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Gentlemen, At lot of testing and measurements has finally paid off. It appears that the alternator was not sliding on the bushing properly. The alternator was angling slightly causing the belt to be pulled off the ac pulley. The fix was to put a washer on the bolt which caused the bushing to tighten up to the alternator. I also had to use a crowbar under the alternator to take the angle out of the pulled on the alternator. I have run it for 45 minutes on three seperate days without any issue. Hopefully, this solution holds. Thank you to everyone who helped, it gave me some good ideas to trouble shoot to solve the issue.

I don't believe the brush issue and belt issue are connected. The spring failed on the brush and it didn't make a connection. I am going to try to get the grooves cleaned on the crank pulley. I need to investigate the AC closer. I have been watching it from the back as that's were the belt is coming off.

Gary, The system has two separate belts, V belt on water pump and then micro V serpentine belt for ALT and A/C. The engine only has 40K miles on it, the original belt was replaced before the RV was put into long term storage a couple years ago. The second belt only has about 10 hours from running the engine while in storage. I can see the grooves are a little wider than the new belt I just bought and put on but the belt looks good. I replaced it because I could not really find the true issue and felt it was worth a shot to replace it. I have checked all the pulleys for play and not found any. I checked the crank pulley as best I can and found no play in it.

I was able to put the laser on to the a/c pulley. I put the other piece onto the ALT, looks like its lined up. I then removed the idler pulley and ran it to the fan, looks like it lined up. Then I went down to the crank from the A/C with the tensioner removed. It looks like there crank pulley is off by one ridge on the pulley. It looks like the pulley needs to come ways from the block by one ridge. The DAYCO belt alignment kit is what I am using. I cannot get it to work on the fan or crank pulley because it looks like the ridges are not high enough to allow the laser on switch to be activated. I checked the tension between the Alt and idler pulley and it was 160 pounds. The other thing I did notice as I was working with the crank pulley, there looks like a lot of built up rubber in the grooves of the crank pulley, much more than the fan pulley. I am going to be limited on how much tear down I can do as I will need to go back to Alaska for a couple weeks soon.

I am going to buy the laser alignment checking too, I need to eliminate that as a cause or find the problem pulley.

One other question, I looked on line and could not find any information on turning a CAT 3126 over by hand. Is there a jacking port someplace? I know on the big ALCO's I used to work on there was a fitting to turn it over.

I checked all pulleys , they turn freely. The belt starts to move off the A/c fairly quickly when turned over with the key, usually second short crank. ( although a couple times it took 4-5 attempts before it came off) I need to check the idler pulley. I have not kept a log book of each attempt because I did not think it would take this long. I do recall that the wear mark on the idler and the belt were not lined up, maybe 3mm movement on earlier attempts.

This CAT tensioner has one bolt and a nipple that inserts into the block, I do not believe it has a way to set the tension. When I start with the belt aligned in all pulleys, the belt is in the middle of the tensioner pulley. When it comes off the A/C inner pulley lip the belt has moved closer to the block on the tensioner. I have cleaned the fan pulley with a wire brush as well as the Crank Pulley each time I remove the belt. I will pull the tensioner again today and recheck that the nipple is in the exact location as the original tensioner, I believe it is but will double check. The belt I am using is a Napa Micro-V AT25-080702, as stated before I bought a new one and the problem continues.

I am from Alaska, but the RV is located in Plymouth MA at my sisters house.

The tensioner was bought from CAT, I brought the old one in, the old one was superseded by the one they gave me. The tit is in the same spot, I wire brushed the surface where the tensioner and the block meet and have rechecked the bolt tightness. Also the problem is almost identical as when the old tensioner was installed. I have sent a package with pictures to Prestolite, they are trying to help. Does anyone have a Beaver with a 3126 that could send a picture of the alternator in place. The one possible solution I would like to rule out is that the upper bracket that holds the ALT adjustment bolt is on the block side of the ALT, I do not think it can go on the other side but I want to rule out every possibility.

This is close to my set up EXCEPT the A/c and Alt are in opposite places.

