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Repairing a Jeep JK AC Compressor Clutch
November 21, 2014

While we were discussing what to do with my son's Ram truck with a case of the death wobbles, he had another car problem. His wife's recently purchased, used, 2012 Jeep Wrangler Sport Unlimited 4 Door started smoking from under the hood. My son, Mike, was able to determine that it was the air conditioning compressor clutch producing the smoke and shut off the AC. The rubber ring in the clutch disk assembly had gotten so hot that it was smoldering. It didn't catch fire, but I have since read of instances where people said that theirs did. He sent me some pictures, but they weren't too clear due to the compressor being hidden beneath the top radiator hose.

He gave me a call and we talked about the possibilities. I suggested that if the compressor wasn't locked up, he might be able to just replace the compressor clutch. He made some calls and later told me that the dealer wanted over $800 parts and labor to replace the compressor. The compressor alone was $510. Apparently, the compressor clutch assembly doesn't come separately. I told him I would look into it and see what we could do to drop the price a bit. There's not a lot of info on this particular air conditioning, but I read as much as I could find. There are a half dozen sites that advertise that they carry a replacement compressor that will fit the 2012 Jeep. There are also a few Ebay sellers that list a replacement, but no one I found sells just the compressor clutch.

I found a seller on Ebay that had a compressor they said would work. Since they had good reviews, I sent them an email with the Jeep's vehicle identification number (VIN) and asked if their compressor was a drop-in replacement. They said yes and I ordered it. At just under $200 with shipping included it seemed to be one of the better deals I had found. I got the compressor quickly, it was a brand new Chinese made compressor and it looked good. If it fit, we were in business. We put it aside until we were ready to replace it.


A shot of the compressor in the Jeep. The larger circles are the AC hose ports and the smaller circles are where the studs secure the hoses.	The tag showing the compressor number is under the compressor. I used a (broken) mirror to find the number.

With the Dodge's death wobbles cured, the day finally came to install the compressor on the Jeep. We pulled enough parts off the Jeep to actually be able to see the AC compressor clearly. This involved draining the engine coolant and removing the top radiator hose and radiator overflow tank to gain some access to the compressor. I got a better look at the compressor and noticed that we had some issues. The electrical plug on the new compressor didn't match the one in the Jeep. This was easy enough to fix, but the next problem wasn't. The AC high and low pressure hoses have formed aluminum connectors where they attach to the compressor. These are formed in a manner that allows them to be installed in one direction only. Each port on the compressor has a stud that aligns the hose connector to the port. The studs on the replacement compressor were in different positions than the ones on the original. If you look at the picture above and the picture of the replacement compressor below, you'll see that the position of the studs on the original compressor don't match the position of the stud holes on the replacement. I suppose that if you really wanted to use the replacement, you could replace the hoses, but that's a whole different can of worms. We'd need to get hoses made up and I'd need to either hard wire the compressor or cut out the waterproof socket on the old compressor to replace the new one that didn't match. I might be up for this on a 10 year old vehicle, but not on their relatively new Jeep that was just out of its warranty. I decided that I needed more info on the compressor so I could try and match it to a replacement. I got out an inspection mirror to see if I could find the part number for the compressor. Here is the info from the sticker I found underneath the compressor.

RS18 F500-CCBAA-02
MFG DATE gEAD SERIAL NO. 0653
OIL FD46XG 120cc MADE IN KOREA
<SKEW>
55111374AB REV

Compressor label -- the compressor number is 55111374AB

As a last ditch effort, we measured the clutch disk on the new compressor to see if we could swap it with the old one. Unfortunately, both the size of the disk and the splined mounting were different from the stock one. We put the Jeep back together and Mike made the long trip back home. Aside from getting to visit with my son, it was a wasted day. I'd contact the Ebay seller and see what we could do to get the proper compressor.


Compare the port and stud location to the picture above. The electrical plug is also wrong. The rear bolt mount may also be wrong.	Once I removed the 8mm headed bolt, the clutch pulls off the shaft. You can see that the rubber is pretty well fried.

