I was able to gather some more data bout my consistent but weird problem. AC systems can be very complex and I'm pretty sure this is an electrical issue, but could also be caused by mechanical issues which then the electrics kick in to protect system. First of all, does anyone have a decent electrical schematic of the system? I looked at factory manual and it's lacking / conflicting info. They claim the diesel version doesn't have an AC pressure transducer yet the schematics show it does, what's going on there. Should just have a low and high pressure cut out, right? And the low pressure cut out also has a "freezup prevention" temperature sensing portion, right?

I have a bluetooth pressure gauge I hooked to the jeep and was able to drive and log data. I'm still trying to figure out the app and see if I can export data, this is kind of chinese thing.

I just did test today so I have fresh data to think about. Plenty of freon in system; no it isn't "low" I've checked all the easy stuff. It could and probably is a little overcharged. 97 degrees ambient temp.

When idoling, the compressor never cuts out when it shouldn't. Maybe I'll make a list to try to explain the data:

AT IDOL:
1. Fan 1-2, ac compressor clutch cycles normally once the interior has cooled to reasonable level. It seemed to cycle between say 30 something and 40 something psi. I thought low pressure switch opens at 25psi? Anyways it stays nice and cool.
2. Fan 3-4, reaches steady state maybe upper 40s lower 50s. I know this is a little high perhaps but I'm just idoling. AC stays COLD, clutch stays ON

AT DRIVING HIGHWAY SPEEDS
1. Fan 3, AC will blow nice and cold humming along stabilizing around 47 or whatever. Randomly, when it wants to, AC clutch appears to cut out and it will climb to say 55 or so then the clutch will kick back on. This is the mystery problem, causing warmer air to blow and the warm air smell / humidity to come. This lasts for maybe about 5-20sec, not sure.
2. Fan 4, AC will blow a little warmer as there is more air, that's understandable. AC CLUTCH DOES NOT CUT OUT! It keeps going, as it should, since the system is fully loaded up.

Thoughts:
As a general rule, more cabin air fan flow raises system pressure high and low side as it's warming the evaporator. More condenser airflow decreases system pressure as it's cooling the condenser.
It is not low side cutting out as if there was maybe too low a freon charge and highway speeds caused system pressure to go low, never went below 45 psi or so. So the theory is maybe high side is cutting it out. I will monitor this next. Or low side is doing it erroneously, but this is pretty reproducible behavior, can't imagine it's random.

HOWEVER, increasing the cabin fan to 4, loading up the system more, increasing pressure more, and then the clutch DOES NOT CUT OUT! This doesn't make sense as the high side would cut out worse, more often, sooner, etc. This isn't the case!

Madness..this has tormented me for 2-3 summers now. I haven't looked at it deeply as it's still fairly comfortable in the cabin, and I can always turn it to 4 and it seems to run steady.

Does anyone have a schematic? I'm pretty sure SOMETHING is cutting out that clutch, but what, and why? Further troubleshooting needed. Hard without a good schematic.

A good test would be to short out the low side to see if behavior continues. I know this is a bit dangerous. Same for high side.

I had an old dodge though the worse case it would mechanically release the freon pressure from a valve somewhere it things get really bad. The dodge would do this as previous owner idiot wired power straight to the clutch lol. It didn't have enough radiator air flow at idol and system pressures would rise.

Is it possible the expansion valve/orifice tube whatever this uses is sort of not acting right? Or the compressor itself. This isn't a variable rate compressor I don't think like some cars where they can change their output dynamically.

I can't seem to export the data in this cheap app, that would be helpful to explain to anyone interested.

Thanks!


This is when clutch cuts out

Have you checked the screen over the cabin air intake to make sure it is not partly stopped up? Located behind the glove box.


This is the wiring diagram for the AC clutch:
2005 Model


This is the AC transducer connector and wiring:

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That screen has zero to do with what is happening.

Those diagrams are what I saw in the FCM, which conflict with the descriptions they state in the HVAC section. There is no pressure transducer they claim on the diesels. Which is it?

Oh the fun adventures!

That screen can have a huge affect on AC cooling, I know this from my own AC's cooling experience. Certainly worth checking just to be sure.

Yes, it has a pressure transductor sensor on the high side line located behind the AC compressor. In some places in the FSM, they refer to it as a "High Pressure Switch", but it is a pressure sensor that supplies line pressure readings to the ECM.

See this old post for more information on the sensor operation and testing.
https://www.lostjeeps.com/forum/phpBB3/ ... 74#p948874

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I do not trust the Front Control Modules on the 2005 KJ CRDs!

All the Jeep Forums I have been on for the last 20 years have reported problems with them and they are apparently hard to get hold of....worth cleaning the contacts on it as snow, water, mud, salt etc. can get into the connector!

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I understand what you mean about the screen but that has nothing to do with the clutch cutting out lol. I'm on recirc mode anyway, screen doesn't apply . Thanks for input though. Yes if it was super dirty the air flow would decrease into cabin, but that isn't what is happening to me.

I read in the FSM that the FCM is used instead of ECM depending on gasoline or diesel! It sounds like the diesel uses the ECM.

