't loose track of where you came from and how you got where you are. Good luck, I am more of the old electro-mechanical systems guy. I know enough to be cautious around the electronic controlled systems.
    I wonder if after a few tests, you can't "bias" the transducer in the direction you want with a fixed resistance from the 5 volt feed? It can never exceed 5 volts so shouldn't cause a problem, just raise the output across the board.
    Last edited by Cornbinder89; 09-23-2018, 10:21 PM.


      Thinking about it some more, the risk I see in manipulating the sensor output, is you may run up against tripping a high pressure cut out, rather than the desired effect of increasing the fan speed. In that, it would be better if you could change the "program" for the fan. Still, there may be room for improvement, The risk is shutting down the system in hot slow moving conditions as the pressure rises above normal. We had one car where the high pressure cut out (program controlled) was lower at road speed than at idle. The risk of telling the "computer" that the high pressure is higher than it is, would be to exceed some other set point.


        I hear ya on favoring electro-mechanical systems!! I much rather walk onto a job that has mechanical controls than an electronic controller. At least II wont have to fight management over the phone to get an access code if something is really wrong with their setpoints. The only access code you need for mechanical controls is a screwdriver! LOL!
        So, I had a little time to try out my rig. I put 2 T-tap connectors in the harness about 4 inches from the plug across the 5V+ and the Signal. I hooked it up and read 1.2V from Sig to Ground with AC on and engine not turning. Cranked the engine and it read 2.2V at idle. Clouds were gathering for a thunderstorm at this point so I didnt have time to get scientific with it. I put the 1K trimmer resistor across the V+ and Sig and raised the Sig voltage to 2.8V and the fan was speeding up and the liquid line going into the TXV was cooling down nicely. I revved the engine and the fan speed increased some more and the liquid line still felt nice!
        Shut it down and pulled the trimmer and measured about 490 ohms. Got a 470 ohm resistor out my junkbox and put male spade lugs on the ends and taped it up nicely and put across the T-taps. Took it for a road test and I am quite happy with the results so far. The fan speed is increased but not to the point that its obvious with the hood shut. I think this is going to work fine. After the rain, the temp is now 83F and the fan is only idling along. I will road test a little more this week and once Im satisfied, I will probably pull those T-taps out and make a more reliable connection with solder and heatshrink. Will let you know how it turns out.


          Yes, please do! That is how we all learn.


            Update... Finally had a chance to put gauges on it late this afternoon as the sun was low and 85F and 85%RH outside.I haven't been driving it much but heres what I found. During the test discharge temps were in the low 70s. Should be [email protected] It was a little low on 134a. I added about 10 ounces to get the head pressure up. Looks like the compressor body gaskets are leaking a little. The fan is spinning MUCH more and pressures never spike over 250psi when you goose the throttle while the fan catches up and then quickly brings it back down to about 200 then modulates around 220psi. Low side is modulating around 35psi. The vent temp after a few moments was 57F. at 1300RPM I drove it around in stop and go slow neighborhood roads to see how it did and you can feel the fan a little more now when as you come on and off the throttle with Recirc OFF and Fan on high. Put the Recirc ON and Vent temps drop quickly to 47F with the driver window open.
            It doesn't look like much of an improvement but I can feel a BIG difference. The fan is turning a little bit faster at idle but not much louder than an electric fan on low. I like this a lot better.


              OK I think? You lucked out just adding refrigerant amount of 10oz is a lot so had to be way off on just that of course would matter. Now just a few #s and when you posted are more like what I would expect (any system that's mostly happy) at 85% RH is high alone makes system erratic some lots of condensate. In short if you can see PERFORMANCE of real 47F vent temp on non recirc, stop and go driving, a window open not much will do better than that IMO and lots of observing other vehicles.

              Now just a question: What did you mean "discharge temps" should be 58F>63F @ 80F and where did you find or calculate how much to add? Good guess? Must be aim for 2.5 X real ambient temp for a high side you hit it. 85F was at least 100F coming thru condenser MY GUESS hit it spot on!

              Stop now but leave thermos in a vent watch while driving for changes many expected assorted driving and call is done I think?
              MetroWest, Boston


                Tom, I'm meaning the discharge air vent temp coming out of the left center vent on the dash at 80F outside. Im going by the Jeep service manual performance chart. Probably good idea for me to post it here for anybody needing it.
                Another Update, I am trying out a 1K resistor now instead of the 470ohm. Fan was running just a bit too much for my liking. Pressures are still much better than before and the liquid line at the receiver tank is barely warm. So far so good.

                Click image for larger version

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                  Charles: That looks like a mix of commercial ideas/info in some attempt to apply to a MVAC = Motor Vehicle Air Conditioning is missing way too much to simplify it to just what to expect for air drop at center vent at what ambient temp.

                  Lacks what refrigerant is used so throw that out it's incomplete.

                  Define what just each item means for examples:
                  What is ambient temp? It's what the condenser is dealing with not much more when you look at performance there and what temps are entering or going thru evaporators. Vehicles are not commercial then move - have quick assorted temps and RPMS of compressors commercial just sits there. Center vents? Which one? That's just the best universal guess at which vent is the shortest duct work from evaporator to where outputs can be read..

                  Vehicles have to change interior air with outside air every so many seconds and many commercial applications so as well but not all. Huge difference. Humidity for sub freezing cooling like a food freezer now all different and not my thing please understand that just speaking for myself know just enough to leave it alone mostly.

                  Crudest basics for vehicles: The system specs for how much refrigerant are generally painstakingly figured out to the closest best for all applications a vehicle is expected to be exposed to put to how much down to ounces/grams with little room to vary once established. Default to empty a system and fill to spec see what you get is and has been step one for a system not performing up to par.