Having worked on a lot of vehicle air conditioning systems, I was curious about the change in compressor clutch design from when I was working on them. All of the AC clutches I had ever seen worked the same way. You flip the control on the dash to turn on the AC and it energizes a coil on the compressor. The coil is an electro-magnet. The magnetic force pulls the clutch plate, which is attached to the compressor shaft with springs. The clutch plate sticks to a machined area on the compressor pulley. With the pulley and compressor shaft locked together by magnetism, the compressor shaft is able to be turned by an engine belt. The difference was that the old AC clutches used three flat steel springs and this clutch used a ring shaped piece of rubber as a spring. I am not sure why the change was made, but my 2006 Dodge Ram also has the rubber spring clutch. Most likely, it was a cost cutting measure.

From the owner's point of view, the change isn't a good one. With the old system of metal springs, if the compressor clutch slipped, the resulting wear would widen the gap between the clutch disk and the pulley/clutch coil. If the gap got too wide, this would prevent the clutch from engaging and you'd have no AC. To fix it, you could reset the air gap between the clutch and pulley by removing some shims from between the clutch hub and compressor shaft. If you found ridges (signs of wear) on the clutch plate, these could be dressed with a file to flatten them out and you were back on the road with a working air conditioner. With the rubber spring clutch, as soon as the clutch starts slipping, the high temperatures (I read 1200° F was possible in less than a minute of slipping.) could char the rubber and make it detach from between the clutch hub and clutch plate. Just like the one we were dealing with. Unless someone can offer me some reasons otherwise, I think this is a poor design choice. Especially considering that the dealer doesn't even sell the clutch disk separately. You need to replace a $510 compressor for what should be a less than $50 clutch disk. It's this kind of stuff that makes me glad that I got out of automotive service field in the 90s.

I exchanged email with the Ebay seller, BLA Autoparts, and they said they couldn't match up the compressor number, but would issue me a pick-up slip so I could return the compressor for a refund. I got the refund and would work with this seller again. Next time, it would be wise for me to send the compressor number rather than the VIN.

I did some more looking on the web and found that most all of the companies who advertised that they had a compressor for a 2012 Jeep JK used the same picture as the one we had purchased. No sense in even contacting them. I did find what appeared to be the correct compressor for $485, but that would be the last resort. I was now thinking that I would try to repair the clutch disk assembly.


A side view of the clutch sitting on a roll of painter's tape	The rubber had burned completely through. When new, these two pieces are supposed to be attached by a ring of rubber.

My first idea was to clean out the old rubber and pour some new rubber in its place. There is a company named Smooth-On that sells all manner of rubbers and plastics and I have used their products to mold various parts when I was racing radio controlled cars. The first problem with this is that I would need to make a guess at the hardness and elasticity of the rubber. After some research, I was guessing that I needed a Shore A hardness of between 65 to 80. However, the more I thought about casting a new rubber ring, the less I liked the idea. My biggest fear was that the rubber would either shear on first use or be so hard that the rubber wouldn't stretch enough to engage the clutch. There were just too many unknowns for me to get it right on the first attempt.

The more I pondered the situation, the more I thought that modifying the clutch to operate like the ones on older cars would be the best way to go. The actual design of the clutch would be pretty simple. There were three rivets in both the hub and the clutch plate and the design called for three flat springs. Nothing to do here except to get the old rivets out, remove the parts used for the rubber ring and install my three springs. Since I have a small lathe and milling machine, doing the modifications would be straight forward. The big unknown would be how thick the springs should be. They would need to be flexible enough to allow the clutch plate to be pulled into the pulley when the AC was on and still have enough tension to be able to return the clutch plate to the disengaged position with the AC turned off or when it was cycling.


The friction portion of the clutch disk. It appeared that this clutch had slipped a few times.	After an hour or so with a razor knife and scraper, I was able to get rid of all of the burnt rubber.

I spent a day working up the design using a couple CAD programs. I ran my ideas past my engineer friend, Scott, who provided some help and some confirmation that what I was attempting was the best plan of action given the circumstances. Some of the design considerations I came up with are as follows.