I'll check out that that transducer for sure. It sounds like I need to jump out the low side and run her hard in the heat, just to rule out the low side pressure switch. However, does it have this de icing circuit as part of it? That sounds suspicious also lol

FCM's sound like a pain for sure! In the BMW world, the footwell control module is similarly problematic

Oh that screen is a recirculate screen? I assumed it was outside air intake, sorry. Still isn't my issue but that's good to check. I watched some youtube videos that showed an outside screen under the cowl that can be clogged with leaves etc.

Yea that's what's confusing, the transducer screenshot you so does not match with what the FSM says for testing contacts on a pressure switch. They are two completely different things. It isn't clear to me if the diesel has a transducer or a pressure switch. I did also notice my fan has a mediumish speed and a high speed depending on the system pressure and heat loading. It would be cool if we could read this "transducer" if that is what it is reading into the ECM over OBD..but this may require the proprietary jeep software not standard OBD stuff.

Maybe it's possible I'm getting some squirlly transducer readings into the ECM, but why would it "go away" at higher operating pressures with cabin fan 4 speed? I can't wait to try to read the high side port to see if it is passing ranges for clutch cutout operation!

Please note; the intake screen I was referring to only comes into play when the HVAC switch is set to "Recirculate". If there is reduced air flow through the evaporator core, it can have an affect on system pressures and operation, just saying.

The 2005 diesel model does indeed have a "Pressure Transducer" on the high pressure AC discharge line coming out of the AC compressor. It is NOT a pressure switch like used on gas models! The pressure signal provided by the transducer sensor is routed directly to the FCM (Front Control Module) which in turn communicates with and provides the ECM with the data via the CAN Bus data network. So in the end, the ECM is making all decisions on electric cooling fan operations and AC compressor clutch operation.
And yes, there is a low side pressure "Switch", its only function is to protect the compressor and prevent it from running if the low side pressure becomes dangerously low.

What you see and read in the FSM in some cases when they are describing ancillary component operations & troubleshooting for common across platform systems will not always be true for diesel models, which can be unique onto themselves.
We (forum members) have found quite a few mistakes in the FSM over the years due to gas model versions terminology being the only information provided and some mistakenly try to apply this this erroneous data to diesel models when it simply does not apply.

I personally have had to trace wiring circuits on my own 2005 diesel and then redraw an existing FSM wiring schematic to fit the actual wiring on my model. I have quite a huge collection of these redrawn wiring diagrams on file of which like the one I shared in the previous post. So bottom line, you have to be very careful when studying the FSM for diesel models unfortunately.

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I'll def check that screen, is it hard to get to under the dash? Doesn't hurt, and I have noticed reduced fan air over time. But that is no different than running your fan at say 1 or 2 setting, as far as "problems" are.

Good to know it's a transducer! Man tracing wires that sounds awful. I feel your pain believe me. Hard to troubleshoot stuff without good schematics and it's expected of us field guys routinely. Super frustrating.

I will have to ultimately determine if the ECM is shutting the clutch down or something else in line with it's output. Tricky stuff. I'll get some high pressure readings to think about soon though that should shed some light.

Thanks for all the help and contribution! When and if I ever figure it out I will report back I won't leave the internets hanging on a good problem

Well I have some very interesting data to present. Hope someone find's this useful and helps me think too!

Summary/conclusion: There is either nothing wrong and I'm OCD, I have slightly wrong charge, or I suspect my low side pressure switch is acting squirrely. It seems to open and close at a wide range of pressures. Not sure what its tolerance is? Depending on the rate of approach of open PSI it seems to change the range when it opens/closes. Some observations: (Pressures always are "ish" and approximate)


1. When RPM increases, discharge pressure increases and so does suction pressure, due to the compressor pumping more I guess. This will stabilize into steady state and decrease after some time. The effect of more wind across condenser seems to take over some time later and/or the compressor stops over pumping? Not sure how they work in conjunction with the orifice tube/expansion valve.
2. The low pressure switch on my system acts differently in different conditions as can be seen below. In steady state, cabin cold, fan 1, it will open around 27 psi and close around 34.5. In fan 3 it seemed to open around 35 and close around 40. It certainly is sloppy the way it behaves. And some of this dynamically changes with the "ramp" of pressure how fast it's approaching these numbers. When I'm driving fast and high heat load on fan 3 it seemed to open at higher pressures and close at higher, whole range shifted up. When calm fan 1 it would shift down. See data below.
3. My high side switch and pressures seemed normal. No high discharge pressure as I thought maybe was happening, so the mechanical side I would say is OK. Possibly have the wrong freon charge in it, I haven't vacuumed and charged it "properly" but just played around with pressures. I don't want to waste the freon in there now to try this it's kind of expensive! It works good enough for me to be happy I think...until it doesnt later on lol.
4. Two wires go to low side pressure switch. Not 4, so the FSM is wrong when it discusses a "de ice" temperature switch. That may be only on the gasoline models.
5. During the high heat load fan 3 setting, I would like to have it drop down lower before it cycles off. Why is this pressure range shifting? This causes the brief warmer air.
6. At fan 4 it did not ever cut off, once again. Enough heat load kept the low side pressure high enough to not trigger the switch.