I checked all the pull-ups and there is no play. I am not even starting the engine only turning it over. It's still riding up on the lip of the AC pulley on the engine side. I am using the new belt. I am not sure what to do next. I have 2 shims in that move the ALT almost 1/4". Again, it's a Lecee-Neville 160A alternator. I have bought a new tensioner pulley and installed it, took idler pulley off and inspected it, and installed new belt. I also installed the 2 shims. I may be installing the ALT incorrectly some how. It only goes in one way for the bolts, but I may be aligning it incorrectly somehow in respect to the tightening process. the top bolt controls the angle of the ALT, the bottom bolt controls the fwd and aft alignment. joe

I bought a new belt today. The old belt, which had 10 hrs on it, has wider grooves from wear than the new out of the box belt. I will install it tomorrow and check if it stays on the pulley.

The belt is fairly new. I pulled it off and will take it to NAPA. I shimmed the alternator as much as I could. I adjusted the adjustment bolt as much as possible and I am still ever so slightly riding up on the inside of the pulley. it is the closest I have been but just slightly out of alignment. When I run a stick across the pullies they appear to be aligned, I am not sure if I should try to change the shims on the a/c as it was never removed. The belt will run up on the lip of the AC pulley and then go back into the track , I am close but not enough to call it done.

Todays efforts included pulling alternator completely off, driving bushing back into hole in rear of Alt, sanding inner surfaces of Alt and cast iron bearing surfaces. I reinstalled with a larger 3/16 shim. I tried it and had it come off 2 channels. I then loosened the adjustment bolt on the top of the bracket as far as I could, this produced the best results of the day with it barely riding up on the rear lip of the pully but enough to eat the belt. I do not believe I can get another shim into the hole as I only have 1/32 to deal with. I maybe adjusting it incorrectly, I am not sure at this point as I followed the directions listed on the ALT. I could not get the pull off the ALT and did not want to fool with it too much as it looks good. My only other option left is to start grinding the cast piece, I want to exhaust the adjustment issue first as an examination of the belt leads me to believe it was aligned before I took it apart. Joe

Ok, I will try that. Heading out now to give it another go.

Bill, I think that is the plan to try today (see if I can file down space for another washer), I have exhausted all other option. I just wish I could have identified the cause since the alternator was the only piece ever removed from the system. Joe

It is a Leece Neville 160A. You do not take the alternator apart to replace the brushes. There is a plate on the voltage regulator box, I removed the 4 screws and the cover and inserted the new brushes in and put the 4 screws and cover back on. Then I reinstalled the alternator into the RV, The alternator can only go on one way, and it can only be aligned in the set up one way to get the pulley into the correct position. The one thing I am not sure of is that the pulley was aligned properly before I took it apart. I did not want to tear into the pulley before I asked on here if anyone had the issue before. I will try to get pictures. It is a pretty standard set up, the 3126 has a cast bracket that holds the alternator. The bottom of the alternator bracket has a cast tube were the bolt goes through. There is a bushing on the aft end of the alternator which I have put flush with the alternator so I could use the space to put a shim on the front end of the cast piece. This helped me cut the distance it is moving in half but I am still 1/16 out of alignment.

Pulley was not taken off shaft. I just changed the brushes.

I have a Patriot Beaver (1999) with a CAT 3126, I pulled the Alternator to replace the brushes. I did not remove the A/C. I kept the bolts and washers together. I reinstalled the Alternator and now the alternator pulley is off by 1/8" and it pulls the belt toward the block and over the lip of the A/C pulley. I replaced the belt tensioner with a new one from CAT because I thought the belt was loose and slipping. I believe I have shimmed the Alternator as much as possible because it is flush between the iron bracket and the alternator, but I am still coming off the pulley by one groove now, down from 2. If anyone has had a similar problem please let me know. I am on day 4 of trying to get the pulleys back in alignment. I am not sure if I can shim the alternator pulley. As noted above, the AC was not touched at all and I verified the alignment of the AC and the tensioner pulley.