Considerations:
1. The springs need to point in a direction that allows the compressor shaft to be pulled, not pushed, by the springs.
2. The minimum air gap between the clutch disk and pulley needed to remain at the stock distance of about 0.020". I could use a wider gap if the new springs had less tension than the original rubber spring.
3. The springs needed to be raised on the center hub enough to allow them to flex at least 0.020" without having to bend around the center hub.
4. The relationship between the height of the center hub and the clutch plate needed to stay the same as the original parts. This worked out to 0.155" between the top hub and clutch plate. (See 2D drawing below.)
5. I would use the 0.044" shim that came under the clutch hub to try and maintain the 0.020" air gap. I would purchase an assortment of additional shims in case the air gap needed to be greater than the original 0.020" gap.
6. I would need to add a disk or washer on top of the springs to prevent the springs from flexing the wrong direction with the AC in the off position.

Unknowns:
1. Amount of magnetic force generated by clutch coil.
2. Amount of force necessary to turn AC compressor.
3. The needed thickness/stiffness of the springs.


2D CAD to figure out the measurements.	3D CAD to get an idea of how the clutch will look.

The biggest design challenge were the three unknowns. Knowing how much magnetic force the clutch coil produced would allow me to size the springs so that with the coil turned on (AC on), the magnetic force would be able to overcome the clutch springs and engage the clutch. I decided that if my springs were a little more flexible, the AC would still work. If my springs were too flexible and the clutch didn't spring back when the AC was shut off, I could always use two springs in each position. For the second unknown, I decided that as long as my springs were not so stiff as to make the clutch slip, it would be OK. I figured that the engineers who designed the clutch assembly had made the coil produce enough force to keep the clutch plate firmly on the pulley. As long as my springs were not less flexible than the rubber spring that was originally used, it should be OK.

The last unknown was the million dollar question. How thick should I make the springs? The thicker they are, the less flexible they are. I didn't have any spring steel to test with, so I used feeler gauges to give me an idea of how much tension I could expect from a particular thickness of steel. I set up a vise to hold the feeler gauge and placed a dial indicator at 1.7" out from the vise. This would be the distance between the rivet holes on the spring. I then used a brass pull scale to try and determine how much force it took to bend the spring 0.020". I then measured the force to bend 40 and 60 thousandths. I tested feeler gauges from 0.010" through 0.040". I found that using a 0.025" feeler took about a pound of force and seemed to flex back to its original shape well. I knew from past experience that spring steel is stiffer than the steel used for feeler gauges of the same thickness and decided to try using 0.020" thick, blue spring steel in a 1/2" width. Since there were going to be 3 springs, the total force that the clutch plate would see would be about triple what I saw with testing only one spring.


Milling the heads off of the three rivets that hold the two pieces that make up the clutch hub.	Here I've used my lathe to cut the bowl shaped piece off of the top portion of the hub.

The next step would be to strip the parts off the clutch assembly that I wouldn't need. I milled the heads off of the rivets on the hub and drove out the remaining bodys of the three rivets. When I got the two pieces of the center hub apart, I found that the upper cup shaped piece was needed -- or at least the center of it was needed. The 1/4" hole in the center of the cup shaped piece was where the bolt goes that retains the clutch hub to the compressor shaft. The bottom section of the hub was shaped into a female 20 tooth spline and was open on the top. Without the top cup-shaped section, the female spline would just slide along the splined compressor shaft. To get my clutch to work, I would need to remove the cup area of the center hub and make the top portion the same diameter as the flange on the lower hub. A few minutes on the lathe and I had this accomplished.

With the clutch hub now the right size, I turned my attention to the clutch plate. There was a stamped metal ring riveted to the clutch plate. This cylindrical piece was where the rubber had been attached. I had to mill the heads off another three rivets and drive out the remaining bits. Now that this was accomplished, I had a flat clutch disk with small raised areas around each of the rivet holes. I assume that these ridges were there to give some space between the stamped metal that held the rubber and the possibly hot clutch plate. I set the disk on a couple boards and put the hub in the center so I could see what it was going to look like. I placed a feeler gauge in the approximate position that the spring would be. Looking at the pieces on the bench, I was thinking that this just might work. I called it quits for the day and went inside to order the parts I'd need.