Maybe I will consider replacing switch, part number? The simple pressure switch shouldn't be too costly. I believe high side is working fine. That's the expensive transducer $140+


What a great little tool, highly recommend. Sometimes it likes to turn itself off, loose batteries or Chinese engineering?


Fan 1, stead state idling in driveway. Open 27 psi; close 35


Fan 3, rolling down highway 60+mph. High side readings. Compressor off at 150 psi; on at 240


Fan 3, rolling down highway 60+mph. Open 34 psi; close 40 psi


Fan 3, rolling down highway 60+mph. Open 34 psi; close 43

Remember these dynamics always changing as sun goes down, cabin cools down, ambient air changes, etc.

Good ole Oreilly

https://www.oreillyauto.com/detail/c/mu ... erty?pos=0

The good thing, you can change the low side pressure switch without discharging the system. It has a schrader valve in the fitting it screws onto.
The LP switch open/closed settings should not vary, they should be very consistent.

Anytime the AC clutch is engaged and the compressor is pumping, the low side pressure should be fairly consistent and the LP switch's status should remain closed unless the freon charge is low and the low side pressure is being pulled to low.

It may be in your best interest to remove all the freon, vacuum the system down, and recharge the system with the correct amount of R135A freon.
It will be interesting to see how the system reacts with a new low side switch.
Good luck!

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Since it was so cheap I changed to rule it out, otherwise I give up forever, is design of system perhaps. I will test it in hot afternoon tomorrow. Texas temps near 100. It was kind of a pain to get on it hard to access without removing everything, but was able to snug it up with small adjustable wrench and some cursing and beer.

Do you know if we can view the reading input from the high side transducer over OBD? Would be good to sanity check that.

Yea, freon charge is def something to check, but that's tricky without recharging entire thing. Even in a "good" charge, in low load conditions like mild weather or fan 1, it will still cycle when working correctly. What concerns me is why it cycles in such different pressure ranges. Maybe something else is commanding it to who knows lol.

Adding freon would def cause the clutch to cycle less, however, in very hot conditions it will trip the system out on overpressure. Ask me how I know...lol. I tried to do that to my BMW and ended up overcharging it. But that creates the "coldest" air pushing it to limit where clutch doesn't cycle much or minimally. The way I think about it is you want the most freon the system will accept in worse case conditions like 100 degrees, without tripping out on overpressure. But then it may freeze up hahah I don't know. Just put right amount I guess.

Since I freshly added freon for the season, maybe next summer..lol . Perhaps it needs a little more! If it annoys me enough I'll flush it down and recharge it properly. I have all the equipment to do it.

I'll report back with any changes tomorrow

I took it for a drive with ambient about 93F. It feels better and colder but I could be imagining? The data shows it cycled between 33psi and 40 or so on fan 3. On fan 4 it did not cycle. But it never got that warm burst of air as bad like it was doing? Remember it was cycling off around 40psi on the old switch. So not sure if it helped.

Something else to keep in mind, after driving playing blasting fan 4, I was about 39psi suction. I switched to outside 93F air and it went up to about 62. After a minute or so the electric fan kicked to high speed also, so that tells me its at its upper range of condenser temp/pressure control under high loading conditions. Too scared to add more. So the game is if I did, perhaps it would run a little colder on fan speed 3, but may also trip itself out when you first start the jeep on a very hot day, we don't want that either. So I'm happy where it's at now, seems overall improved. Time will tell . At least I did my due research and can rest that it's doing the best it can.

It does also have a slow leak lol. So I add freon maybe once or twice a year. I wish I had someone else's good operating system to compare suction pressure. Wwdiesel have you ever looked at yours? Fan 4, outside air, hottest day you can try?

Is that recirc screen hard to clean?
EDIT: I removed globox and there was some dust built up but nothing too terrible; there was a bunch of napkins that sort of got sucked behind there that probably didn't help things lol. Thx!

What is that small cable thing on the bottom plastic piece below glove box? On the right side it goes into the firewall so I cannot remove that plastic piece fully.

No, I have never tried to monitor AC system pressure while driving the vehicle.
I have had gauges on it at idle and added a little freon. As long as the low side pressure stays between 20-30 psig and the suction line going back to the compressor is good and cold, when touched, I am happy.

You don't need to remove the air intake screen, just vacuum it off good with a shop vac.


This picture is one I found online, it is not of my intake screen.

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Yea I cleaned screen. Wow 20-30 psi? Is that running fan full blast outside air ? And what ambient ? That’s pretty low. That’s for mild temps usually 70-80s ambient maybe.

It’s possible my orifice tube is clogged or whatever. That would be my next game to replace it and re vaccum system. Maybe the accumulator too. Maybe I have flow issues. Like in my house AC the expansion valve was bad so I compensated it for years by overcharging it. Is what HVAC guy said. Lol. Proper system with good valve works great now. And doesn’t trip out in heat pump mode (it did before due to overcharge in summer).

Save this for future reference, it may be of help?

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Yea I’m aware of all that. It doesn’t always apply as many experts will tell you. Just a rough rule of thumb.