                  It never ends: What then is "par" for the course. OMG - hair pulling twist of observations all at once to include sun, surface temp where vehicle is located, engine heat invasion not so applicable to a commercial setting varies all over the place - cold engine up to hot engine usually layouts are against the firewall where ductwork is also plus time with exhaust parts all but glowing or even really glowing hot.

                  Engines cool with a radiator that can radiate heat against airflow thru heating a condenser for A/C.

                  Difficult to be concise there's just everything to consider for the moment you are taking observations.

                  The end all is if performance (have to say IMO) is somewhere in the 40s at a vent and goes no lower ever and only over 50F at extremes you probably are not going to do much better and system are NOT designed to.

                  The failsafes and computer controls when working properly do quickly adjust to constant changes of conditions using sensors everywhere for everything if perfect can help and if failing a nightmare to locate where the problem is. Wiring and controls just are substandard materials exposed to heat, wet, corrosion and vibrations do or don't do well over time also varies.

                  I'll run out of space. If you are happy and comfortable, system is not self destructing or noisy it's time to stop there and leave it alone. Good luck it's not as simple as any one chart could possibly do. Good luck Charles. If not happy with it later I or we will try to zoom in on where to look for a reason and fix,
                  MetroWest, Boston


                    Tom, I think I'm right where I want to be with it now. Before even with a weighed-in charge it just didnt cool well and the high side pressures were just too high for my liking, And the liquid line was hot at an idle. I got to drive it around today after giving it a flush and fill wilth new PS fluid and I couldn't be happier. This is going to work out great!

                    The chart in the picture I have is from the factory Jeep service manual. It outlines every last detail to run the test correctly and thats what I followed. Maybe all that is wrong with my system is that $60 high side pressure sensor is off by 20psi. Either way this 1KOhm resistor is telling the computer just what I want it to and running much better at idle and hot slow traffic.


                      Sure seems you've found the flaw and outsmarted it! Kudos to you. I have my share or more of distrust of both specs and if the item in fact is what was specified. About anything. Even if or when reduced to "swaptronics" is the item or the tester giving you the real info. Credit goes to you with this in the real world if this whole thing was new and wasn't right nobody would have the time allowed nor be the driver in all the assorted conditions is in fact it behaving? There's just no way to argue with what ended up working if performance is, and it is, the end all objective you achieved it. Good for you.

                      Other and a surprise: Changing out PS fluid is almost always a good idea with some time and miles is one of a few things not usually listed as something that should be done periodically or upon first notice of anything not quite right.

                      It's now decades of assorted vehicles that run hotter and these fluids do break down from original properties. Happened to one of my own now some years ago did in fact change it by constant dilution rather than flush took longer did show steering binding like some odd something was preventing smooth, even plain steering? Strong guess changed it out now about 10 years ago problem did quit it not to return so guess I was right - that nothing to do with A/C alone there's just more heat trapped trying to cool in tighter and tighter space was in fact at fault. That never would have been found if I just sent that out with a complaint as it wouldn't do it for long or predictably - just another example of many.

                      That just a side note but another whole type of vehicle did the same thing but not in front of me well described (hard to get that accurately for the routine driver most of the time) did that again and also fixed for years now. TMK that isn't written in any service as a correction rather dump all parts involved at insane expense too often the answer.

                      Back and think we can wrap this up as solved. Said if with A/C which is it's own bird if you will if all works, no odd noises as said already call it fixed.

                      Good job and let it be a good archive of a fix that would elude almost anyone,
                      MetroWest, Boston


                        I think Jeep is trying to minimize fan drag, eliminating viscus or on- off electric with a variable speed drive and then turning only as fast as they think it needs to get the job done. They likely would have done better if the increased the capacity of the condenser as well to compensate for the less air flow over it. I notice the high side pressure is lower at the highest temp on the chart than the next one down, I would guess that at that heat, the fan is full on.
                        The one reservation I have, is in real hot weather, with the resistor increasing the sensor output, you might trip a HPCO limit in the system. The sensor telling the controller that the high side is over 400 psi, and shutting down the system. But that is something you can address if and when it shows up, just be aware that it might occur. It would be nice to be able to go in and just change the fan control map.


                          Agreed - there are times I wish I could control the power of the fan for the situation right then not some average of what works when and saves by other methods. Face it since the oil embargo of 1973 there wasn't much motivation for saving energy rather how to just make nuke powered everything at any cost of weight first then any other possible thing that consumes power not helping to move the dang vehicle now leaving no stone untouched.

                          A really strong high pitched fan with a perfect clutch even with 400+ HP Land Yachts you could feel the drag and wasted HP to cool engine first. No engine, no A/C of course.

                          Hidden is use anything light as possible and wind drag of a vehicle. Fans, electric use we don't think of as much just feel a hard working alternator get so hot you can't touch it you are paying for that too in HP.

                          Downside mix - there's a problem making room inside the dang thing for passengers or cargo. What a battle to have it all and add don't emit anything no matter what form of power you use - arg! Oh - let's not forget how much it costs per vehicle to add safety features now an opinion is wasted when the vehicle is junked vs plain fixing dangerous road areas and more permanent fixes I do see just real slow to come of course all that is costly as well,
                          MetroWest, Boston


                            Tom, ain't that the truth! Or the cars they build now that the plastic parts are so thin that they will degrade before they ever make it to the junkyard.


                              See if this picture posts? It's been all downhill since the Flintstone Car -ZERO plastics, no A/C needed with plenty of ventilation. Where did we (Mankind) go wrong? Smile hope picture shows below. If not you know what I mean,

                              Last edited by Tom Greenleaf; 10-08-2018, 10:22 AM.
                              MetroWest, Boston


                                HAHA!! Good one!