The top part of the hub has been turned on the lathe to match the diameter of the lower portion of the center hub.	After machining off the rivets on the clutch plate I set up the parts using a feeler gauge in place of the spring for the mock-up.

The next day, my parts arrived. I sure do like McMaster-Carr's fast shipping. I had ordered the smallest roll of 1/2" wide 0.020" thick blue spring stock I could get. That was ten feet. Ten feet and I will use less than seven inches. Does anyone need a clutch disk made? I received my shims and some washers to make the spacers that would hold the clutch disk and hub to be 0.155" apart. I also received 100 1/2" long 3/16" solid steel, flat head rivets. I am going to have a few of these left over as well. If I don't mess any of them up trying to set them, I should only use six rivets. I also ordered a 13/64" solid carbide end mill to cut the holes in the spring steel. The six holes on the clutch parts were all 13/64" diameter and I wanted to keep the holes in the springs the same size. I probably wouldn't have had a problem cutting the spring steel (Rockwell hardness C44) with a high speed steel (M2) end mill (Rockwell hardness C66), but the carbide mill is harder still at around C79 and with some care will last a long time. The total cost for the order was about $50. If I can make a clutch disk that works for $50 in parts, I will be way ahead of even the $200 compressor that didn't fit. We shall see.


My modified clutch disk is almost finished. Blue steel flat springs take the place of the rubber ring.	The bottom of the clutch plate has been dressed with a file. One ugly rivet needs to be replaced.

With the springs cut and drilled, the assembly of the clutch went quickly. So quickly that I forgot to take pictures. In order for the clutch plate to end up twenty thousandths of an inch from the compressor pulley, I needed the top of the center hub to be 0.155" higher than the top of the clutch plate. The outer spacers/washers would need to be this thickness minus the height of the little ridges around each of the three holes on the plate. I had computed the thickness of the washers I would use to accomplish this when I drew up the CAD plans. Now it was just a matter of measuring the washers and comparing the thickness with my plans. With the correct thickness washers laid out in six piles, I was almost ready to assemble the clutch.

I still needed a large washer to go over the top of the springs on the hub. I dug though my odds and ends box and found a two inch washer to put on top of the hub. I drilled it to match the holes on the hub, enlarged the center hole to accept an 8mm socket (so we could tighten the mounting bolt) and mounted it on top of the springs. This washer should stop the clutch plate and springs from springing the wrong way (away from the pulley) when the AC is not running. I am not sure that it's necessary, but most (all?) of the old style clutch disks on the import compressors from the 70s and 80s had them. The washer was a little scratched, but should do the job just fine. I then added the spring spacers under the springs and set the rivets on the hub by using my shop hydraulic press. Next, I needed to add the spacers between the other end of the springs and the clutch plate. Since the rivets on the clutch plate had to seat in chamfered pockets, I set these rivets with a punch and a hammer. I got two out of the three rivets seated perfectly and miss-struck the third rivet on the first blow. All three are tight, but the one rivet doesn't look as nice as the other two. While I took some pictures, I decided that I didn't like the look of the one rivet, so I milled the head off and replaced it. I also decided that it needed a coat of paint to look its best, so I degreased it with some lacquer thinner and gave it a couple coats of gloss black enamel.

So there we have it. The clutch has been changed from a rubber spring clutch to a flat steel spring clutch. The only question that still needs to be answered is, "Will it will work?"


I decided that a couple of coats of paint would make the clutch look a little nicer.	The finished clutch disk along with the pieces I removed to make the new design work.

