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Discussion Starter #1
Howdy ROF, it's been a while since I've posted.

I notice the following sitting at idle with AC turned on (2006, 4.0L). As the AC compressor cycles on and off the engine RPM drops so low the Jeep shudders like it's about to stall, then revs up to 2500 or so. If I'm driving and turn on the AC, it cycles between a significant loss of power and a huge surge of speed (kinda dangerous!).

My first thought was to replace the AC clutch but Mopar discontinued the part. I wanted to solicit a few opinions before I bring it in to a shop and drop big bucks on (I expect) a new compressor, AC recharge, shop fees, etc.

Thanks all!
-Tommy
 

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Hey Tommy welcome back. The issue of the revs dropping so low it shutters used to happen to me in my 97 and was due to heat soak and was fixed by adding hood louvers to let a lot of the hot air escape from under the hood. I dunno if this would also be related to what you're experiencing. In all my vehicles I've noticed when the AC compressor cycles the rpms change but not to a point of shuttering. I'll be following this post and hopefully someone more knowledgeable and experienced can chime in and help ya out.
 

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If your AC is blowing cold in all probability your clutch and compressor are fine. When the compressor turns on it puts a load on the engine thus dropping idle speed and the computer compensates by upping the idle. Something in that system is the culprit I am thinking.
 

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My first thought is to check your IAC. If it is not reacting quick/smooth enough, it could cause the symptoms you are seeing.

My logic (and it could be wrong) tells me that even though you are driving (throttle plate open X%) as the load increases, the computer will increase fuel to help compensate. Add fuel with no additional airflow just creates a rich condition. The IAC can allow more air to enter the motor and therefore achieving the desired A/F ratio.

Add the fact that it dips dramatically at idle....IAC seems a good candidate
 

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Check the pressure on the system and if its working and cooling then I would start with cleaning the IAC and throttle body before replacing parts on the A/C compressor. Check the hi low pressure switches on the A/C system. Look at items related to the PCM first- the clutch is activated through the pcm, it can start or delay based on engine conditions.
 

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Discussion Starter #6
Some great suggestions here, THANK YOU all so much! I'll do some testing. Always preferable to throwing parts at the problem blindly or going into a shop if it turns out to be something I can fix myself.
 

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If you are going to remove the throttle body, might as well replace the IAC at the same time. They aren't that expensive.
 

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Discussion Starter #8
Fixed it! It was subtle, and other TJs could experience this as they start getting up in years. The short version: the blades on the 10A fuse (in the fuse block under the hood) had the thinnest layer of oxidation on them.

How I found it: The FSM suggests measuring voltage and current. I started by measuring the voltage at the battery. When the AC compressor engaged it drew the engine RPMs down to almost 0 and the voltage dropped to 10-ish. I poked around at a few other things and decided I should probably throw in the towel and bring it to a shop. I decided to pull the AC fuse so that I could use split-level or defroster vent settings without the AC lugging the engine, until I can get it to a shop. As I pulled the fuse, it occurred to me that I can easily check the current by using my ammeter in place of the fuse (current measurements require putting your ammeter in series with the power supply). With everything running, I probed the contacts with the ammeter and the AC engaged and purred like a kitten. I then looked at the fuse and the contacts had an iridescent blue sheen. It was scraped off a bit from pulling the fuse out. I sanded the contacts with fine-grit emery paper and replaced the fuse, and problem solved.

I then sanded the contacts of each fuse in that block. They all had corrosion to some degree. My Rubicon is 10 years old this summer and I've never pulled any of those fuses.

Thanks again for the replies. I hope my solution is useful to others with similar, mysterious electrical issues.
-Tommy
 
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Fixed it! It was subtle, and other TJs could experience this as they start getting up in years. The short version: the blades on the 10A fuse (in the fuse block under the hood) had the thinnest layer of oxidation on them.

How I found it: The FSM suggests measuring voltage and current. I started by measuring the voltage at the battery. When the AC compressor engaged it drew the engine RPMs down to almost 0 and the voltage dropped to 10-ish. I poked around at a few other things and decided I should probably throw in the towel and bring it to a shop. I decided to pull the AC fuse so that I could use split-level or defroster vent settings without the AC lugging the engine, until I can get it to a shop. As I pulled the fuse, it occurred to me that I can easily check the current by using my ammeter in place of the fuse (current measurements require putting your ammeter in series with the power supply). With everything running, I probed the contacts with the ammeter and the AC engaged and purred like a kitten. I then looked at the fuse and the contacts had an iridescent blue sheen. It was scraped off a bit from pulling the fuse out. I sanded the contacts with fine-grit emery paper and replaced the fuse, and problem solved.

I then sanded the contacts of each fuse in that block. They all had corrosion to some degree. My Rubicon is 10 years old this summer and I've never pulled any of those fuses.

Thanks again for the replies. I hope my solution is useful to others with similar, mysterious electrical issues.
-Tommy
Thanks for the update
 

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Glad it's fixed. Your update is appreciated. Now, if I can only remember it in case of future problems I may have.
 

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Good find! Its not often that a corroded fuse causes an issue like that. But its a great reminder of issued that can be.
 
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