Mike and his family made the trip back up to our home after having been in town at my wife's parents the day before for Thanksgiving. Today was the day that we'd see if the redesigned clutch would actually work. We bolted the clutch to the compressor and used only the 0.044" shim that had been on the compressor shaft originally. My measuring had been pretty close and the air gap was about 0.018" instead of the 0.020" I had planned for. Considering that I had measure the clutch hub on compressor while it was in the Jeep, I figure that I did pretty well. Once the clutch bolt was tight, I grabbed a hold of the clutch disk to see how much pressure it took to push the clutch into the pulley. It felt like I had expected it would feel when I pushed the clutch plate evenly. However when I pushed on only one side of the clutch plate, it moved a bit too easily for there to be only 29 thousandths gap, so I decided to increase the air gap to about 0.040" to help ensure that the clutch disk would be less likely to rub against the pulley with the clutch disengaged (air conditioner turned off). I added a couple 0.010" washers between the clutch hub and the compressor shaft and we again tightened up the bolt that holds the clutch in place.

It was time to start the engine and give the clutch a try. As it was 30° outside and a rather chilly 40° in the shop, Mike had the Jeep's heater running. He switched the mode to defrost. The defrost mode runs the air conditioner along with heat and helps prevent the windshield from fogging up. As soon as he selected defrost, we heard the "clack" of the clutch engaging with the pulley face. He shut the fan off and the clutch disengaged. He turned the thermostat from hot to cold and turned on the air conditioner. This time I saw the clutch engage as we heard the clack of the clutch plate being magnetically pulled into the pulley. We messed around with it for another few minutes and tried engaging the clutch at higher engine RPMs. It worked just like an AC clutch should. It appears that we have the clutch problem fixed.


The engine is running at about 1500 RPM and the clutch is engaged. You can see that the springs didn't have to flex very much.	I tried to get a good front view pic of the clutch spinning the compressor, but the camera barely fit between the radiator and compressor.

The nice thing about a clutch using the flat spring design is that it won't self-destruct if the clutch plate gets hot. All of the parts on the clutch disk assembly are steel. Unlike the rubber "spring" on the original clutch, overheating the clutch plate will not ruin the new steel springs. This clutch should last for the life of the air conditioning system.

Update: May 2015
The new compressor clutch worked for about 2 months, then broke two of the clutch springs. I was a bit baffled by this. The breaks occurred were I had bored the holes to attach the spring to the outer disk with steel rivets. The breaks appeared to be from an undue amount of stress at this connection. The steel rivets had almost been sheared. Very strange. I checked the compressor and the shaft turned freely, so I assumed that it hadn't locked up. I made some new springs from thicker stock - some 0.032" feeler gauges - and we put it back together and sent my son on his way. Two months later, we had the same situation again. This time one rivet had sheared and two of the three springs had broken. My son sent me the clutch through the mail and I didn't have a chance to check the compressor this time. I rebuilt the clutch yet again and waited until the next time he'd be in town to install it.

When he came up, I went to install the clutch again, we found that the compressor shaft was locked up tight when trying to rotate it clockwise. I was able to turn it counter clockwise. When I again tried to turn it clockwise, it now turned. It now appeared that we had an internal problem in the compressor. The whole situation was now beginning to make more sense. The compressor was the cause of the first burned up clutch, as well as the two subsequent clutches.

Michael ordered a new compressor from http://www.justforjeeps.com/accompressor1.html and also ordered a new receiver drier in case some metal shavings got into the lines. We got the compressor and receiver drier installed last week. I would have liked to have stripped down the compressor and find out what had happened, but there was a core charge on the old compressor that he didn't want to lose out on. So I guess that we'll never know the reason for the compressor locking up randomly, but I guess that's life.

To add insult to injury, the air conditioning clutch coil on my Dodge 2500 died a few days before we installed his compressor. The coil is inexpensive as an aftermarket part, but the coil is controlled by the fuse box which is properly named the Totally Integrated Power Module (TIPM). The TIPM saw that the coil was drawing too much amperage and has shut down that circuit. I have the coil installed, but am presently waiting on some software from AutoEnginiuity that may allow me to reset the TIPM. If this proves successful, I may write up the procedure. Always something to keep me busy.